xref: /linux/kernel/sched/ext.c (revision de95ad90fb19e4b7778a0c27115a4639c7c8b186)

/* SPDX-License-Identifier: GPL-2.0 */
/*
 * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst
 *
 * Copyright (c) 2022 Meta Platforms, Inc. and affiliates.
 * Copyright (c) 2022 Tejun Heo <tj@kernel.org>
 * Copyright (c) 2022 David Vernet <dvernet@meta.com>
 */
#include <linux/btf_ids.h>
#include "ext_idle.h"

static DEFINE_RAW_SPINLOCK(scx_sched_lock);

/*
 * NOTE: sched_ext is in the process of growing multiple scheduler support and
 * scx_root usage is in a transitional state. Naked dereferences are safe if the
 * caller is one of the tasks attached to SCX and explicit RCU dereference is
 * necessary otherwise. Naked scx_root dereferences trigger sparse warnings but
 * are used as temporary markers to indicate that the dereferences need to be
 * updated to point to the associated scheduler instances rather than scx_root.
 */
struct scx_sched __rcu *scx_root;

/*
 * All scheds, writers must hold both scx_enable_mutex and scx_sched_lock.
 * Readers can hold either or rcu_read_lock().
 */
static LIST_HEAD(scx_sched_all);

#ifdef CONFIG_EXT_SUB_SCHED
static const struct rhashtable_params scx_sched_hash_params = {
	.key_len		= sizeof_field(struct scx_sched, ops.sub_cgroup_id),
	.key_offset		= offsetof(struct scx_sched, ops.sub_cgroup_id),
	.head_offset		= offsetof(struct scx_sched, hash_node),
	.insecure_elasticity	= true,	/* inserted under scx_sched_lock */
};

static struct rhashtable scx_sched_hash;
#endif

/*
 * During exit, a task may schedule after losing its PIDs. When disabling the
 * BPF scheduler, we need to be able to iterate tasks in every state to
 * guarantee system safety. Maintain a dedicated task list which contains every
 * task between its fork and eventual free.
 */
static DEFINE_RAW_SPINLOCK(scx_tasks_lock);
static LIST_HEAD(scx_tasks);

/* ops enable/disable */
static DEFINE_MUTEX(scx_enable_mutex);
DEFINE_STATIC_KEY_FALSE(__scx_enabled);
DEFINE_STATIC_PERCPU_RWSEM(scx_fork_rwsem);
static atomic_t scx_enable_state_var = ATOMIC_INIT(SCX_DISABLED);
static DEFINE_RAW_SPINLOCK(scx_bypass_lock);
static bool scx_init_task_enabled;
static bool scx_switching_all;
DEFINE_STATIC_KEY_FALSE(__scx_switched_all);

static atomic_long_t scx_nr_rejected = ATOMIC_LONG_INIT(0);
static atomic_long_t scx_hotplug_seq = ATOMIC_LONG_INIT(0);

#ifdef CONFIG_EXT_SUB_SCHED
/*
 * The sub sched being enabled. Used by scx_disable_and_exit_task() to exit
 * tasks for the sub-sched being enabled. Use a global variable instead of a
 * per-task field as all enables are serialized.
 */
static struct scx_sched *scx_enabling_sub_sched;
#else
#define scx_enabling_sub_sched	(struct scx_sched *)NULL
#endif	/* CONFIG_EXT_SUB_SCHED */

/*
 * A monotonically increasing sequence number that is incremented every time a
 * scheduler is enabled. This can be used to check if any custom sched_ext
 * scheduler has ever been used in the system.
 */
static atomic_long_t scx_enable_seq = ATOMIC_LONG_INIT(0);

/*
 * Watchdog interval. All scx_sched's share a single watchdog timer and the
 * interval is half of the shortest sch->watchdog_timeout.
 */
static unsigned long scx_watchdog_interval;

/*
 * The last time the delayed work was run. This delayed work relies on
 * ksoftirqd being able to run to service timer interrupts, so it's possible
 * that this work itself could get wedged. To account for this, we check that
 * it's not stalled in the timer tick, and trigger an error if it is.
 */
static unsigned long scx_watchdog_timestamp = INITIAL_JIFFIES;

static struct delayed_work scx_watchdog_work;

/*
 * For %SCX_KICK_WAIT: Each CPU has a pointer to an array of kick_sync sequence
 * numbers. The arrays are allocated with kvzalloc() as size can exceed percpu
 * allocator limits on large machines. O(nr_cpu_ids^2) allocation, allocated
 * lazily when enabling and freed when disabling to avoid waste when sched_ext
 * isn't active.
 */
struct scx_kick_syncs {
	struct rcu_head		rcu;
	unsigned long		syncs[];
};

static DEFINE_PER_CPU(struct scx_kick_syncs __rcu *, scx_kick_syncs);

/*
 * Direct dispatch marker.
 *
 * Non-NULL values are used for direct dispatch from enqueue path. A valid
 * pointer points to the task currently being enqueued. An ERR_PTR value is used
 * to indicate that direct dispatch has already happened.
 */
static DEFINE_PER_CPU(struct task_struct *, direct_dispatch_task);

static const struct rhashtable_params dsq_hash_params = {
	.key_len		= sizeof_field(struct scx_dispatch_q, id),
	.key_offset		= offsetof(struct scx_dispatch_q, id),
	.head_offset		= offsetof(struct scx_dispatch_q, hash_node),
};

static LLIST_HEAD(dsqs_to_free);

/* string formatting from BPF */
struct scx_bstr_buf {
	u64			data[MAX_BPRINTF_VARARGS];
	char			line[SCX_EXIT_MSG_LEN];
};

static DEFINE_RAW_SPINLOCK(scx_exit_bstr_buf_lock);
static struct scx_bstr_buf scx_exit_bstr_buf;

/* ops debug dump */
static DEFINE_RAW_SPINLOCK(scx_dump_lock);

struct scx_dump_data {
	s32			cpu;
	bool			first;
	s32			cursor;
	struct seq_buf		*s;
	const char		*prefix;
	struct scx_bstr_buf	buf;
};

static struct scx_dump_data scx_dump_data = {
	.cpu			= -1,
};

/* /sys/kernel/sched_ext interface */
static struct kset *scx_kset;

/*
 * Parameters that can be adjusted through /sys/module/sched_ext/parameters.
 * There usually is no reason to modify these as normal scheduler operation
 * shouldn't be affected by them. The knobs are primarily for debugging.
 */
static unsigned int scx_slice_bypass_us = SCX_SLICE_BYPASS / NSEC_PER_USEC;
static unsigned int scx_bypass_lb_intv_us = SCX_BYPASS_LB_DFL_INTV_US;

static int set_slice_us(const char *val, const struct kernel_param *kp)
{
	return param_set_uint_minmax(val, kp, 100, 100 * USEC_PER_MSEC);
}

static const struct kernel_param_ops slice_us_param_ops = {
	.set = set_slice_us,
	.get = param_get_uint,
};

static int set_bypass_lb_intv_us(const char *val, const struct kernel_param *kp)
{
	return param_set_uint_minmax(val, kp, 0, 10 * USEC_PER_SEC);
}

static const struct kernel_param_ops bypass_lb_intv_us_param_ops = {
	.set = set_bypass_lb_intv_us,
	.get = param_get_uint,
};

#undef MODULE_PARAM_PREFIX
#define MODULE_PARAM_PREFIX	"sched_ext."

module_param_cb(slice_bypass_us, &slice_us_param_ops, &scx_slice_bypass_us, 0600);
MODULE_PARM_DESC(slice_bypass_us, "bypass slice in microseconds, applied on [un]load (100us to 100ms)");
module_param_cb(bypass_lb_intv_us, &bypass_lb_intv_us_param_ops, &scx_bypass_lb_intv_us, 0600);
MODULE_PARM_DESC(bypass_lb_intv_us, "bypass load balance interval in microseconds (0 (disable) to 10s)");

#undef MODULE_PARAM_PREFIX

#define CREATE_TRACE_POINTS
#include <trace/events/sched_ext.h>

static void run_deferred(struct rq *rq);
static bool task_dead_and_done(struct task_struct *p);
static void scx_kick_cpu(struct scx_sched *sch, s32 cpu, u64 flags);
static void scx_disable(struct scx_sched *sch, enum scx_exit_kind kind);
static bool scx_vexit(struct scx_sched *sch, enum scx_exit_kind kind,
		      s64 exit_code, const char *fmt, va_list args);

static __printf(4, 5) bool scx_exit(struct scx_sched *sch,
				    enum scx_exit_kind kind, s64 exit_code,
				    const char *fmt, ...)
{
	va_list args;
	bool ret;

	va_start(args, fmt);
	ret = scx_vexit(sch, kind, exit_code, fmt, args);
	va_end(args);

	return ret;
}

#define scx_error(sch, fmt, args...)	scx_exit((sch), SCX_EXIT_ERROR, 0, fmt, ##args)
#define scx_verror(sch, fmt, args)	scx_vexit((sch), SCX_EXIT_ERROR, 0, fmt, args)

#define SCX_HAS_OP(sch, op)	test_bit(SCX_OP_IDX(op), (sch)->has_op)

static long jiffies_delta_msecs(unsigned long at, unsigned long now)
{
	if (time_after(at, now))
		return jiffies_to_msecs(at - now);
	else
		return -(long)jiffies_to_msecs(now - at);
}

static bool u32_before(u32 a, u32 b)
{
	return (s32)(a - b) < 0;
}

#ifdef CONFIG_EXT_SUB_SCHED
/**
 * scx_parent - Find the parent sched
 * @sch: sched to find the parent of
 *
 * Returns the parent scheduler or %NULL if @sch is root.
 */
static struct scx_sched *scx_parent(struct scx_sched *sch)
{
	if (sch->level)
		return sch->ancestors[sch->level - 1];
	else
		return NULL;
}

/**
 * scx_next_descendant_pre - find the next descendant for pre-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: sched whose descendants to walk
 *
 * To be used by scx_for_each_descendant_pre(). Find the next descendant to
 * visit for pre-order traversal of @root's descendants. @root is included in
 * the iteration and the first node to be visited.
 */
static struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos,
						 struct scx_sched *root)
{
	struct scx_sched *next;

	lockdep_assert(lockdep_is_held(&scx_enable_mutex) ||
		       lockdep_is_held(&scx_sched_lock));

	/* if first iteration, visit @root */
	if (!pos)
		return root;

	/* visit the first child if exists */
	next = list_first_entry_or_null(&pos->children, struct scx_sched, sibling);
	if (next)
		return next;

	/* no child, visit my or the closest ancestor's next sibling */
	while (pos != root) {
		if (!list_is_last(&pos->sibling, &scx_parent(pos)->children))
			return list_next_entry(pos, sibling);
		pos = scx_parent(pos);
	}

	return NULL;
}

static struct scx_sched *scx_find_sub_sched(u64 cgroup_id)
{
	return rhashtable_lookup(&scx_sched_hash, &cgroup_id,
				 scx_sched_hash_params);
}

static void scx_set_task_sched(struct task_struct *p, struct scx_sched *sch)
{
	rcu_assign_pointer(p->scx.sched, sch);
}
#else	/* CONFIG_EXT_SUB_SCHED */
static struct scx_sched *scx_parent(struct scx_sched *sch) { return NULL; }
static struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos, struct scx_sched *root) { return pos ? NULL : root; }
static struct scx_sched *scx_find_sub_sched(u64 cgroup_id) { return NULL; }
static void scx_set_task_sched(struct task_struct *p, struct scx_sched *sch) {}
#endif	/* CONFIG_EXT_SUB_SCHED */

/**
 * scx_is_descendant - Test whether sched is a descendant
 * @sch: sched to test
 * @ancestor: ancestor sched to test against
 *
 * Test whether @sch is a descendant of @ancestor.
 */
static bool scx_is_descendant(struct scx_sched *sch, struct scx_sched *ancestor)
{
	if (sch->level < ancestor->level)
		return false;
	return sch->ancestors[ancestor->level] == ancestor;
}

/**
 * scx_for_each_descendant_pre - pre-order walk of a sched's descendants
 * @pos: iteration cursor
 * @root: sched to walk the descendants of
 *
 * Walk @root's descendants. @root is included in the iteration and the first
 * node to be visited. Must be called with either scx_enable_mutex or
 * scx_sched_lock held.
 */
#define scx_for_each_descendant_pre(pos, root)					\
	for ((pos) = scx_next_descendant_pre(NULL, (root)); (pos);		\
	     (pos) = scx_next_descendant_pre((pos), (root)))

static struct scx_dispatch_q *find_global_dsq(struct scx_sched *sch, s32 cpu)
{
	return &sch->pnode[cpu_to_node(cpu)]->global_dsq;
}

static struct scx_dispatch_q *find_user_dsq(struct scx_sched *sch, u64 dsq_id)
{
	return rhashtable_lookup(&sch->dsq_hash, &dsq_id, dsq_hash_params);
}

static const struct sched_class *scx_setscheduler_class(struct task_struct *p)
{
	if (p->sched_class == &stop_sched_class)
		return &stop_sched_class;

	return __setscheduler_class(p->policy, p->prio);
}

static struct scx_dispatch_q *bypass_dsq(struct scx_sched *sch, s32 cpu)
{
	return &per_cpu_ptr(sch->pcpu, cpu)->bypass_dsq;
}

static struct scx_dispatch_q *bypass_enq_target_dsq(struct scx_sched *sch, s32 cpu)
{
#ifdef CONFIG_EXT_SUB_SCHED
	/*
	 * If @sch is a sub-sched which is bypassing, its tasks should go into
	 * the bypass DSQs of the nearest ancestor which is not bypassing. The
	 * not-bypassing ancestor is responsible for scheduling all tasks from
	 * bypassing sub-trees. If all ancestors including root are bypassing,
	 * all tasks should go to the root's bypass DSQs.
	 *
	 * Whenever a sched starts bypassing, all runnable tasks in its subtree
	 * are re-enqueued after scx_bypassing() is turned on, guaranteeing that
	 * all tasks are transferred to the right DSQs.
	 */
	while (scx_parent(sch) && scx_bypassing(sch, cpu))
		sch = scx_parent(sch);
#endif	/* CONFIG_EXT_SUB_SCHED */

	return bypass_dsq(sch, cpu);
}

/**
 * bypass_dsp_enabled - Check if bypass dispatch path is enabled
 * @sch: scheduler to check
 *
 * When a descendant scheduler enters bypass mode, bypassed tasks are scheduled
 * by the nearest non-bypassing ancestor, or the root scheduler if all ancestors
 * are bypassing. In the former case, the ancestor is not itself bypassing but
 * its bypass DSQs will be populated with bypassed tasks from descendants. Thus,
 * the ancestor's bypass dispatch path must be active even though its own
 * bypass_depth remains zero.
 *
 * This function checks bypass_dsp_enable_depth which is managed separately from
 * bypass_depth to enable this decoupling. See enable_bypass_dsp() and
 * disable_bypass_dsp().
 */
static bool bypass_dsp_enabled(struct scx_sched *sch)
{
	return unlikely(atomic_read(&sch->bypass_dsp_enable_depth));
}

/**
 * rq_is_open - Is the rq available for immediate execution of an SCX task?
 * @rq: rq to test
 * @enq_flags: optional %SCX_ENQ_* of the task being enqueued
 *
 * Returns %true if @rq is currently open for executing an SCX task. After a
 * %false return, @rq is guaranteed to invoke SCX dispatch path at least once
 * before going to idle and not inserting a task into @rq's local DSQ after a
 * %false return doesn't cause @rq to stall.
 */
static bool rq_is_open(struct rq *rq, u64 enq_flags)
{
	lockdep_assert_rq_held(rq);

	/*
	 * A higher-priority class task is either running or in the process of
	 * waking up on @rq.
	 */
	if (sched_class_above(rq->next_class, &ext_sched_class))
		return false;

	/*
	 * @rq is either in transition to or in idle and there is no
	 * higher-priority class task waking up on it.
	 */
	if (sched_class_above(&ext_sched_class, rq->next_class))
		return true;

	/*
	 * @rq is either picking, in transition to, or running an SCX task.
	 */

	/*
	 * If we're in the dispatch path holding rq lock, $curr may or may not
	 * be ready depending on whether the on-going dispatch decides to extend
	 * $curr's slice. We say yes here and resolve it at the end of dispatch.
	 * See balance_one().
	 */
	if (rq->scx.flags & SCX_RQ_IN_BALANCE)
		return true;

	/*
	 * %SCX_ENQ_PREEMPT clears $curr's slice if on SCX and kicks dispatch,
	 * so allow it to avoid spuriously triggering reenq on a combined
	 * PREEMPT|IMMED insertion.
	 */
	if (enq_flags & SCX_ENQ_PREEMPT)
		return true;

	/*
	 * @rq is either in transition to or running an SCX task and can't go
	 * idle without another SCX dispatch cycle.
	 */
	return false;
}

/*
 * Track the rq currently locked.
 *
 * This allows kfuncs to safely operate on rq from any scx ops callback,
 * knowing which rq is already locked.
 */
DEFINE_PER_CPU(struct rq *, scx_locked_rq_state);

static inline void update_locked_rq(struct rq *rq)
{
	/*
	 * Check whether @rq is actually locked. This can help expose bugs
	 * or incorrect assumptions about the context in which a kfunc or
	 * callback is executed.
	 */
	if (rq)
		lockdep_assert_rq_held(rq);
	__this_cpu_write(scx_locked_rq_state, rq);
}

/*
 * SCX ops can recurse via scx_bpf_sub_dispatch() - the inner call must not
 * clobber the outer's scx_locked_rq_state. Save it on entry, restore on exit.
 */
#define SCX_CALL_OP(sch, op, locked_rq, args...)				\
do {										\
	struct rq *__prev_locked_rq;						\
										\
	if (locked_rq) {							\
		__prev_locked_rq = scx_locked_rq();				\
		update_locked_rq(locked_rq);					\
	}									\
	(sch)->ops.op(args);							\
	if (locked_rq)								\
		update_locked_rq(__prev_locked_rq);				\
} while (0)

#define SCX_CALL_OP_RET(sch, op, locked_rq, args...)				\
({										\
	struct rq *__prev_locked_rq;						\
	__typeof__((sch)->ops.op(args)) __ret;					\
										\
	if (locked_rq) {							\
		__prev_locked_rq = scx_locked_rq();				\
		update_locked_rq(locked_rq);					\
	}									\
	__ret = (sch)->ops.op(args);						\
	if (locked_rq)								\
		update_locked_rq(__prev_locked_rq);				\
	__ret;									\
})

/*
 * SCX_CALL_OP_TASK*() invokes an SCX op that takes one or two task arguments
 * and records them in current->scx.kf_tasks[] for the duration of the call. A
 * kfunc invoked from inside such an op can then use
 * scx_kf_arg_task_ok() to verify that its task argument is one of
 * those subject tasks.
 *
 * Every SCX_CALL_OP_TASK*() call site invokes its op with @p's rq lock held -
 * either via the @locked_rq argument here, or (for ops.select_cpu()) via @p's
 * pi_lock held by try_to_wake_up() with rq tracking via scx_rq.in_select_cpu.
 * So if kf_tasks[] is set, @p's scheduler-protected fields are stable.
 *
 * kf_tasks[] can not stack, so task-based SCX ops must not nest. The
 * WARN_ON_ONCE() in each macro catches a re-entry of any of the three variants
 * while a previous one is still in progress.
 */
#define SCX_CALL_OP_TASK(sch, op, locked_rq, task, args...)			\
do {										\
	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
	current->scx.kf_tasks[0] = task;					\
	SCX_CALL_OP((sch), op, locked_rq, task, ##args);			\
	current->scx.kf_tasks[0] = NULL;					\
} while (0)

#define SCX_CALL_OP_TASK_RET(sch, op, locked_rq, task, args...)			\
({										\
	__typeof__((sch)->ops.op(task, ##args)) __ret;				\
	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
	current->scx.kf_tasks[0] = task;					\
	__ret = SCX_CALL_OP_RET((sch), op, locked_rq, task, ##args);		\
	current->scx.kf_tasks[0] = NULL;					\
	__ret;									\
})

#define SCX_CALL_OP_2TASKS_RET(sch, op, locked_rq, task0, task1, args...)	\
({										\
	__typeof__((sch)->ops.op(task0, task1, ##args)) __ret;			\
	WARN_ON_ONCE(current->scx.kf_tasks[0]);					\
	current->scx.kf_tasks[0] = task0;					\
	current->scx.kf_tasks[1] = task1;					\
	__ret = SCX_CALL_OP_RET((sch), op, locked_rq, task0, task1, ##args);	\
	current->scx.kf_tasks[0] = NULL;					\
	current->scx.kf_tasks[1] = NULL;					\
	__ret;									\
})

/* see SCX_CALL_OP_TASK() */
static __always_inline bool scx_kf_arg_task_ok(struct scx_sched *sch,
							struct task_struct *p)
{
	if (unlikely((p != current->scx.kf_tasks[0] &&
		      p != current->scx.kf_tasks[1]))) {
		scx_error(sch, "called on a task not being operated on");
		return false;
	}

	return true;
}

enum scx_dsq_iter_flags {
	/* iterate in the reverse dispatch order */
	SCX_DSQ_ITER_REV		= 1U << 16,

	__SCX_DSQ_ITER_HAS_SLICE	= 1U << 30,
	__SCX_DSQ_ITER_HAS_VTIME	= 1U << 31,

	__SCX_DSQ_ITER_USER_FLAGS	= SCX_DSQ_ITER_REV,
	__SCX_DSQ_ITER_ALL_FLAGS	= __SCX_DSQ_ITER_USER_FLAGS |
					  __SCX_DSQ_ITER_HAS_SLICE |
					  __SCX_DSQ_ITER_HAS_VTIME,
};

/**
 * nldsq_next_task - Iterate to the next task in a non-local DSQ
 * @dsq: non-local dsq being iterated
 * @cur: current position, %NULL to start iteration
 * @rev: walk backwards
 *
 * Returns %NULL when iteration is finished.
 */
static struct task_struct *nldsq_next_task(struct scx_dispatch_q *dsq,
					   struct task_struct *cur, bool rev)
{
	struct list_head *list_node;
	struct scx_dsq_list_node *dsq_lnode;

	lockdep_assert_held(&dsq->lock);

	if (cur)
		list_node = &cur->scx.dsq_list.node;
	else
		list_node = &dsq->list;

	/* find the next task, need to skip BPF iteration cursors */
	do {
		if (rev)
			list_node = list_node->prev;
		else
			list_node = list_node->next;

		if (list_node == &dsq->list)
			return NULL;

		dsq_lnode = container_of(list_node, struct scx_dsq_list_node,
					 node);
	} while (dsq_lnode->flags & SCX_DSQ_LNODE_ITER_CURSOR);

	return container_of(dsq_lnode, struct task_struct, scx.dsq_list);
}

#define nldsq_for_each_task(p, dsq)						\
	for ((p) = nldsq_next_task((dsq), NULL, false); (p);			\
	     (p) = nldsq_next_task((dsq), (p), false))

/**
 * nldsq_cursor_next_task - Iterate to the next task given a cursor in a non-local DSQ
 * @cursor: scx_dsq_list_node initialized with INIT_DSQ_LIST_CURSOR()
 * @dsq: non-local dsq being iterated
 *
 * Find the next task in a cursor based iteration. The caller must have
 * initialized @cursor using INIT_DSQ_LIST_CURSOR() and can release the DSQ lock
 * between the iteration steps.
 *
 * Only tasks which were queued before @cursor was initialized are visible. This
 * bounds the iteration and guarantees that vtime never jumps in the other
 * direction while iterating.
 */
static struct task_struct *nldsq_cursor_next_task(struct scx_dsq_list_node *cursor,
						  struct scx_dispatch_q *dsq)
{
	bool rev = cursor->flags & SCX_DSQ_ITER_REV;
	struct task_struct *p;

	lockdep_assert_held(&dsq->lock);
	BUG_ON(!(cursor->flags & SCX_DSQ_LNODE_ITER_CURSOR));

	if (list_empty(&cursor->node))
		p = NULL;
	else
		p = container_of(cursor, struct task_struct, scx.dsq_list);

	/* skip cursors and tasks that were queued after @cursor init */
	do {
		p = nldsq_next_task(dsq, p, rev);
	} while (p && unlikely(u32_before(cursor->priv, p->scx.dsq_seq)));

	if (p) {
		if (rev)
			list_move_tail(&cursor->node, &p->scx.dsq_list.node);
		else
			list_move(&cursor->node, &p->scx.dsq_list.node);
	} else {
		list_del_init(&cursor->node);
	}

	return p;
}

/**
 * nldsq_cursor_lost_task - Test whether someone else took the task since iteration
 * @cursor: scx_dsq_list_node initialized with INIT_DSQ_LIST_CURSOR()
 * @rq: rq @p was on
 * @dsq: dsq @p was on
 * @p: target task
 *
 * @p is a task returned by nldsq_cursor_next_task(). The locks may have been
 * dropped and re-acquired inbetween. Verify that no one else took or is in the
 * process of taking @p from @dsq.
 *
 * On %false return, the caller can assume full ownership of @p.
 */
static bool nldsq_cursor_lost_task(struct scx_dsq_list_node *cursor,
				   struct rq *rq, struct scx_dispatch_q *dsq,
				   struct task_struct *p)
{
	lockdep_assert_rq_held(rq);
	lockdep_assert_held(&dsq->lock);

	/*
	 * @p could have already left $src_dsq, got re-enqueud, or be in the
	 * process of being consumed by someone else.
	 */
	if (unlikely(p->scx.dsq != dsq ||
		     u32_before(cursor->priv, p->scx.dsq_seq) ||
		     p->scx.holding_cpu >= 0))
		return true;

	/* if @p has stayed on @dsq, its rq couldn't have changed */
	if (WARN_ON_ONCE(rq != task_rq(p)))
		return true;

	return false;
}

/*
 * BPF DSQ iterator. Tasks in a non-local DSQ can be iterated in [reverse]
 * dispatch order. BPF-visible iterator is opaque and larger to allow future
 * changes without breaking backward compatibility. Can be used with
 * bpf_for_each(). See bpf_iter_scx_dsq_*().
 */
struct bpf_iter_scx_dsq_kern {
	struct scx_dsq_list_node	cursor;
	struct scx_dispatch_q		*dsq;
	u64				slice;
	u64				vtime;
} __attribute__((aligned(8)));

struct bpf_iter_scx_dsq {
	u64				__opaque[6];
} __attribute__((aligned(8)));


/*
 * SCX task iterator.
 */
struct scx_task_iter {
	struct sched_ext_entity		cursor;
	struct task_struct		*locked_task;
	struct rq			*rq;
	struct rq_flags			rf;
	u32				cnt;
	bool				list_locked;
#ifdef CONFIG_EXT_SUB_SCHED
	struct cgroup			*cgrp;
	struct cgroup_subsys_state	*css_pos;
	struct css_task_iter		css_iter;
#endif
};

/**
 * scx_task_iter_start - Lock scx_tasks_lock and start a task iteration
 * @iter: iterator to init
 * @cgrp: Optional root of cgroup subhierarchy to iterate
 *
 * Initialize @iter. Once initialized, @iter must eventually be stopped with
 * scx_task_iter_stop().
 *
 * If @cgrp is %NULL, scx_tasks is used for iteration and this function returns
 * with scx_tasks_lock held and @iter->cursor inserted into scx_tasks.
 *
 * If @cgrp is not %NULL, @cgrp and its descendants' tasks are walked using
 * @iter->css_iter. The caller must be holding cgroup_lock() to prevent cgroup
 * task migrations.
 *
 * The two modes of iterations are largely independent and it's likely that
 * scx_tasks can be removed in favor of always using cgroup iteration if
 * CONFIG_SCHED_CLASS_EXT depends on CONFIG_CGROUPS.
 *
 * scx_tasks_lock and the rq lock may be released using scx_task_iter_unlock()
 * between this and the first next() call or between any two next() calls. If
 * the locks are released between two next() calls, the caller is responsible
 * for ensuring that the task being iterated remains accessible either through
 * RCU read lock or obtaining a reference count.
 *
 * All tasks which existed when the iteration started are guaranteed to be
 * visited as long as they are not dead.
 */
static void scx_task_iter_start(struct scx_task_iter *iter, struct cgroup *cgrp)
{
	memset(iter, 0, sizeof(*iter));

#ifdef CONFIG_EXT_SUB_SCHED
	if (cgrp) {
		lockdep_assert_held(&cgroup_mutex);
		iter->cgrp = cgrp;
		iter->css_pos = css_next_descendant_pre(NULL, &iter->cgrp->self);
		css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
				    &iter->css_iter);
		return;
	}
#endif
	raw_spin_lock_irq(&scx_tasks_lock);

	iter->cursor = (struct sched_ext_entity){ .flags = SCX_TASK_CURSOR };
	list_add(&iter->cursor.tasks_node, &scx_tasks);
	iter->list_locked = true;
}

static void __scx_task_iter_rq_unlock(struct scx_task_iter *iter)
{
	if (iter->locked_task) {
		__balance_callbacks(iter->rq, &iter->rf);
		task_rq_unlock(iter->rq, iter->locked_task, &iter->rf);
		iter->locked_task = NULL;
	}
}

/**
 * scx_task_iter_unlock - Unlock rq and scx_tasks_lock held by a task iterator
 * @iter: iterator to unlock
 *
 * If @iter is in the middle of a locked iteration, it may be locking the rq of
 * the task currently being visited in addition to scx_tasks_lock. Unlock both.
 * This function can be safely called anytime during an iteration. The next
 * iterator operation will automatically restore the necessary locking.
 */
static void scx_task_iter_unlock(struct scx_task_iter *iter)
{
	__scx_task_iter_rq_unlock(iter);
	if (iter->list_locked) {
		iter->list_locked = false;
		raw_spin_unlock_irq(&scx_tasks_lock);
	}
}

static void __scx_task_iter_maybe_relock(struct scx_task_iter *iter)
{
	if (!iter->list_locked) {
		raw_spin_lock_irq(&scx_tasks_lock);
		iter->list_locked = true;
	}
}

/**
 * scx_task_iter_stop - Stop a task iteration and unlock scx_tasks_lock
 * @iter: iterator to exit
 *
 * Exit a previously initialized @iter. Must be called with scx_tasks_lock held
 * which is released on return. If the iterator holds a task's rq lock, that rq
 * lock is also released. See scx_task_iter_start() for details.
 */
static void scx_task_iter_stop(struct scx_task_iter *iter)
{
#ifdef CONFIG_EXT_SUB_SCHED
	if (iter->cgrp) {
		if (iter->css_pos)
			css_task_iter_end(&iter->css_iter);
		__scx_task_iter_rq_unlock(iter);
		return;
	}
#endif
	__scx_task_iter_maybe_relock(iter);
	list_del_init(&iter->cursor.tasks_node);
	scx_task_iter_unlock(iter);
}

/**
 * scx_task_iter_next - Next task
 * @iter: iterator to walk
 *
 * Visit the next task. See scx_task_iter_start() for details. Locks are dropped
 * and re-acquired every %SCX_TASK_ITER_BATCH iterations to avoid causing stalls
 * by holding scx_tasks_lock for too long.
 */
static struct task_struct *scx_task_iter_next(struct scx_task_iter *iter)
{
	struct list_head *cursor = &iter->cursor.tasks_node;
	struct sched_ext_entity *pos;

	if (!(++iter->cnt % SCX_TASK_ITER_BATCH)) {
		scx_task_iter_unlock(iter);
		cond_resched();
	}

#ifdef CONFIG_EXT_SUB_SCHED
	if (iter->cgrp) {
		while (iter->css_pos) {
			struct task_struct *p;

			p = css_task_iter_next(&iter->css_iter);
			if (p)
				return p;

			css_task_iter_end(&iter->css_iter);
			iter->css_pos = css_next_descendant_pre(iter->css_pos,
								&iter->cgrp->self);
			if (iter->css_pos)
				css_task_iter_start(iter->css_pos, CSS_TASK_ITER_WITH_DEAD,
						    &iter->css_iter);
		}
		return NULL;
	}
#endif
	__scx_task_iter_maybe_relock(iter);

	list_for_each_entry(pos, cursor, tasks_node) {
		if (&pos->tasks_node == &scx_tasks)
			return NULL;
		if (!(pos->flags & SCX_TASK_CURSOR)) {
			list_move(cursor, &pos->tasks_node);
			return container_of(pos, struct task_struct, scx);
		}
	}

	/* can't happen, should always terminate at scx_tasks above */
	BUG();
}

/**
 * scx_task_iter_next_locked - Next non-idle task with its rq locked
 * @iter: iterator to walk
 *
 * Visit the non-idle task with its rq lock held. Allows callers to specify
 * whether they would like to filter out dead tasks. See scx_task_iter_start()
 * for details.
 */
static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
{
	struct task_struct *p;

	__scx_task_iter_rq_unlock(iter);

	while ((p = scx_task_iter_next(iter))) {
		/*
		 * scx_task_iter is used to prepare and move tasks into SCX
		 * while loading the BPF scheduler and vice-versa while
		 * unloading. The init_tasks ("swappers") should be excluded
		 * from the iteration because:
		 *
		 * - It's unsafe to use __setschduler_prio() on an init_task to
		 *   determine the sched_class to use as it won't preserve its
		 *   idle_sched_class.
		 *
		 * - ops.init/exit_task() can easily be confused if called with
		 *   init_tasks as they, e.g., share PID 0.
		 *
		 * As init_tasks are never scheduled through SCX, they can be
		 * skipped safely. Note that is_idle_task() which tests %PF_IDLE
		 * doesn't work here:
		 *
		 * - %PF_IDLE may not be set for an init_task whose CPU hasn't
		 *   yet been onlined.
		 *
		 * - %PF_IDLE can be set on tasks that are not init_tasks. See
		 *   play_idle_precise() used by CONFIG_IDLE_INJECT.
		 *
		 * Test for idle_sched_class as only init_tasks are on it.
		 */
		if (p->sched_class == &idle_sched_class)
			continue;

		iter->rq = task_rq_lock(p, &iter->rf);
		iter->locked_task = p;

		/*
		 * cgroup_task_dead() removes the dead tasks from cset->tasks
		 * after sched_ext_dead() and cgroup iteration may see tasks
		 * which already finished sched_ext_dead(). %SCX_TASK_OFF_TASKS
		 * is set by sched_ext_dead() under @p's rq lock. Test it to
		 * avoid visiting tasks which are already dead from SCX POV.
		 */
		if (p->scx.flags & SCX_TASK_OFF_TASKS) {
			__scx_task_iter_rq_unlock(iter);
			continue;
		}

		return p;
	}
	return NULL;
}

/**
 * scx_add_event - Increase an event counter for 'name' by 'cnt'
 * @sch: scx_sched to account events for
 * @name: an event name defined in struct scx_event_stats
 * @cnt: the number of the event occurred
 *
 * This can be used when preemption is not disabled.
 */
#define scx_add_event(sch, name, cnt) do {					\
	this_cpu_add((sch)->pcpu->event_stats.name, (cnt));			\
	trace_sched_ext_event(#name, (cnt));					\
} while(0)

/**
 * __scx_add_event - Increase an event counter for 'name' by 'cnt'
 * @sch: scx_sched to account events for
 * @name: an event name defined in struct scx_event_stats
 * @cnt: the number of the event occurred
 *
 * This should be used only when preemption is disabled.
 */
#define __scx_add_event(sch, name, cnt) do {					\
	__this_cpu_add((sch)->pcpu->event_stats.name, (cnt));			\
	trace_sched_ext_event(#name, cnt);					\
} while(0)

/**
 * scx_agg_event - Aggregate an event counter 'kind' from 'src_e' to 'dst_e'
 * @dst_e: destination event stats
 * @src_e: source event stats
 * @kind: a kind of event to be aggregated
 */
#define scx_agg_event(dst_e, src_e, kind) do {					\
	(dst_e)->kind += READ_ONCE((src_e)->kind);				\
} while(0)

/**
 * scx_dump_event - Dump an event 'kind' in 'events' to 's'
 * @s: output seq_buf
 * @events: event stats
 * @kind: a kind of event to dump
 */
#define scx_dump_event(s, events, kind) do {					\
	dump_line(&(s), "%40s: %16lld", #kind, (events)->kind);			\
} while (0)


static void scx_read_events(struct scx_sched *sch,
			    struct scx_event_stats *events);

static enum scx_enable_state scx_enable_state(void)
{
	return atomic_read(&scx_enable_state_var);
}

static enum scx_enable_state scx_set_enable_state(enum scx_enable_state to)
{
	return atomic_xchg(&scx_enable_state_var, to);
}

static bool scx_tryset_enable_state(enum scx_enable_state to,
				    enum scx_enable_state from)
{
	int from_v = from;

	return atomic_try_cmpxchg(&scx_enable_state_var, &from_v, to);
}

/**
 * wait_ops_state - Busy-wait the specified ops state to end
 * @p: target task
 * @opss: state to wait the end of
 *
 * Busy-wait for @p to transition out of @opss. This can only be used when the
 * state part of @opss is %SCX_QUEUEING or %SCX_DISPATCHING. This function also
 * has load_acquire semantics to ensure that the caller can see the updates made
 * in the enqueueing and dispatching paths.
 */
static void wait_ops_state(struct task_struct *p, unsigned long opss)
{
	do {
		cpu_relax();
	} while (atomic_long_read_acquire(&p->scx.ops_state) == opss);
}

static inline bool __cpu_valid(s32 cpu)
{
	return likely(cpu >= 0 && cpu < nr_cpu_ids && cpu_possible(cpu));
}

/**
 * ops_cpu_valid - Verify a cpu number, to be used on ops input args
 * @sch: scx_sched to abort on error
 * @cpu: cpu number which came from a BPF ops
 * @where: extra information reported on error
 *
 * @cpu is a cpu number which came from the BPF scheduler and can be any value.
 * Verify that it is in range and one of the possible cpus. If invalid, trigger
 * an ops error.
 */
static bool ops_cpu_valid(struct scx_sched *sch, s32 cpu, const char *where)
{
	if (__cpu_valid(cpu)) {
		return true;
	} else {
		scx_error(sch, "invalid CPU %d%s%s", cpu, where ? " " : "", where ?: "");
		return false;
	}
}

/**
 * ops_sanitize_err - Sanitize a -errno value
 * @sch: scx_sched to error out on error
 * @ops_name: operation to blame on failure
 * @err: -errno value to sanitize
 *
 * Verify @err is a valid -errno. If not, trigger scx_error() and return
 * -%EPROTO. This is necessary because returning a rogue -errno up the chain can
 * cause misbehaviors. For an example, a large negative return from
 * ops.init_task() triggers an oops when passed up the call chain because the
 * value fails IS_ERR() test after being encoded with ERR_PTR() and then is
 * handled as a pointer.
 */
static int ops_sanitize_err(struct scx_sched *sch, const char *ops_name, s32 err)
{
	if (err < 0 && err >= -MAX_ERRNO)
		return err;

	scx_error(sch, "ops.%s() returned an invalid errno %d", ops_name, err);
	return -EPROTO;
}

static void deferred_bal_cb_workfn(struct rq *rq)
{
	run_deferred(rq);
}

static void deferred_irq_workfn(struct irq_work *irq_work)
{
	struct rq *rq = container_of(irq_work, struct rq, scx.deferred_irq_work);

	raw_spin_rq_lock(rq);
	run_deferred(rq);
	raw_spin_rq_unlock(rq);
}

/**
 * schedule_deferred - Schedule execution of deferred actions on an rq
 * @rq: target rq
 *
 * Schedule execution of deferred actions on @rq. Deferred actions are executed
 * with @rq locked but unpinned, and thus can unlock @rq to e.g. migrate tasks
 * to other rqs.
 */
static void schedule_deferred(struct rq *rq)
{
	/*
	 * This is the fallback when schedule_deferred_locked() can't use
	 * the cheaper balance callback or wakeup hook paths (the target
	 * CPU is not in balance or wakeup). Currently, this is primarily
	 * hit by reenqueue operations targeting a remote CPU.
	 *
	 * Queue on the target CPU. The deferred work can run from any CPU
	 * correctly - the _locked() path already processes remote rqs from
	 * the calling CPU - but targeting the owning CPU allows IPI delivery
	 * without waiting for the calling CPU to re-enable IRQs and is
	 * cheaper as the reenqueue runs locally.
	 */
	irq_work_queue_on(&rq->scx.deferred_irq_work, cpu_of(rq));
}

/**
 * schedule_deferred_locked - Schedule execution of deferred actions on an rq
 * @rq: target rq
 *
 * Schedule execution of deferred actions on @rq. Equivalent to
 * schedule_deferred() but requires @rq to be locked and can be more efficient.
 */
static void schedule_deferred_locked(struct rq *rq)
{
	lockdep_assert_rq_held(rq);

	/*
	 * If in the middle of waking up a task, task_woken_scx() will be called
	 * afterwards which will then run the deferred actions, no need to
	 * schedule anything.
	 */
	if (rq->scx.flags & SCX_RQ_IN_WAKEUP)
		return;

	/* Don't do anything if there already is a deferred operation. */
	if (rq->scx.flags & SCX_RQ_BAL_CB_PENDING)
		return;

	/*
	 * If in balance, the balance callbacks will be called before rq lock is
	 * released. Schedule one.
	 *
	 *
	 * We can't directly insert the callback into the
	 * rq's list: The call can drop its lock and make the pending balance
	 * callback visible to unrelated code paths that call rq_pin_lock().
	 *
	 * Just let balance_one() know that it must do it itself.
	 */
	if (rq->scx.flags & SCX_RQ_IN_BALANCE) {
		rq->scx.flags |= SCX_RQ_BAL_CB_PENDING;
		return;
	}

	/*
	 * No scheduler hooks available. Use the generic irq_work path. The
	 * above WAKEUP and BALANCE paths should cover most of the cases and the
	 * time to IRQ re-enable shouldn't be long.
	 */
	schedule_deferred(rq);
}

static void schedule_dsq_reenq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
			       u64 reenq_flags, struct rq *locked_rq)
{
	struct rq *rq;

	/*
	 * Allowing reenqueues doesn't make sense while bypassing. This also
	 * blocks from new reenqueues to be scheduled on dead scheds.
	 */
	if (unlikely(READ_ONCE(sch->bypass_depth)))
		return;

	if (dsq->id == SCX_DSQ_LOCAL) {
		rq = container_of(dsq, struct rq, scx.local_dsq);

		struct scx_sched_pcpu *sch_pcpu = per_cpu_ptr(sch->pcpu, cpu_of(rq));
		struct scx_deferred_reenq_local *drl = &sch_pcpu->deferred_reenq_local;

		/*
		 * Pairs with smp_mb() in process_deferred_reenq_locals() and
		 * guarantees that there is a reenq_local() afterwards.
		 */
		smp_mb();

		if (list_empty(&drl->node) ||
		    (READ_ONCE(drl->flags) & reenq_flags) != reenq_flags) {

			guard(raw_spinlock_irqsave)(&rq->scx.deferred_reenq_lock);

			if (list_empty(&drl->node))
				list_move_tail(&drl->node, &rq->scx.deferred_reenq_locals);
			WRITE_ONCE(drl->flags, drl->flags | reenq_flags);
		}
	} else if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN)) {
		rq = this_rq();

		struct scx_dsq_pcpu *dsq_pcpu = per_cpu_ptr(dsq->pcpu, cpu_of(rq));
		struct scx_deferred_reenq_user *dru = &dsq_pcpu->deferred_reenq_user;

		/*
		 * Pairs with smp_mb() in process_deferred_reenq_users() and
		 * guarantees that there is a reenq_user() afterwards.
		 */
		smp_mb();

		if (list_empty(&dru->node) ||
		    (READ_ONCE(dru->flags) & reenq_flags) != reenq_flags) {

			guard(raw_spinlock_irqsave)(&rq->scx.deferred_reenq_lock);

			if (list_empty(&dru->node))
				list_move_tail(&dru->node, &rq->scx.deferred_reenq_users);
			WRITE_ONCE(dru->flags, dru->flags | reenq_flags);
		}
	} else {
		scx_error(sch, "DSQ 0x%llx not allowed for reenq", dsq->id);
		return;
	}

	if (rq == locked_rq)
		schedule_deferred_locked(rq);
	else
		schedule_deferred(rq);
}

static void schedule_reenq_local(struct rq *rq, u64 reenq_flags)
{
	struct scx_sched *root = rcu_dereference_sched(scx_root);

	if (WARN_ON_ONCE(!root))
		return;

	schedule_dsq_reenq(root, &rq->scx.local_dsq, reenq_flags, rq);
}

/**
 * touch_core_sched - Update timestamp used for core-sched task ordering
 * @rq: rq to read clock from, must be locked
 * @p: task to update the timestamp for
 *
 * Update @p->scx.core_sched_at timestamp. This is used by scx_prio_less() to
 * implement global or local-DSQ FIFO ordering for core-sched. Should be called
 * when a task becomes runnable and its turn on the CPU ends (e.g. slice
 * exhaustion).
 */
static void touch_core_sched(struct rq *rq, struct task_struct *p)
{
	lockdep_assert_rq_held(rq);

#ifdef CONFIG_SCHED_CORE
	/*
	 * It's okay to update the timestamp spuriously. Use
	 * sched_core_disabled() which is cheaper than enabled().
	 *
	 * As this is used to determine ordering between tasks of sibling CPUs,
	 * it may be better to use per-core dispatch sequence instead.
	 */
	if (!sched_core_disabled())
		p->scx.core_sched_at = sched_clock_cpu(cpu_of(rq));
#endif
}

/**
 * touch_core_sched_dispatch - Update core-sched timestamp on dispatch
 * @rq: rq to read clock from, must be locked
 * @p: task being dispatched
 *
 * If the BPF scheduler implements custom core-sched ordering via
 * ops.core_sched_before(), @p->scx.core_sched_at is used to implement FIFO
 * ordering within each local DSQ. This function is called from dispatch paths
 * and updates @p->scx.core_sched_at if custom core-sched ordering is in effect.
 */
static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
{
	lockdep_assert_rq_held(rq);

#ifdef CONFIG_SCHED_CORE
	if (unlikely(SCX_HAS_OP(scx_root, core_sched_before)))
		touch_core_sched(rq, p);
#endif
}

static void update_curr_scx(struct rq *rq)
{
	struct task_struct *curr = rq->curr;
	s64 delta_exec;

	delta_exec = update_curr_common(rq);
	if (unlikely(delta_exec <= 0))
		return;

	if (curr->scx.slice != SCX_SLICE_INF) {
		curr->scx.slice -= min_t(u64, curr->scx.slice, delta_exec);
		if (!curr->scx.slice)
			touch_core_sched(rq, curr);
	}

	dl_server_update(&rq->ext_server, delta_exec);
}

static bool scx_dsq_priq_less(struct rb_node *node_a,
			      const struct rb_node *node_b)
{
	const struct task_struct *a =
		container_of(node_a, struct task_struct, scx.dsq_priq);
	const struct task_struct *b =
		container_of(node_b, struct task_struct, scx.dsq_priq);

	return time_before64(a->scx.dsq_vtime, b->scx.dsq_vtime);
}

static void dsq_inc_nr(struct scx_dispatch_q *dsq, struct task_struct *p, u64 enq_flags)
{
	/* scx_bpf_dsq_nr_queued() reads ->nr without locking, use WRITE_ONCE() */
	WRITE_ONCE(dsq->nr, dsq->nr + 1);

	/*
	 * Once @p reaches a local DSQ, it can only leave it by being dispatched
	 * to the CPU or dequeued. In both cases, the only way @p can go back to
	 * the BPF sched is through enqueueing. If being inserted into a local
	 * DSQ with IMMED, persist the state until the next enqueueing event in
	 * do_enqueue_task() so that we can maintain IMMED protection through
	 * e.g. SAVE/RESTORE cycles and slice extensions.
	 */
	if (enq_flags & SCX_ENQ_IMMED) {
		if (unlikely(dsq->id != SCX_DSQ_LOCAL)) {
			WARN_ON_ONCE(!(enq_flags & SCX_ENQ_GDSQ_FALLBACK));
			return;
		}
		p->scx.flags |= SCX_TASK_IMMED;
	}

	if (p->scx.flags & SCX_TASK_IMMED) {
		struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);

		if (WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL))
			return;

		rq->scx.nr_immed++;

		/*
		 * If @rq already had other tasks or the current task is not
		 * done yet, @p can't go on the CPU immediately. Re-enqueue.
		 */
		if (unlikely(dsq->nr > 1 || !rq_is_open(rq, enq_flags)))
			schedule_reenq_local(rq, 0);
	}
}

static void dsq_dec_nr(struct scx_dispatch_q *dsq, struct task_struct *p)
{
	/* see dsq_inc_nr() */
	WRITE_ONCE(dsq->nr, dsq->nr - 1);

	if (p->scx.flags & SCX_TASK_IMMED) {
		struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);

		if (WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL) ||
		    WARN_ON_ONCE(rq->scx.nr_immed <= 0))
			return;

		rq->scx.nr_immed--;
	}
}

static void refill_task_slice_dfl(struct scx_sched *sch, struct task_struct *p)
{
	p->scx.slice = READ_ONCE(sch->slice_dfl);
	__scx_add_event(sch, SCX_EV_REFILL_SLICE_DFL, 1);
}

/*
 * Return true if @p is moving due to an internal SCX migration, false
 * otherwise.
 */
static inline bool task_scx_migrating(struct task_struct *p)
{
	/*
	 * We only need to check sticky_cpu: it is set to the destination
	 * CPU in move_remote_task_to_local_dsq() before deactivate_task()
	 * and cleared when the task is enqueued on the destination, so it
	 * is only non-negative during an internal SCX migration.
	 */
	return p->scx.sticky_cpu >= 0;
}

/*
 * Call ops.dequeue() if the task is in BPF custody and not migrating.
 * Clears %SCX_TASK_IN_CUSTODY when the callback is invoked.
 */
static void call_task_dequeue(struct scx_sched *sch, struct rq *rq,
			      struct task_struct *p, u64 deq_flags)
{
	if (!(p->scx.flags & SCX_TASK_IN_CUSTODY) || task_scx_migrating(p))
		return;

	if (SCX_HAS_OP(sch, dequeue))
		SCX_CALL_OP_TASK(sch, dequeue, rq, p, deq_flags);

	p->scx.flags &= ~SCX_TASK_IN_CUSTODY;
}

static void local_dsq_post_enq(struct scx_sched *sch, struct scx_dispatch_q *dsq,
			       struct task_struct *p, u64 enq_flags)
{
	struct rq *rq = container_of(dsq, struct rq, scx.local_dsq);

	call_task_dequeue(sch, rq, p, 0);

	/*
	 * Note that @rq's lock may be dropped between this enqueue and @p
	 * actually getting on CPU. This gives higher-class tasks (e.g. RT)
	 * an opportunity to wake up on @rq and prevent @p from running.
	 * Here are some concrete examples:
	 *
	 * Example 1:
	 *
	 * We dispatch two tasks from a single ops.dispatch():
	 * - First, a local task to this CPU's local DSQ;
	 * - Second, a local/remote task to a remote CPU's local DSQ.
	 * We must drop the local rq lock in order to finish the second
	 * dispatch. In that time, an RT task can wake up on the local rq.
	 *
	 * Example 2:
	 *
	 * We dispatch a local/remote task to a remote CPU's local DSQ.
	 * We must drop the remote rq lock before the dispatched task can run,
	 * which gives an RT task an opportunity to wake up on the remote rq.
	 *
	 * Both examples work the same if we replace dispatching with moving
	 * the tasks from a user-created DSQ.
	 *
	 * We must detect these wakeups so that we can re-enqueue IMMED tasks
	 * from @rq's local DSQ. scx_wakeup_preempt() serves exactly this
	 * purpose, but for it to be invoked, we must ensure that we bump
	 * @rq->next_class to &ext_sched_class if it's currently idle.
	 *
	 * wakeup_preempt() does the bumping, and since we only invoke it if
	 * @rq->next_class is below &ext_sched_class, it will also
	 * resched_curr(rq).
	 */
	if (sched_class_above(p->sched_class, rq->next_class))
		wakeup_preempt(rq, p, 0);

	/*
	 * If @rq is in balance, the CPU is already vacant and looking for the
	 * next task to run. No need to preempt or trigger resched after moving
	 * @p into its local DSQ.
	 * Note that the wakeup_preempt() above may have already triggered
	 * a resched if @rq->next_class was idle. It's harmless, since
	 * need_resched is cleared immediately after task pick.
	 */
	if (rq->scx.flags & SCX_RQ_IN_BALANCE)
		return;

	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
	    rq->curr->sched_class == &ext_sched_class) {
		rq->curr->scx.slice = 0;
		resched_curr(rq);
	}
}

static void dispatch_enqueue(struct scx_sched *sch, struct rq *rq,
			     struct scx_dispatch_q *dsq, struct task_struct *p,
			     u64 enq_flags)
{
	bool is_local = dsq->id == SCX_DSQ_LOCAL;

	WARN_ON_ONCE(p->scx.dsq || !list_empty(&p->scx.dsq_list.node));
	WARN_ON_ONCE((p->scx.dsq_flags & SCX_TASK_DSQ_ON_PRIQ) ||
		     !RB_EMPTY_NODE(&p->scx.dsq_priq));

	if (!is_local) {
		raw_spin_lock_nested(&dsq->lock,
			(enq_flags & SCX_ENQ_NESTED) ? SINGLE_DEPTH_NESTING : 0);

		if (unlikely(dsq->id == SCX_DSQ_INVALID)) {
			scx_error(sch, "attempting to dispatch to a destroyed dsq");
			/* fall back to the global dsq */
			raw_spin_unlock(&dsq->lock);
			dsq = find_global_dsq(sch, task_cpu(p));
			raw_spin_lock(&dsq->lock);
		}
	}

	if (unlikely((dsq->id & SCX_DSQ_FLAG_BUILTIN) &&
		     (enq_flags & SCX_ENQ_DSQ_PRIQ))) {
		/*
		 * SCX_DSQ_LOCAL and SCX_DSQ_GLOBAL DSQs always consume from
		 * their FIFO queues. To avoid confusion and accidentally
		 * starving vtime-dispatched tasks by FIFO-dispatched tasks, we
		 * disallow any internal DSQ from doing vtime ordering of
		 * tasks.
		 */
		scx_error(sch, "cannot use vtime ordering for built-in DSQs");
		enq_flags &= ~SCX_ENQ_DSQ_PRIQ;
	}

	if (enq_flags & SCX_ENQ_DSQ_PRIQ) {
		struct rb_node *rbp;

		/*
		 * A PRIQ DSQ shouldn't be using FIFO enqueueing. As tasks are
		 * linked to both the rbtree and list on PRIQs, this can only be
		 * tested easily when adding the first task.
		 */
		if (unlikely(RB_EMPTY_ROOT(&dsq->priq) &&
			     nldsq_next_task(dsq, NULL, false)))
			scx_error(sch, "DSQ ID 0x%016llx already had FIFO-enqueued tasks",
				  dsq->id);

		p->scx.dsq_flags |= SCX_TASK_DSQ_ON_PRIQ;
		rb_add(&p->scx.dsq_priq, &dsq->priq, scx_dsq_priq_less);

		/*
		 * Find the previous task and insert after it on the list so
		 * that @dsq->list is vtime ordered.
		 */
		rbp = rb_prev(&p->scx.dsq_priq);
		if (rbp) {
			struct task_struct *prev =
				container_of(rbp, struct task_struct,
					     scx.dsq_priq);
			list_add(&p->scx.dsq_list.node, &prev->scx.dsq_list.node);
			/* first task unchanged - no update needed */
		} else {
			list_add(&p->scx.dsq_list.node, &dsq->list);
			/* not builtin and new task is at head - use fastpath */
			rcu_assign_pointer(dsq->first_task, p);
		}
	} else {
		/* a FIFO DSQ shouldn't be using PRIQ enqueuing */
		if (unlikely(!RB_EMPTY_ROOT(&dsq->priq)))
			scx_error(sch, "DSQ ID 0x%016llx already had PRIQ-enqueued tasks",
				  dsq->id);

		if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT)) {
			list_add(&p->scx.dsq_list.node, &dsq->list);
			/* new task inserted at head - use fastpath */
			if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN))
				rcu_assign_pointer(dsq->first_task, p);
		} else {
			/*
			 * dsq->list can contain parked BPF iterator cursors, so
			 * list_empty() here isn't a reliable proxy for "no real
			 * task in the DSQ". Test dsq->first_task directly.
			 */
			list_add_tail(&p->scx.dsq_list.node, &dsq->list);
			if (!dsq->first_task && !(dsq->id & SCX_DSQ_FLAG_BUILTIN))
				rcu_assign_pointer(dsq->first_task, p);
		}
	}

	/* seq records the order tasks are queued, used by BPF DSQ iterator */
	WRITE_ONCE(dsq->seq, dsq->seq + 1);
	p->scx.dsq_seq = dsq->seq;

	dsq_inc_nr(dsq, p, enq_flags);
	p->scx.dsq = dsq;

	/*
	 * Update custody and call ops.dequeue() before clearing ops_state:
	 * once ops_state is cleared, waiters in ops_dequeue() can proceed
	 * and dequeue_task_scx() will RMW p->scx.flags. If we clear
	 * ops_state first, both sides would modify p->scx.flags
	 * concurrently in a non-atomic way.
	 */
	if (is_local) {
		local_dsq_post_enq(sch, dsq, p, enq_flags);
	} else {
		/*
		 * Task on global/bypass DSQ: leave custody, task on
		 * non-terminal DSQ: enter custody.
		 */
		if (dsq->id == SCX_DSQ_GLOBAL || dsq->id == SCX_DSQ_BYPASS)
			call_task_dequeue(sch, rq, p, 0);
		else
			p->scx.flags |= SCX_TASK_IN_CUSTODY;

		raw_spin_unlock(&dsq->lock);
	}

	/*
	 * We're transitioning out of QUEUEING or DISPATCHING. store_release to
	 * match waiters' load_acquire.
	 */
	if (enq_flags & SCX_ENQ_CLEAR_OPSS)
		atomic_long_set_release(&p->scx.ops_state, SCX_OPSS_NONE);
}

static void task_unlink_from_dsq(struct task_struct *p,
				 struct scx_dispatch_q *dsq)
{
	WARN_ON_ONCE(list_empty(&p->scx.dsq_list.node));

	if (p->scx.dsq_flags & SCX_TASK_DSQ_ON_PRIQ) {
		rb_erase(&p->scx.dsq_priq, &dsq->priq);
		RB_CLEAR_NODE(&p->scx.dsq_priq);
		p->scx.dsq_flags &= ~SCX_TASK_DSQ_ON_PRIQ;
	}

	list_del_init(&p->scx.dsq_list.node);
	dsq_dec_nr(dsq, p);

	if (!(dsq->id & SCX_DSQ_FLAG_BUILTIN) && dsq->first_task == p) {
		struct task_struct *first_task;

		first_task = nldsq_next_task(dsq, NULL, false);
		rcu_assign_pointer(dsq->first_task, first_task);
	}
}

static void dispatch_dequeue(struct rq *rq, struct task_struct *p)
{
	struct scx_dispatch_q *dsq = p->scx.dsq;
	bool is_local = dsq == &rq->scx.local_dsq;

	lockdep_assert_rq_held(rq);

	if (!dsq) {
		/*
		 * If !dsq && on-list, @p is on @rq's ddsp_deferred_locals.
		 * Unlinking is all that's needed to cancel.
		 */
		if (unlikely(!list_empty(&p->scx.dsq_list.node)))
			list_del_init(&p->scx.dsq_list.node);

		/*
		 * When dispatching directly from the BPF scheduler to a local
		 * DSQ, the task isn't associated with any DSQ but
		 * @p->scx.holding_cpu may be set under the protection of
		 * %SCX_OPSS_DISPATCHING.
		 */
		if (p->scx.holding_cpu >= 0)
			p->scx.holding_cpu = -1;

		return;
	}

	if (!is_local)
		raw_spin_lock(&dsq->lock);

	/*
	 * Now that we hold @dsq->lock, @p->holding_cpu and @p->scx.dsq_* can't
	 * change underneath us.
	*/
	if (p->scx.holding_cpu < 0) {
		/* @p must still be on @dsq, dequeue */
		task_unlink_from_dsq(p, dsq);
	} else {
		/*
		 * We're racing against dispatch_to_local_dsq() which already
		 * removed @p from @dsq and set @p->scx.holding_cpu. Clear the
		 * holding_cpu which tells dispatch_to_local_dsq() that it lost
		 * the race.
		 */
		WARN_ON_ONCE(!list_empty(&p->scx.dsq_list.node));
		p->scx.holding_cpu = -1;
	}
	p->scx.dsq = NULL;

	if (!is_local)
		raw_spin_unlock(&dsq->lock);
}

/*
 * Abbreviated version of dispatch_dequeue() that can be used when both @p's rq
 * and dsq are locked.
 */
static void dispatch_dequeue_locked(struct task_struct *p,
				    struct scx_dispatch_q *dsq)
{
	lockdep_assert_rq_held(task_rq(p));
	lockdep_assert_held(&dsq->lock);

	task_unlink_from_dsq(p, dsq);
	p->scx.dsq = NULL;
}

static struct scx_dispatch_q *find_dsq_for_dispatch(struct scx_sched *sch,
						    struct rq *rq, u64 dsq_id,
						    s32 tcpu)
{
	struct scx_dispatch_q *dsq;

	if (dsq_id == SCX_DSQ_LOCAL)
		return &rq->scx.local_dsq;

	if ((dsq_id & SCX_DSQ_LOCAL_ON) == SCX_DSQ_LOCAL_ON) {
		s32 cpu = dsq_id & SCX_DSQ_LOCAL_CPU_MASK;

		if (!ops_cpu_valid(sch, cpu, "in SCX_DSQ_LOCAL_ON dispatch verdict"))
			return find_global_dsq(sch, tcpu);

		return &cpu_rq(cpu)->scx.local_dsq;
	}

	if (dsq_id == SCX_DSQ_GLOBAL)
		dsq = find_global_dsq(sch, tcpu);
	else
		dsq = find_user_dsq(sch, dsq_id);

	if (unlikely(!dsq)) {
		scx_error(sch, "non-existent DSQ 0x%llx", dsq_id);
		return find_global_dsq(sch, tcpu);
	}

	return dsq;
}

static void mark_direct_dispatch(struct scx_sched *sch,
				 struct task_struct *ddsp_task,
				 struct task_struct *p, u64 dsq_id,
				 u64 enq_flags)
{
	/*
	 * Mark that dispatch already happened from ops.select_cpu() or
	 * ops.enqueue() by spoiling direct_dispatch_task with a non-NULL value
	 * which can never match a valid task pointer.
	 */
	__this_cpu_write(direct_dispatch_task, ERR_PTR(-ESRCH));

	/* @p must match the task on the enqueue path */
	if (unlikely(p != ddsp_task)) {
		if (IS_ERR(ddsp_task))
			scx_error(sch, "%s[%d] already direct-dispatched",
				  p->comm, p->pid);
		else
			scx_error(sch, "scheduling for %s[%d] but trying to direct-dispatch %s[%d]",
				  ddsp_task->comm, ddsp_task->pid,
				  p->comm, p->pid);
		return;
	}

	WARN_ON_ONCE(p->scx.ddsp_dsq_id != SCX_DSQ_INVALID);
	WARN_ON_ONCE(p->scx.ddsp_enq_flags);

	p->scx.ddsp_dsq_id = dsq_id;
	p->scx.ddsp_enq_flags = enq_flags;
}

/*
 * Clear @p direct dispatch state when leaving the scheduler.
 *
 * Direct dispatch state must be cleared in the following cases:
 *  - direct_dispatch(): cleared on the synchronous enqueue path, deferred
 *    dispatch keeps the state until consumed
 *  - process_ddsp_deferred_locals(): cleared after consuming deferred state,
 *  - do_enqueue_task(): cleared on enqueue fallbacks where the dispatch
 *    verdict is ignored (local/global/bypass)
 *  - dequeue_task_scx(): cleared after dispatch_dequeue(), covering deferred
 *    cancellation and holding_cpu races
 *  - scx_disable_task(): cleared for queued wakeup tasks, which are excluded by
 *    the scx_bypass() loop, so that stale state is not reused by a subsequent
 *    scheduler instance
 */
static inline void clear_direct_dispatch(struct task_struct *p)
{
	p->scx.ddsp_dsq_id = SCX_DSQ_INVALID;
	p->scx.ddsp_enq_flags = 0;
}

static void direct_dispatch(struct scx_sched *sch, struct task_struct *p,
			    u64 enq_flags)
{
	struct rq *rq = task_rq(p);
	struct scx_dispatch_q *dsq =
		find_dsq_for_dispatch(sch, rq, p->scx.ddsp_dsq_id, task_cpu(p));
	u64 ddsp_enq_flags;

	touch_core_sched_dispatch(rq, p);

	p->scx.ddsp_enq_flags |= enq_flags;

	/*
	 * We are in the enqueue path with @rq locked and pinned, and thus can't
	 * double lock a remote rq and enqueue to its local DSQ. For
	 * DSQ_LOCAL_ON verdicts targeting the local DSQ of a remote CPU, defer
	 * the enqueue so that it's executed when @rq can be unlocked.
	 */
	if (dsq->id == SCX_DSQ_LOCAL && dsq != &rq->scx.local_dsq) {
		unsigned long opss;

		opss = atomic_long_read(&p->scx.ops_state) & SCX_OPSS_STATE_MASK;

		switch (opss & SCX_OPSS_STATE_MASK) {
		case SCX_OPSS_NONE:
			break;
		case SCX_OPSS_QUEUEING:
			/*
			 * As @p was never passed to the BPF side, _release is
			 * not strictly necessary. Still do it for consistency.
			 */
			atomic_long_set_release(&p->scx.ops_state, SCX_OPSS_NONE);
			break;
		default:
			WARN_ONCE(true, "sched_ext: %s[%d] has invalid ops state 0x%lx in direct_dispatch()",
				  p->comm, p->pid, opss);
			atomic_long_set_release(&p->scx.ops_state, SCX_OPSS_NONE);
			break;
		}

		WARN_ON_ONCE(p->scx.dsq || !list_empty(&p->scx.dsq_list.node));
		list_add_tail(&p->scx.dsq_list.node,
			      &rq->scx.ddsp_deferred_locals);
		schedule_deferred_locked(rq);
		return;
	}

	ddsp_enq_flags = p->scx.ddsp_enq_flags;
	clear_direct_dispatch(p);

	dispatch_enqueue(sch, rq, dsq, p, ddsp_enq_flags | SCX_ENQ_CLEAR_OPSS);
}

static bool scx_rq_online(struct rq *rq)
{
	/*
	 * Test both cpu_active() and %SCX_RQ_ONLINE. %SCX_RQ_ONLINE indicates
	 * the online state as seen from the BPF scheduler. cpu_active() test
	 * guarantees that, if this function returns %true, %SCX_RQ_ONLINE will
	 * stay set until the current scheduling operation is complete even if
	 * we aren't locking @rq.
	 */
	return likely((rq->scx.flags & SCX_RQ_ONLINE) && cpu_active(cpu_of(rq)));
}

static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
			    int sticky_cpu)
{
	struct scx_sched *sch = scx_task_sched(p);
	struct task_struct **ddsp_taskp;
	struct scx_dispatch_q *dsq;
	unsigned long qseq;

	WARN_ON_ONCE(!(p->scx.flags & SCX_TASK_QUEUED));

	/* internal movements - rq migration / RESTORE */
	if (sticky_cpu == cpu_of(rq))
		goto local_norefill;

	/*
	 * Clear persistent TASK_IMMED for fresh enqueues, see dsq_inc_nr().
	 * Note that exiting and migration-disabled tasks that skip
	 * ops.enqueue() below will lose IMMED protection unless
	 * %SCX_OPS_ENQ_EXITING / %SCX_OPS_ENQ_MIGRATION_DISABLED are set.
	 */
	p->scx.flags &= ~SCX_TASK_IMMED;

	/*
	 * If !scx_rq_online(), we already told the BPF scheduler that the CPU
	 * is offline and are just running the hotplug path. Don't bother the
	 * BPF scheduler.
	 */
	if (!scx_rq_online(rq))
		goto local;

	if (scx_bypassing(sch, cpu_of(rq))) {
		__scx_add_event(sch, SCX_EV_BYPASS_DISPATCH, 1);
		goto bypass;
	}

	if (p->scx.ddsp_dsq_id != SCX_DSQ_INVALID)
		goto direct;

	/* see %SCX_OPS_ENQ_EXITING */
	if (!(sch->ops.flags & SCX_OPS_ENQ_EXITING) &&
	    unlikely(p->flags & PF_EXITING)) {
		__scx_add_event(sch, SCX_EV_ENQ_SKIP_EXITING, 1);
		goto local;
	}

	/* see %SCX_OPS_ENQ_MIGRATION_DISABLED */
	if (!(sch->ops.flags & SCX_OPS_ENQ_MIGRATION_DISABLED) &&
	    is_migration_disabled(p)) {
		__scx_add_event(sch, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED, 1);
		goto local;
	}

	if (unlikely(!SCX_HAS_OP(sch, enqueue)))
		goto global;

	/* DSQ bypass didn't trigger, enqueue on the BPF scheduler */
	qseq = rq->scx.ops_qseq++ << SCX_OPSS_QSEQ_SHIFT;

	WARN_ON_ONCE(atomic_long_read(&p->scx.ops_state) != SCX_OPSS_NONE);
	atomic_long_set(&p->scx.ops_state, SCX_OPSS_QUEUEING | qseq);

	ddsp_taskp = this_cpu_ptr(&direct_dispatch_task);
	WARN_ON_ONCE(*ddsp_taskp);
	*ddsp_taskp = p;

	SCX_CALL_OP_TASK(sch, enqueue, rq, p, enq_flags);

	*ddsp_taskp = NULL;
	if (p->scx.ddsp_dsq_id != SCX_DSQ_INVALID)
		goto direct;

	/*
	 * Task is now in BPF scheduler's custody. Set %SCX_TASK_IN_CUSTODY
	 * so ops.dequeue() is called when it leaves custody.
	 */
	p->scx.flags |= SCX_TASK_IN_CUSTODY;

	/*
	 * If not directly dispatched, QUEUEING isn't clear yet and dispatch or
	 * dequeue may be waiting. The store_release matches their load_acquire.
	 */
	atomic_long_set_release(&p->scx.ops_state, SCX_OPSS_QUEUED | qseq);
	return;

direct:
	direct_dispatch(sch, p, enq_flags);
	return;
local_norefill:
	dispatch_enqueue(sch, rq, &rq->scx.local_dsq, p, enq_flags);
	return;
local:
	dsq = &rq->scx.local_dsq;
	goto enqueue;
global:
	dsq = find_global_dsq(sch, task_cpu(p));
	goto enqueue;
bypass:
	dsq = bypass_enq_target_dsq(sch, task_cpu(p));
	goto enqueue;

enqueue:
	/*
	 * For task-ordering, slice refill must be treated as implying the end
	 * of the current slice. Otherwise, the longer @p stays on the CPU, the
	 * higher priority it becomes from scx_prio_less()'s POV.
	 */
	touch_core_sched(rq, p);
	refill_task_slice_dfl(sch, p);
	clear_direct_dispatch(p);
	dispatch_enqueue(sch, rq, dsq, p, enq_flags);
}

static bool task_runnable(const struct task_struct *p)
{
	return !list_empty(&p->scx.runnable_node);
}

static void set_task_runnable(struct rq *rq, struct task_struct *p)
{
	lockdep_assert_rq_held(rq);

	if (p->scx.flags & SCX_TASK_RESET_RUNNABLE_AT) {
		p->scx.runnable_at = jiffies;
		p->scx.flags &= ~SCX_TASK_RESET_RUNNABLE_AT;
	}

	/*
	 * list_add_tail() must be used. scx_bypass() depends on tasks being
	 * appended to the runnable_list.
	 */
	list_add_tail(&p->scx.runnable_node, &rq->scx.runnable_list);
}

static void clr_task_runnable(struct task_struct *p, bool reset_runnable_at)
{
	list_del_init(&p->scx.runnable_node);
	if (reset_runnable_at)
		p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
}

static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int core_enq_flags)
{
	struct scx_sched *sch = scx_task_sched(p);
	int sticky_cpu = p->scx.sticky_cpu;
	u64 enq_flags = core_enq_flags | rq->scx.extra_enq_flags;

	if (enq_flags & ENQUEUE_WAKEUP)
		rq->scx.flags |= SCX_RQ_IN_WAKEUP;

	/*
	 * Restoring a running task will be immediately followed by
	 * set_next_task_scx() which expects the task to not be on the BPF
	 * scheduler as tasks can only start running through local DSQs. Force
	 * direct-dispatch into the local DSQ by setting the sticky_cpu.
	 */
	if (unlikely(enq_flags & ENQUEUE_RESTORE) && task_current(rq, p))
		sticky_cpu = cpu_of(rq);

	if (p->scx.flags & SCX_TASK_QUEUED) {
		WARN_ON_ONCE(!task_runnable(p));
		goto out;
	}

	set_task_runnable(rq, p);
	p->scx.flags |= SCX_TASK_QUEUED;
	rq->scx.nr_running++;
	add_nr_running(rq, 1);

	if (SCX_HAS_OP(sch, runnable) && !task_on_rq_migrating(p))
		SCX_CALL_OP_TASK(sch, runnable, rq, p, enq_flags);

	if (enq_flags & SCX_ENQ_WAKEUP)
		touch_core_sched(rq, p);

	/* Start dl_server if this is the first task being enqueued */
	if (rq->scx.nr_running == 1)
		dl_server_start(&rq->ext_server);

	do_enqueue_task(rq, p, enq_flags, sticky_cpu);

	if (sticky_cpu >= 0)
		p->scx.sticky_cpu = -1;
out:
	rq->scx.flags &= ~SCX_RQ_IN_WAKEUP;

	if ((enq_flags & SCX_ENQ_CPU_SELECTED) &&
	    unlikely(cpu_of(rq) != p->scx.selected_cpu))
		__scx_add_event(sch, SCX_EV_SELECT_CPU_FALLBACK, 1);
}

static void ops_dequeue(struct rq *rq, struct task_struct *p, u64 deq_flags)
{
	struct scx_sched *sch = scx_task_sched(p);
	unsigned long opss;

	/* dequeue is always temporary, don't reset runnable_at */
	clr_task_runnable(p, false);

	/* acquire ensures that we see the preceding updates on QUEUED */
	opss = atomic_long_read_acquire(&p->scx.ops_state);

	switch (opss & SCX_OPSS_STATE_MASK) {
	case SCX_OPSS_NONE:
		break;
	case SCX_OPSS_QUEUEING:
		/*
		 * QUEUEING is started and finished while holding @p's rq lock.
		 * As we're holding the rq lock now, we shouldn't see QUEUEING.
		 */
		BUG();
	case SCX_OPSS_QUEUED:
		/* A queued task must always be in BPF scheduler's custody */
		WARN_ON_ONCE(!(p->scx.flags & SCX_TASK_IN_CUSTODY));
		if (atomic_long_try_cmpxchg(&p->scx.ops_state, &opss,
					    SCX_OPSS_NONE))
			break;
		fallthrough;
	case SCX_OPSS_DISPATCHING:
		/*
		 * If @p is being dispatched from the BPF scheduler to a DSQ,
		 * wait for the transfer to complete so that @p doesn't get
		 * added to its DSQ after dequeueing is complete.
		 *
		 * As we're waiting on DISPATCHING with the rq locked, the
		 * dispatching side shouldn't try to lock the rq while
		 * DISPATCHING is set. See dispatch_to_local_dsq().
		 *
		 * DISPATCHING shouldn't have qseq set and control can reach
		 * here with NONE @opss from the above QUEUED case block.
		 * Explicitly wait on %SCX_OPSS_DISPATCHING instead of @opss.
		 */
		wait_ops_state(p, SCX_OPSS_DISPATCHING);
		BUG_ON(atomic_long_read(&p->scx.ops_state) != SCX_OPSS_NONE);
		break;
	}

	/*
	 * Call ops.dequeue() if the task is still in BPF custody.
	 *
	 * The code that clears ops_state to %SCX_OPSS_NONE does not always
	 * clear %SCX_TASK_IN_CUSTODY: in dispatch_to_local_dsq(), when
	 * we're moving a task that was in %SCX_OPSS_DISPATCHING to a
	 * remote CPU's local DSQ, we only set ops_state to %SCX_OPSS_NONE
	 * so that a concurrent dequeue can proceed, but we clear
	 * %SCX_TASK_IN_CUSTODY only when we later enqueue or move the
	 * task. So we can see NONE + IN_CUSTODY here and we must handle
	 * it. Similarly, after waiting on %SCX_OPSS_DISPATCHING we see
	 * NONE but the task may still have %SCX_TASK_IN_CUSTODY set until
	 * it is enqueued on the destination.
	 */
	call_task_dequeue(sch, rq, p, deq_flags);
}

static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int core_deq_flags)
{
	struct scx_sched *sch = scx_task_sched(p);
	u64 deq_flags = core_deq_flags;

	/*
	 * Set %SCX_DEQ_SCHED_CHANGE when the dequeue is due to a property
	 * change (not sleep or core-sched pick).
	 */
	if (!(deq_flags & (DEQUEUE_SLEEP | SCX_DEQ_CORE_SCHED_EXEC)))
		deq_flags |= SCX_DEQ_SCHED_CHANGE;

	if (!(p->scx.flags & SCX_TASK_QUEUED)) {
		WARN_ON_ONCE(task_runnable(p));
		return true;
	}

	ops_dequeue(rq, p, deq_flags);

	/*
	 * A currently running task which is going off @rq first gets dequeued
	 * and then stops running. As we want running <-> stopping transitions
	 * to be contained within runnable <-> quiescent transitions, trigger
	 * ->stopping() early here instead of in put_prev_task_scx().
	 *
	 * @p may go through multiple stopping <-> running transitions between
	 * here and put_prev_task_scx() if task attribute changes occur while
	 * balance_one() leaves @rq unlocked. However, they don't contain any
	 * information meaningful to the BPF scheduler and can be suppressed by
	 * skipping the callbacks if the task is !QUEUED.
	 */
	if (SCX_HAS_OP(sch, stopping) && task_current(rq, p)) {
		update_curr_scx(rq);
		SCX_CALL_OP_TASK(sch, stopping, rq, p, false);
	}

	if (SCX_HAS_OP(sch, quiescent) && !task_on_rq_migrating(p))
		SCX_CALL_OP_TASK(sch, quiescent, rq, p, deq_flags);

	if (deq_flags & SCX_DEQ_SLEEP)
		p->scx.flags |= SCX_TASK_DEQD_FOR_SLEEP;
	else
		p->scx.flags &= ~SCX_TASK_DEQD_FOR_SLEEP;

	p->scx.flags &= ~SCX_TASK_QUEUED;
	rq->scx.nr_running--;
	sub_nr_running(rq, 1);

	dispatch_dequeue(rq, p);
	clear_direct_dispatch(p);
	return true;
}

static void yield_task_scx(struct rq *rq)
{
	struct task_struct *p = rq->donor;
	struct scx_sched *sch = scx_task_sched(p);

	if (SCX_HAS_OP(sch, yield))
		SCX_CALL_OP_2TASKS_RET(sch, yield, rq, p, NULL);
	else
		p->scx.slice = 0;
}

static bool yield_to_task_scx(struct rq *rq, struct task_struct *to)
{
	struct task_struct *from = rq->donor;
	struct scx_sched *sch = scx_task_sched(from);

	if (SCX_HAS_OP(sch, yield) && sch == scx_task_sched(to))
		return SCX_CALL_OP_2TASKS_RET(sch, yield, rq, from, to);
	else
		return false;
}

static void wakeup_preempt_scx(struct rq *rq, struct task_struct *p, int wake_flags)
{
	/*
	 * Preemption between SCX tasks is implemented by resetting the victim
	 * task's slice to 0 and triggering reschedule on the target CPU.
	 * Nothing to do.
	 */
	if (p->sched_class == &ext_sched_class)
		return;

	/*
	 * Getting preempted by a higher-priority class. Reenqueue IMMED tasks.
	 * This captures all preemption cases including:
	 *
	 * - A SCX task is currently running.
	 *
	 * - @rq is waking from idle due to a SCX task waking to it.
	 *
	 * - A higher-priority wakes up while SCX dispatch is in progress.
	 */
	if (rq->scx.nr_immed)
		schedule_reenq_local(rq, 0);
}

static void move_local_task_to_local_dsq(struct scx_sched *sch,
					 struct task_struct *p, u64 enq_flags,
					 struct scx_dispatch_q *src_dsq,
					 struct rq *dst_rq)
{
	struct scx_dispatch_q *dst_dsq = &dst_rq->scx.local_dsq;

	/* @dsq is locked and @p is on @dst_rq */
	lockdep_assert_held(&src_dsq->lock);
	lockdep_assert_rq_held(dst_rq);

	WARN_ON_ONCE(p->scx.holding_cpu >= 0);

	if (enq_flags & (SCX_ENQ_HEAD | SCX_ENQ_PREEMPT))
		list_add(&p->scx.dsq_list.node, &dst_dsq->list);
	else
		list_add_tail(&p->scx.dsq_list.node, &dst_dsq->list);

	dsq_inc_nr(dst_dsq, p, enq_flags);
	p->scx.dsq = dst_dsq;

	local_dsq_post_enq(sch, dst_dsq, p, enq_flags);
}

/**
 * move_remote_task_to_local_dsq - Move a task from a foreign rq to a local DSQ
 * @p: task to move
 * @enq_flags: %SCX_ENQ_*
 * @src_rq: rq to move the task from, locked on entry, released on return
 * @dst_rq: rq to move the task into, locked on return
 *
 * Move @p which is currently on @src_rq to @dst_rq's local DSQ.
 */
static void move_remote_task_to_local_dsq(struct task_struct *p, u64 enq_flags,
					  struct rq *src_rq, struct rq *dst_rq)
{
	lockdep_assert_rq_held(src_rq);

	/*
	 * Set sticky_cpu before deactivate_task() to properly mark the
	 * beginning of an SCX-internal migration.
	 */
	p->scx.sticky_cpu = cpu_of(dst_rq);
	deactivate_task(src_rq, p, 0);
	set_task_cpu(p, cpu_of(dst_rq));

	raw_spin_rq_unlock(src_rq);
	raw_spin_rq_lock(dst_rq);

	/*
	 * We want to pass scx-specific enq_flags but activate_task() will
	 * truncate the upper 32 bit. As we own @rq, we can pass them through
	 * @rq->scx.extra_enq_flags instead.
	 */
	WARN_ON_ONCE(!cpumask_test_cpu(cpu_of(dst_rq), p->cpus_ptr));
	WARN_ON_ONCE(dst_rq->scx.extra_enq_flags);
	dst_rq->scx.extra_enq_flags = enq_flags;
	activate_task(dst_rq, p, 0);
	dst_rq->scx.extra_enq_flags = 0;
}

/*
 * Similar to kernel/sched/core.c::is_cpu_allowed(). However, there are two
 * differences:
 *
 * - is_cpu_allowed() asks "Can this task run on this CPU?" while
 *   task_can_run_on_remote_rq() asks "Can the BPF scheduler migrate the task to
 *   this CPU?".
 *
 *   While migration is disabled, is_cpu_allowed() has to say "yes" as the task
 *   must be allowed to finish on the CPU that it's currently on regardless of
 *   the CPU state. However, task_can_run_on_remote_rq() must say "no" as the
 *   BPF scheduler shouldn't attempt to migrate a task which has migration
 *   disabled.
 *
 * - The BPF scheduler is bypassed while the rq is offline and we can always say
 *   no to the BPF scheduler initiated migrations while offline.
 *
 * The caller must ensure that @p and @rq are on different CPUs.
 */
static bool task_can_run_on_remote_rq(struct scx_sched *sch,
				      struct task_struct *p, struct rq *rq,
				      bool enforce)
{
	s32 cpu = cpu_of(rq);

	WARN_ON_ONCE(task_cpu(p) == cpu);

	/*
	 * If @p has migration disabled, @p->cpus_ptr is updated to contain only
	 * the pinned CPU in migrate_disable_switch() while @p is being switched
	 * out. However, put_prev_task_scx() is called before @p->cpus_ptr is
	 * updated and thus another CPU may see @p on a DSQ inbetween leading to
	 * @p passing the below task_allowed_on_cpu() check while migration is
	 * disabled.
	 *
	 * Test the migration disabled state first as the race window is narrow
	 * and the BPF scheduler failing to check migration disabled state can
	 * easily be masked if task_allowed_on_cpu() is done first.
	 */
	if (unlikely(is_migration_disabled(p))) {
		if (enforce)
			scx_error(sch, "SCX_DSQ_LOCAL[_ON] cannot move migration disabled %s[%d] from CPU %d to %d",
				  p->comm, p->pid, task_cpu(p), cpu);
		return false;
	}

	/*
	 * We don't require the BPF scheduler to avoid dispatching to offline
	 * CPUs mostly for convenience but also because CPUs can go offline
	 * between scx_bpf_dsq_insert() calls and here. Trigger error iff the
	 * picked CPU is outside the allowed mask.
	 */
	if (!task_allowed_on_cpu(p, cpu)) {
		if (enforce)
			scx_error(sch, "SCX_DSQ_LOCAL[_ON] target CPU %d not allowed for %s[%d]",
				  cpu, p->comm, p->pid);
		return false;
	}

	if (!scx_rq_online(rq)) {
		if (enforce)
			__scx_add_event(sch, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE, 1);
		return false;
	}

	return true;
}

/**
 * unlink_dsq_and_lock_src_rq() - Unlink task from its DSQ and lock its task_rq
 * @p: target task
 * @dsq: locked DSQ @p is currently on
 * @src_rq: rq @p is currently on, stable with @dsq locked
 *
 * Called with @dsq locked but no rq's locked. We want to move @p to a different
 * DSQ, including any local DSQ, but are not locking @src_rq. Locking @src_rq is
 * required when transferring into a local DSQ. Even when transferring into a
 * non-local DSQ, it's better to use the same mechanism to protect against
 * dequeues and maintain the invariant that @p->scx.dsq can only change while
 * @src_rq is locked, which e.g. scx_dump_task() depends on.
 *
 * We want to grab @src_rq but that can deadlock if we try while locking @dsq,
 * so we want to unlink @p from @dsq, drop its lock and then lock @src_rq. As
 * this may race with dequeue, which can't drop the rq lock or fail, do a little
 * dancing from our side.
 *
 * @p->scx.holding_cpu is set to this CPU before @dsq is unlocked. If @p gets
 * dequeued after we unlock @dsq but before locking @src_rq, the holding_cpu
 * would be cleared to -1. While other cpus may have updated it to different
 * values afterwards, as this operation can't be preempted or recurse, the
 * holding_cpu can never become this CPU again before we're done. Thus, we can
 * tell whether we lost to dequeue by testing whether the holding_cpu still
 * points to this CPU. See dispatch_dequeue() for the counterpart.
 *
 * On return, @dsq is unlocked and @src_rq is locked. Returns %true if @p is
 * still valid. %false if lost to dequeue.
 */
static bool unlink_dsq_and_lock_src_rq(struct task_struct *p,
				       struct scx_dispatch_q *dsq,
				       struct rq *src_rq)
{
	s32 cpu = raw_smp_processor_id();

	lockdep_assert_held(&dsq->lock);

	WARN_ON_ONCE(p->scx.holding_cpu >= 0);
	task_unlink_from_dsq(p, dsq);
	p->scx.holding_cpu = cpu;

	raw_spin_unlock(&dsq->lock);
	raw_spin_rq_lock(src_rq);

	/* task_rq couldn't have changed if we're still the holding cpu */
	return likely(p->scx.holding_cpu == cpu) &&
		!WARN_ON_ONCE(src_rq != task_rq(p));
}

static bool consume_remote_task(struct rq *this_rq,
				struct task_struct *p, u64 enq_flags,
				struct scx_dispatch_q *dsq, struct rq *src_rq)
{
	raw_spin_rq_unlock(this_rq);

	if (unlink_dsq_and_lock_src_rq(p, dsq, src_rq)) {
		move_remote_task_to_local_dsq(p, enq_flags, src_rq, this_rq);
		return true;
	} else {
		raw_spin_rq_unlock(src_rq);
		raw_spin_rq_lock(this_rq);
		return false;
	}
}

/**
 * move_task_between_dsqs() - Move a task from one DSQ to another
 * @sch: scx_sched being operated on
 * @p: target task
 * @enq_flags: %SCX_ENQ_*
 * @src_dsq: DSQ @p is currently on, must not be a local DSQ
 * @dst_dsq: DSQ @p is being moved to, can be any DSQ
 *
 * Must be called with @p's task_rq and @src_dsq locked. If @dst_dsq is a local
 * DSQ and @p is on a different CPU, @p will be migrated and thus its task_rq
 * will change. As @p's task_rq is locked, this function doesn't need to use the
 * holding_cpu mechanism.
 *
 * On return, @src_dsq is unlocked and only @p's new task_rq, which is the
 * return value, is locked.
 */
static struct rq *move_task_between_dsqs(struct scx_sched *sch,
					 struct task_struct *p, u64 enq_flags,
					 struct scx_dispatch_q *src_dsq,
					 struct scx_dispatch_q *dst_dsq)
{
	struct rq *src_rq = task_rq(p), *dst_rq;

	BUG_ON(src_dsq->id == SCX_DSQ_LOCAL);
	lockdep_assert_held(&src_dsq->lock);
	lockdep_assert_rq_held(src_rq);

	if (dst_dsq->id == SCX_DSQ_LOCAL) {
		dst_rq = container_of(dst_dsq, struct rq, scx.local_dsq);
		if (src_rq != dst_rq &&
		    unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
			dst_dsq = find_global_dsq(sch, task_cpu(p));
			dst_rq = src_rq;
			enq_flags |= SCX_ENQ_GDSQ_FALLBACK;
		}
	} else {
		/* no need to migrate if destination is a non-local DSQ */
		dst_rq = src_rq;
	}

	/*
	 * Move @p into $dst_dsq. If $dst_dsq is the local DSQ of a different
	 * CPU, @p will be migrated.
	 */
	if (dst_dsq->id == SCX_DSQ_LOCAL) {
		/* @p is going from a non-local DSQ to a local DSQ */
		if (src_rq == dst_rq) {
			task_unlink_from_dsq(p, src_dsq);
			move_local_task_to_local_dsq(sch, p, enq_flags,
						     src_dsq, dst_rq);
			raw_spin_unlock(&src_dsq->lock);
		} else {
			raw_spin_unlock(&src_dsq->lock);
			move_remote_task_to_local_dsq(p, enq_flags,
						      src_rq, dst_rq);
		}
	} else {
		/*
		 * @p is going from a non-local DSQ to a non-local DSQ. As
		 * $src_dsq is already locked, do an abbreviated dequeue.
		 */
		dispatch_dequeue_locked(p, src_dsq);
		raw_spin_unlock(&src_dsq->lock);

		dispatch_enqueue(sch, dst_rq, dst_dsq, p, enq_flags);
	}

	return dst_rq;
}

static bool consume_dispatch_q(struct scx_sched *sch, struct rq *rq,
			       struct scx_dispatch_q *dsq, u64 enq_flags)
{
	struct task_struct *p;
retry:
	/*
	 * The caller can't expect to successfully consume a task if the task's
	 * addition to @dsq isn't guaranteed to be visible somehow. Test
	 * @dsq->list without locking and skip if it seems empty.
	 */
	if (list_empty(&dsq->list))
		return false;

	raw_spin_lock(&dsq->lock);

	nldsq_for_each_task(p, dsq) {
		struct rq *task_rq = task_rq(p);

		/*
		 * This loop can lead to multiple lockup scenarios, e.g. the BPF
		 * scheduler can put an enormous number of affinitized tasks into
		 * a contended DSQ, or the outer retry loop can repeatedly race
		 * against scx_bypass() dequeueing tasks from @dsq trying to put
		 * the system into the bypass mode. This can easily live-lock the
		 * machine. If aborting, exit from all non-bypass DSQs.
		 */
		if (unlikely(READ_ONCE(sch->aborting)) && dsq->id != SCX_DSQ_BYPASS)
			break;

		if (rq == task_rq) {
			task_unlink_from_dsq(p, dsq);
			move_local_task_to_local_dsq(sch, p, enq_flags, dsq, rq);
			raw_spin_unlock(&dsq->lock);
			return true;
		}

		if (task_can_run_on_remote_rq(sch, p, rq, false)) {
			if (likely(consume_remote_task(rq, p, enq_flags, dsq, task_rq)))
				return true;
			goto retry;
		}
	}

	raw_spin_unlock(&dsq->lock);
	return false;
}

static bool consume_global_dsq(struct scx_sched *sch, struct rq *rq)
{
	int node = cpu_to_node(cpu_of(rq));

	return consume_dispatch_q(sch, rq, &sch->pnode[node]->global_dsq, 0);
}

/**
 * dispatch_to_local_dsq - Dispatch a task to a local dsq
 * @sch: scx_sched being operated on
 * @rq: current rq which is locked
 * @dst_dsq: destination DSQ
 * @p: task to dispatch
 * @enq_flags: %SCX_ENQ_*
 *
 * We're holding @rq lock and want to dispatch @p to @dst_dsq which is a local
 * DSQ. This function performs all the synchronization dancing needed because
 * local DSQs are protected with rq locks.
 *
 * The caller must have exclusive ownership of @p (e.g. through
 * %SCX_OPSS_DISPATCHING).
 */
static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
				  struct scx_dispatch_q *dst_dsq,
				  struct task_struct *p, u64 enq_flags)
{
	struct rq *src_rq = task_rq(p);
	struct rq *dst_rq = container_of(dst_dsq, struct rq, scx.local_dsq);
	struct rq *locked_rq = rq;

	/*
	 * We're synchronized against dequeue through DISPATCHING. As @p can't
	 * be dequeued, its task_rq and cpus_allowed are stable too.
	 *
	 * If dispatching to @rq that @p is already on, no lock dancing needed.
	 */
	if (rq == src_rq && rq == dst_rq) {
		dispatch_enqueue(sch, rq, dst_dsq, p,
				 enq_flags | SCX_ENQ_CLEAR_OPSS);
		return;
	}

	if (src_rq != dst_rq &&
	    unlikely(!task_can_run_on_remote_rq(sch, p, dst_rq, true))) {
		dispatch_enqueue(sch, rq, find_global_dsq(sch, task_cpu(p)), p,
				 enq_flags | SCX_ENQ_CLEAR_OPSS | SCX_ENQ_GDSQ_FALLBACK);
		return;
	}

	/*
	 * @p is on a possibly remote @src_rq which we need to lock to move the
	 * task. If dequeue is in progress, it'd be locking @src_rq and waiting
	 * on DISPATCHING, so we can't grab @src_rq lock while holding
	 * DISPATCHING.
	 *
	 * As DISPATCHING guarantees that @p is wholly ours, we can pretend that
	 * we're moving from a DSQ and use the same mechanism - mark the task
	 * under transfer with holding_cpu, release DISPATCHING and then follow
	 * the same protocol. See unlink_dsq_and_lock_src_rq().
	 */
	p->scx.holding_cpu = raw_smp_processor_id();

	/* store_release ensures that dequeue sees the above */
	atomic_long_set_release(&p->scx.ops_state, SCX_OPSS_NONE);

	/* switch to @src_rq lock */
	if (locked_rq != src_rq) {
		raw_spin_rq_unlock(locked_rq);
		locked_rq = src_rq;
		raw_spin_rq_lock(src_rq);
	}

	/* task_rq couldn't have changed if we're still the holding cpu */
	if (likely(p->scx.holding_cpu == raw_smp_processor_id()) &&
	    !WARN_ON_ONCE(src_rq != task_rq(p))) {
		/*
		 * If @p is staying on the same rq, there's no need to go
		 * through the full deactivate/activate cycle. Optimize by
		 * abbreviating move_remote_task_to_local_dsq().
		 */
		if (src_rq == dst_rq) {
			p->scx.holding_cpu = -1;
			dispatch_enqueue(sch, dst_rq, &dst_rq->scx.local_dsq, p,
					 enq_flags);
		} else {
			move_remote_task_to_local_dsq(p, enq_flags,
						      src_rq, dst_rq);
			/* task has been moved to dst_rq, which is now locked */
			locked_rq = dst_rq;
		}

		/* if the destination CPU is idle, wake it up */
		if (sched_class_above(p->sched_class, dst_rq->curr->sched_class))
			resched_curr(dst_rq);
	}

	/* switch back to @rq lock */
	if (locked_rq != rq) {
		raw_spin_rq_unlock(locked_rq);
		raw_spin_rq_lock(rq);
	}
}

/**
 * finish_dispatch - Asynchronously finish dispatching a task
 * @rq: current rq which is locked
 * @p: task to finish dispatching
 * @qseq_at_dispatch: qseq when @p started getting dispatched
 * @dsq_id: destination DSQ ID
 * @enq_flags: %SCX_ENQ_*
 *
 * Dispatching to local DSQs may need to wait for queueing to complete or
 * require rq lock dancing. As we don't wanna do either while inside
 * ops.dispatch() to avoid locking order inversion, we split dispatching into
 * two parts. scx_bpf_dsq_insert() which is called by ops.dispatch() records the
 * task and its qseq. Once ops.dispatch() returns, this function is called to
 * finish up.
 *
 * There is no guarantee that @p is still valid for dispatching or even that it
 * was valid in the first place. Make sure that the task is still owned by the
 * BPF scheduler and claim the ownership before dispatching.
 */
static void finish_dispatch(struct scx_sched *sch, struct rq *rq,
			    struct task_struct *p,
			    unsigned long qseq_at_dispatch,
			    u64 dsq_id, u64 enq_flags)
{
	struct scx_dispatch_q *dsq;
	unsigned long opss;

	touch_core_sched_dispatch(rq, p);
retry:
	/*
	 * No need for _acquire here. @p is accessed only after a successful
	 * try_cmpxchg to DISPATCHING.
	 */
	opss = atomic_long_read(&p->scx.ops_state);

	switch (opss & SCX_OPSS_STATE_MASK) {
	case SCX_OPSS_DISPATCHING:
	case SCX_OPSS_NONE:
		/* someone else already got to it */
		return;
	case SCX_OPSS_QUEUED:
		/*
		 * If qseq doesn't match, @p has gone through at least one
		 * dispatch/dequeue and re-enqueue cycle between
		 * scx_bpf_dsq_insert() and here and we have no claim on it.
		 */
		if ((opss & SCX_OPSS_QSEQ_MASK) != qseq_at_dispatch)
			return;

		/* see SCX_EV_INSERT_NOT_OWNED definition */
		if (unlikely(!scx_task_on_sched(sch, p))) {
			__scx_add_event(sch, SCX_EV_INSERT_NOT_OWNED, 1);
			return;
		}

		/*
		 * While we know @p is accessible, we don't yet have a claim on
		 * it - the BPF scheduler is allowed to dispatch tasks
		 * spuriously and there can be a racing dequeue attempt. Let's
		 * claim @p by atomically transitioning it from QUEUED to
		 * DISPATCHING.
		 */
		if (likely(atomic_long_try_cmpxchg(&p->scx.ops_state, &opss,
						   SCX_OPSS_DISPATCHING)))
			break;
		goto retry;
	case SCX_OPSS_QUEUEING:
		/*
		 * do_enqueue_task() is in the process of transferring the task
		 * to the BPF scheduler while holding @p's rq lock. As we aren't
		 * holding any kernel or BPF resource that the enqueue path may
		 * depend upon, it's safe to wait.
		 */
		wait_ops_state(p, opss);
		goto retry;
	}

	BUG_ON(!(p->scx.flags & SCX_TASK_QUEUED));

	dsq = find_dsq_for_dispatch(sch, this_rq(), dsq_id, task_cpu(p));

	if (dsq->id == SCX_DSQ_LOCAL)
		dispatch_to_local_dsq(sch, rq, dsq, p, enq_flags);
	else
		dispatch_enqueue(sch, rq, dsq, p, enq_flags | SCX_ENQ_CLEAR_OPSS);
}

static void flush_dispatch_buf(struct scx_sched *sch, struct rq *rq)
{
	struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;
	u32 u;

	for (u = 0; u < dspc->cursor; u++) {
		struct scx_dsp_buf_ent *ent = &dspc->buf[u];

		finish_dispatch(sch, rq, ent->task, ent->qseq, ent->dsq_id,
				ent->enq_flags);
	}

	dspc->nr_tasks += dspc->cursor;
	dspc->cursor = 0;
}

static inline void maybe_queue_balance_callback(struct rq *rq)
{
	lockdep_assert_rq_held(rq);

	if (!(rq->scx.flags & SCX_RQ_BAL_CB_PENDING))
		return;

	queue_balance_callback(rq, &rq->scx.deferred_bal_cb,
				deferred_bal_cb_workfn);

	rq->scx.flags &= ~SCX_RQ_BAL_CB_PENDING;
}

/*
 * One user of this function is scx_bpf_dispatch() which can be called
 * recursively as sub-sched dispatches nest. Always inline to reduce stack usage
 * from the call frame.
 */
static __always_inline bool
scx_dispatch_sched(struct scx_sched *sch, struct rq *rq,
		   struct task_struct *prev, bool nested)
{
	struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;
	int nr_loops = SCX_DSP_MAX_LOOPS;
	s32 cpu = cpu_of(rq);
	bool prev_on_sch = (prev->sched_class == &ext_sched_class) &&
		scx_task_on_sched(sch, prev);

	if (consume_global_dsq(sch, rq))
		return true;

	if (bypass_dsp_enabled(sch)) {
		/* if @sch is bypassing, only the bypass DSQs are active */
		if (scx_bypassing(sch, cpu))
			return consume_dispatch_q(sch, rq, bypass_dsq(sch, cpu), 0);

#ifdef CONFIG_EXT_SUB_SCHED
		/*
		 * If @sch isn't bypassing but its children are, @sch is
		 * responsible for making forward progress for both its own
		 * tasks that aren't bypassing and the bypassing descendants'
		 * tasks. The following implements a simple built-in behavior -
		 * let each CPU try to run the bypass DSQ every Nth time.
		 *
		 * Later, if necessary, we can add an ops flag to suppress the
		 * auto-consumption and a kfunc to consume the bypass DSQ and,
		 * so that the BPF scheduler can fully control scheduling of
		 * bypassed tasks.
		 */
		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);

		if (!(pcpu->bypass_host_seq++ % SCX_BYPASS_HOST_NTH) &&
		    consume_dispatch_q(sch, rq, bypass_dsq(sch, cpu), 0)) {
			__scx_add_event(sch, SCX_EV_SUB_BYPASS_DISPATCH, 1);
			return true;
		}
#endif	/* CONFIG_EXT_SUB_SCHED */
	}

	if (unlikely(!SCX_HAS_OP(sch, dispatch)) || !scx_rq_online(rq))
		return false;

	dspc->rq = rq;

	/*
	 * The dispatch loop. Because flush_dispatch_buf() may drop the rq lock,
	 * the local DSQ might still end up empty after a successful
	 * ops.dispatch(). If the local DSQ is empty even after ops.dispatch()
	 * produced some tasks, retry. The BPF scheduler may depend on this
	 * looping behavior to simplify its implementation.
	 */
	do {
		dspc->nr_tasks = 0;

		if (nested) {
			SCX_CALL_OP(sch, dispatch, rq, cpu, prev_on_sch ? prev : NULL);
		} else {
			/* stash @prev so that nested invocations can access it */
			rq->scx.sub_dispatch_prev = prev;
			SCX_CALL_OP(sch, dispatch, rq, cpu, prev_on_sch ? prev : NULL);
			rq->scx.sub_dispatch_prev = NULL;
		}

		flush_dispatch_buf(sch, rq);

		if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice) {
			rq->scx.flags |= SCX_RQ_BAL_KEEP;
			return true;
		}
		if (rq->scx.local_dsq.nr)
			return true;
		if (consume_global_dsq(sch, rq))
			return true;

		/*
		 * ops.dispatch() can trap us in this loop by repeatedly
		 * dispatching ineligible tasks. Break out once in a while to
		 * allow the watchdog to run. As IRQ can't be enabled in
		 * balance(), we want to complete this scheduling cycle and then
		 * start a new one. IOW, we want to call resched_curr() on the
		 * next, most likely idle, task, not the current one. Use
		 * __scx_bpf_kick_cpu() for deferred kicking.
		 */
		if (unlikely(!--nr_loops)) {
			scx_kick_cpu(sch, cpu, 0);
			break;
		}
	} while (dspc->nr_tasks);

	/*
	 * Prevent the CPU from going idle while bypassed descendants have tasks
	 * queued. Without this fallback, bypassed tasks could stall if the host
	 * scheduler's ops.dispatch() doesn't yield any tasks.
	 */
	if (bypass_dsp_enabled(sch))
		return consume_dispatch_q(sch, rq, bypass_dsq(sch, cpu), 0);

	return false;
}

static int balance_one(struct rq *rq, struct task_struct *prev)
{
	struct scx_sched *sch = scx_root;
	s32 cpu = cpu_of(rq);

	lockdep_assert_rq_held(rq);
	rq->scx.flags |= SCX_RQ_IN_BALANCE;
	rq->scx.flags &= ~SCX_RQ_BAL_KEEP;

	if ((sch->ops.flags & SCX_OPS_HAS_CPU_PREEMPT) &&
	    unlikely(rq->scx.cpu_released)) {
		/*
		 * If the previous sched_class for the current CPU was not SCX,
		 * notify the BPF scheduler that it again has control of the
		 * core. This callback complements ->cpu_release(), which is
		 * emitted in switch_class().
		 */
		if (SCX_HAS_OP(sch, cpu_acquire))
			SCX_CALL_OP(sch, cpu_acquire, rq, cpu, NULL);
		rq->scx.cpu_released = false;
	}

	if (prev->sched_class == &ext_sched_class) {
		update_curr_scx(rq);

		/*
		 * If @prev is runnable & has slice left, it has priority and
		 * fetching more just increases latency for the fetched tasks.
		 * Tell pick_task_scx() to keep running @prev. If the BPF
		 * scheduler wants to handle this explicitly, it should
		 * implement ->cpu_release().
		 *
		 * See scx_disable_workfn() for the explanation on the bypassing
		 * test.
		 */
		if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice &&
		    !scx_bypassing(sch, cpu)) {
			rq->scx.flags |= SCX_RQ_BAL_KEEP;
			goto has_tasks;
		}
	}

	/* if there already are tasks to run, nothing to do */
	if (rq->scx.local_dsq.nr)
		goto has_tasks;

	if (scx_dispatch_sched(sch, rq, prev, false))
		goto has_tasks;

	/*
	 * Didn't find another task to run. Keep running @prev unless
	 * %SCX_OPS_ENQ_LAST is in effect.
	 */
	if ((prev->scx.flags & SCX_TASK_QUEUED) &&
	    (!(sch->ops.flags & SCX_OPS_ENQ_LAST) || scx_bypassing(sch, cpu))) {
		rq->scx.flags |= SCX_RQ_BAL_KEEP;
		__scx_add_event(sch, SCX_EV_DISPATCH_KEEP_LAST, 1);
		goto has_tasks;
	}
	rq->scx.flags &= ~SCX_RQ_IN_BALANCE;
	return false;

has_tasks:
	/*
	 * @rq may have extra IMMED tasks without reenq scheduled:
	 *
	 * - rq_is_open() can't reliably tell when and how slice is going to be
	 *   modified for $curr and allows IMMED tasks to be queued while
	 *   dispatch is in progress.
	 *
	 * - A non-IMMED HEAD task can get queued in front of an IMMED task
	 *   between the IMMED queueing and the subsequent scheduling event.
	 */
	if (unlikely(rq->scx.local_dsq.nr > 1 && rq->scx.nr_immed))
		schedule_reenq_local(rq, 0);

	rq->scx.flags &= ~SCX_RQ_IN_BALANCE;
	return true;
}

static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
{
	struct scx_sched *sch = scx_task_sched(p);

	if (p->scx.flags & SCX_TASK_QUEUED) {
		/*
		 * Core-sched might decide to execute @p before it is
		 * dispatched. Call ops_dequeue() to notify the BPF scheduler.
		 */
		ops_dequeue(rq, p, SCX_DEQ_CORE_SCHED_EXEC);
		dispatch_dequeue(rq, p);
	}

	p->se.exec_start = rq_clock_task(rq);

	/* see dequeue_task_scx() on why we skip when !QUEUED */
	if (SCX_HAS_OP(sch, running) && (p->scx.flags & SCX_TASK_QUEUED))
		SCX_CALL_OP_TASK(sch, running, rq, p);

	clr_task_runnable(p, true);

	/*
	 * @p is getting newly scheduled or got kicked after someone updated its
	 * slice. Refresh whether tick can be stopped. See scx_can_stop_tick().
	 */
	if ((p->scx.slice == SCX_SLICE_INF) !=
	    (bool)(rq->scx.flags & SCX_RQ_CAN_STOP_TICK)) {
		if (p->scx.slice == SCX_SLICE_INF)
			rq->scx.flags |= SCX_RQ_CAN_STOP_TICK;
		else
			rq->scx.flags &= ~SCX_RQ_CAN_STOP_TICK;

		sched_update_tick_dependency(rq);

		/*
		 * For now, let's refresh the load_avgs just when transitioning
		 * in and out of nohz. In the future, we might want to add a
		 * mechanism which calls the following periodically on
		 * tick-stopped CPUs.
		 */
		update_other_load_avgs(rq);
	}
}

static enum scx_cpu_preempt_reason
preempt_reason_from_class(const struct sched_class *class)
{
	if (class == &stop_sched_class)
		return SCX_CPU_PREEMPT_STOP;
	if (class == &dl_sched_class)
		return SCX_CPU_PREEMPT_DL;
	if (class == &rt_sched_class)
		return SCX_CPU_PREEMPT_RT;
	return SCX_CPU_PREEMPT_UNKNOWN;
}

static void switch_class(struct rq *rq, struct task_struct *next)
{
	struct scx_sched *sch = scx_root;
	const struct sched_class *next_class = next->sched_class;

	if (!(sch->ops.flags & SCX_OPS_HAS_CPU_PREEMPT))
		return;

	/*
	 * The callback is conceptually meant to convey that the CPU is no
	 * longer under the control of SCX. Therefore, don't invoke the callback
	 * if the next class is below SCX (in which case the BPF scheduler has
	 * actively decided not to schedule any tasks on the CPU).
	 */
	if (sched_class_above(&ext_sched_class, next_class))
		return;

	/*
	 * At this point we know that SCX was preempted by a higher priority
	 * sched_class, so invoke the ->cpu_release() callback if we have not
	 * done so already. We only send the callback once between SCX being
	 * preempted, and it regaining control of the CPU.
	 *
	 * ->cpu_release() complements ->cpu_acquire(), which is emitted the
	 *  next time that balance_one() is invoked.
	 */
	if (!rq->scx.cpu_released) {
		if (SCX_HAS_OP(sch, cpu_release)) {
			struct scx_cpu_release_args args = {
				.reason = preempt_reason_from_class(next_class),
				.task = next,
			};

			SCX_CALL_OP(sch, cpu_release, rq, cpu_of(rq), &args);
		}
		rq->scx.cpu_released = true;
	}
}

static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
			      struct task_struct *next)
{
	struct scx_sched *sch = scx_task_sched(p);

	/* see kick_sync_wait_bal_cb() */
	smp_store_release(&rq->scx.kick_sync, rq->scx.kick_sync + 1);

	update_curr_scx(rq);

	/* see dequeue_task_scx() on why we skip when !QUEUED */
	if (SCX_HAS_OP(sch, stopping) && (p->scx.flags & SCX_TASK_QUEUED))
		SCX_CALL_OP_TASK(sch, stopping, rq, p, true);

	if (p->scx.flags & SCX_TASK_QUEUED) {
		set_task_runnable(rq, p);

		/*
		 * If @p has slice left and is being put, @p is getting
		 * preempted by a higher priority scheduler class or core-sched
		 * forcing a different task. Leave it at the head of the local
		 * DSQ unless it was an IMMED task. IMMED tasks should not
		 * linger on a busy CPU, reenqueue them to the BPF scheduler.
		 */
		if (p->scx.slice && !scx_bypassing(sch, cpu_of(rq))) {
			if (p->scx.flags & SCX_TASK_IMMED) {
				p->scx.flags |= SCX_TASK_REENQ_PREEMPTED;
				do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);
				p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
			} else {
				dispatch_enqueue(sch, rq, &rq->scx.local_dsq, p, SCX_ENQ_HEAD);
			}
			goto switch_class;
		}

		/*
		 * If @p is runnable but we're about to enter a lower
		 * sched_class, %SCX_OPS_ENQ_LAST must be set. Tell
		 * ops.enqueue() that @p is the only one available for this cpu,
		 * which should trigger an explicit follow-up scheduling event.
		 */
		if (next && sched_class_above(&ext_sched_class, next->sched_class)) {
			WARN_ON_ONCE(!(sch->ops.flags & SCX_OPS_ENQ_LAST));
			do_enqueue_task(rq, p, SCX_ENQ_LAST, -1);
		} else {
			do_enqueue_task(rq, p, 0, -1);
		}
	}

switch_class:
	if (next && next->sched_class != &ext_sched_class)
		switch_class(rq, next);
}

static void kick_sync_wait_bal_cb(struct rq *rq)
{
	struct scx_kick_syncs __rcu *ks = __this_cpu_read(scx_kick_syncs);
	unsigned long *ksyncs = rcu_dereference_sched(ks)->syncs;
	bool waited;
	s32 cpu;

	/*
	 * Drop rq lock and enable IRQs while waiting. IRQs must be enabled
	 * — a target CPU may be waiting for us to process an IPI (e.g. TLB
	 * flush) while we wait for its kick_sync to advance.
	 *
	 * Also, keep advancing our own kick_sync so that new kick_sync waits
	 * targeting us, which can start after we drop the lock, cannot form
	 * cyclic dependencies.
	 */
retry:
	waited = false;
	for_each_cpu(cpu, rq->scx.cpus_to_sync) {
		/*
		 * smp_load_acquire() pairs with smp_store_release() on
		 * kick_sync updates on the target CPUs.
		 */
		if (cpu == cpu_of(rq) ||
		    smp_load_acquire(&cpu_rq(cpu)->scx.kick_sync) != ksyncs[cpu]) {
			cpumask_clear_cpu(cpu, rq->scx.cpus_to_sync);
			continue;
		}

		raw_spin_rq_unlock_irq(rq);
		while (READ_ONCE(cpu_rq(cpu)->scx.kick_sync) == ksyncs[cpu]) {
			smp_store_release(&rq->scx.kick_sync, rq->scx.kick_sync + 1);
			cpu_relax();
		}
		raw_spin_rq_lock_irq(rq);
		waited = true;
	}

	if (waited)
		goto retry;
}

static struct task_struct *first_local_task(struct rq *rq)
{
	return list_first_entry_or_null(&rq->scx.local_dsq.list,
					struct task_struct, scx.dsq_list.node);
}

static struct task_struct *
do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
{
	struct task_struct *prev = rq->curr;
	bool keep_prev;
	struct task_struct *p;

	/* see kick_sync_wait_bal_cb() */
	smp_store_release(&rq->scx.kick_sync, rq->scx.kick_sync + 1);

	rq_modified_begin(rq, &ext_sched_class);

	rq_unpin_lock(rq, rf);
	balance_one(rq, prev);
	rq_repin_lock(rq, rf);
	maybe_queue_balance_callback(rq);

	/*
	 * Defer to a balance callback which can drop rq lock and enable
	 * IRQs. Waiting directly in the pick path would deadlock against
	 * CPUs sending us IPIs (e.g. TLB flushes) while we wait for them.
	 */
	if (unlikely(rq->scx.kick_sync_pending)) {
		rq->scx.kick_sync_pending = false;
		queue_balance_callback(rq, &rq->scx.kick_sync_bal_cb,
				       kick_sync_wait_bal_cb);
	}

	/*
	 * If any higher-priority sched class enqueued a runnable task on
	 * this rq during balance_one(), abort and return RETRY_TASK, so
	 * that the scheduler loop can restart.
	 *
	 * If @force_scx is true, always try to pick a SCHED_EXT task,
	 * regardless of any higher-priority sched classes activity.
	 */
	if (!force_scx && rq_modified_above(rq, &ext_sched_class))
		return RETRY_TASK;

	keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
	if (unlikely(keep_prev &&
		     prev->sched_class != &ext_sched_class)) {
		WARN_ON_ONCE(scx_enable_state() == SCX_ENABLED);
		keep_prev = false;
	}

	/*
	 * If balance_one() is telling us to keep running @prev, replenish slice
	 * if necessary and keep running @prev. Otherwise, pop the first one
	 * from the local DSQ.
	 */
	if (keep_prev) {
		p = prev;
		if (!p->scx.slice)
			refill_task_slice_dfl(scx_task_sched(p), p);
	} else {
		p = first_local_task(rq);
		if (!p)
			return NULL;

		if (unlikely(!p->scx.slice)) {
			struct scx_sched *sch = scx_task_sched(p);

			if (!scx_bypassing(sch, cpu_of(rq)) &&
			    !sch->warned_zero_slice) {
				printk_deferred(KERN_WARNING "sched_ext: %s[%d] has zero slice in %s()\n",
						p->comm, p->pid, __func__);
				sch->warned_zero_slice = true;
			}
			refill_task_slice_dfl(sch, p);
		}
	}

	return p;
}

static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
{
	return do_pick_task_scx(rq, rf, false);
}

/*
 * Select the next task to run from the ext scheduling class.
 *
 * Use do_pick_task_scx() directly with @force_scx enabled, since the
 * dl_server must always select a sched_ext task.
 */
static struct task_struct *
ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
{
	if (!scx_enabled())
		return NULL;

	return do_pick_task_scx(dl_se->rq, rf, true);
}

/*
 * Initialize the ext server deadline entity.
 */
void ext_server_init(struct rq *rq)
{
	struct sched_dl_entity *dl_se = &rq->ext_server;

	init_dl_entity(dl_se);

	dl_server_init(dl_se, rq, ext_server_pick_task);
}

#ifdef CONFIG_SCHED_CORE
/**
 * scx_prio_less - Task ordering for core-sched
 * @a: task A
 * @b: task B
 * @in_fi: in forced idle state
 *
 * Core-sched is implemented as an additional scheduling layer on top of the
 * usual sched_class'es and needs to find out the expected task ordering. For
 * SCX, core-sched calls this function to interrogate the task ordering.
 *
 * Unless overridden by ops.core_sched_before(), @p->scx.core_sched_at is used
 * to implement the default task ordering. The older the timestamp, the higher
 * priority the task - the global FIFO ordering matching the default scheduling
 * behavior.
 *
 * When ops.core_sched_before() is enabled, @p->scx.core_sched_at is used to
 * implement FIFO ordering within each local DSQ. See pick_task_scx().
 */
bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
		   bool in_fi)
{
	struct scx_sched *sch_a = scx_task_sched(a);
	struct scx_sched *sch_b = scx_task_sched(b);

	/*
	 * The const qualifiers are dropped from task_struct pointers when
	 * calling ops.core_sched_before(). Accesses are controlled by the
	 * verifier.
	 */
	if (sch_a == sch_b && SCX_HAS_OP(sch_a, core_sched_before) &&
	    !scx_bypassing(sch_a, task_cpu(a)))
		return SCX_CALL_OP_2TASKS_RET(sch_a, core_sched_before,
					      task_rq(a),
					      (struct task_struct *)a,
					      (struct task_struct *)b);
	else
		return time_after64(a->scx.core_sched_at, b->scx.core_sched_at);
}
#endif	/* CONFIG_SCHED_CORE */

static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flags)
{
	struct scx_sched *sch = scx_task_sched(p);
	bool bypassing;

	/*
	 * sched_exec() calls with %WF_EXEC when @p is about to exec(2) as it
	 * can be a good migration opportunity with low cache and memory
	 * footprint. Returning a CPU different than @prev_cpu triggers
	 * immediate rq migration. However, for SCX, as the current rq
	 * association doesn't dictate where the task is going to run, this
	 * doesn't fit well. If necessary, we can later add a dedicated method
	 * which can decide to preempt self to force it through the regular
	 * scheduling path.
	 */
	if (unlikely(wake_flags & WF_EXEC))
		return prev_cpu;

	bypassing = scx_bypassing(sch, task_cpu(p));
	if (likely(SCX_HAS_OP(sch, select_cpu)) && !bypassing) {
		s32 cpu;
		struct task_struct **ddsp_taskp;

		ddsp_taskp = this_cpu_ptr(&direct_dispatch_task);
		WARN_ON_ONCE(*ddsp_taskp);
		*ddsp_taskp = p;

		this_rq()->scx.in_select_cpu = true;
		cpu = SCX_CALL_OP_TASK_RET(sch, select_cpu, NULL, p, prev_cpu, wake_flags);
		this_rq()->scx.in_select_cpu = false;
		p->scx.selected_cpu = cpu;
		*ddsp_taskp = NULL;
		if (ops_cpu_valid(sch, cpu, "from ops.select_cpu()"))
			return cpu;
		else
			return prev_cpu;
	} else {
		s32 cpu;

		cpu = scx_select_cpu_dfl(p, prev_cpu, wake_flags, NULL, 0);
		if (cpu >= 0) {
			refill_task_slice_dfl(sch, p);
			p->scx.ddsp_dsq_id = SCX_DSQ_LOCAL;
		} else {
			cpu = prev_cpu;
		}
		p->scx.selected_cpu = cpu;

		if (bypassing)
			__scx_add_event(sch, SCX_EV_BYPASS_DISPATCH, 1);
		return cpu;
	}
}

static void task_woken_scx(struct rq *rq, struct task_struct *p)
{
	run_deferred(rq);
}

static void set_cpus_allowed_scx(struct task_struct *p,
				 struct affinity_context *ac)
{
	struct scx_sched *sch = scx_task_sched(p);

	set_cpus_allowed_common(p, ac);

	if (task_dead_and_done(p))
		return;

	/*
	 * The effective cpumask is stored in @p->cpus_ptr which may temporarily
	 * differ from the configured one in @p->cpus_mask. Always tell the bpf
	 * scheduler the effective one.
	 *
	 * Fine-grained memory write control is enforced by BPF making the const
	 * designation pointless. Cast it away when calling the operation.
	 */
	if (SCX_HAS_OP(sch, set_cpumask))
		SCX_CALL_OP_TASK(sch, set_cpumask, task_rq(p), p, (struct cpumask *)p->cpus_ptr);
}

static void handle_hotplug(struct rq *rq, bool online)
{
	struct scx_sched *sch = scx_root;
	s32 cpu = cpu_of(rq);

	atomic_long_inc(&scx_hotplug_seq);

	/*
	 * scx_root updates are protected by cpus_read_lock() and will stay
	 * stable here. Note that we can't depend on scx_enabled() test as the
	 * hotplug ops need to be enabled before __scx_enabled is set.
	 */
	if (unlikely(!sch))
		return;

	if (scx_enabled())
		scx_idle_update_selcpu_topology(&sch->ops);

	if (online && SCX_HAS_OP(sch, cpu_online))
		SCX_CALL_OP(sch, cpu_online, NULL, cpu);
	else if (!online && SCX_HAS_OP(sch, cpu_offline))
		SCX_CALL_OP(sch, cpu_offline, NULL, cpu);
	else
		scx_exit(sch, SCX_EXIT_UNREG_KERN,
			 SCX_ECODE_ACT_RESTART | SCX_ECODE_RSN_HOTPLUG,
			 "cpu %d going %s, exiting scheduler", cpu,
			 online ? "online" : "offline");
}

void scx_rq_activate(struct rq *rq)
{
	handle_hotplug(rq, true);
}

void scx_rq_deactivate(struct rq *rq)
{
	handle_hotplug(rq, false);
}

static void rq_online_scx(struct rq *rq)
{
	rq->scx.flags |= SCX_RQ_ONLINE;
}

static void rq_offline_scx(struct rq *rq)
{
	rq->scx.flags &= ~SCX_RQ_ONLINE;
}

static bool check_rq_for_timeouts(struct rq *rq)
{
	struct scx_sched *sch;
	struct task_struct *p;
	struct rq_flags rf;
	bool timed_out = false;

	rq_lock_irqsave(rq, &rf);
	sch = rcu_dereference_bh(scx_root);
	if (unlikely(!sch))
		goto out_unlock;

	list_for_each_entry(p, &rq->scx.runnable_list, scx.runnable_node) {
		struct scx_sched *sch = scx_task_sched(p);
		unsigned long last_runnable = p->scx.runnable_at;

		if (unlikely(time_after(jiffies,
					last_runnable + READ_ONCE(sch->watchdog_timeout)))) {
			u32 dur_ms = jiffies_to_msecs(jiffies - last_runnable);

			scx_exit(sch, SCX_EXIT_ERROR_STALL, 0,
				 "%s[%d] failed to run for %u.%03us",
				 p->comm, p->pid, dur_ms / 1000, dur_ms % 1000);
			timed_out = true;
			break;
		}
	}
out_unlock:
	rq_unlock_irqrestore(rq, &rf);
	return timed_out;
}

static void scx_watchdog_workfn(struct work_struct *work)
{
	unsigned long intv;
	int cpu;

	WRITE_ONCE(scx_watchdog_timestamp, jiffies);

	for_each_online_cpu(cpu) {
		if (unlikely(check_rq_for_timeouts(cpu_rq(cpu))))
			break;

		cond_resched();
	}

	intv = READ_ONCE(scx_watchdog_interval);
	if (intv < ULONG_MAX)
		queue_delayed_work(system_dfl_wq, to_delayed_work(work), intv);
}

void scx_tick(struct rq *rq)
{
	struct scx_sched *root;
	unsigned long last_check;

	if (!scx_enabled())
		return;

	root = rcu_dereference_bh(scx_root);
	if (unlikely(!root))
		return;

	last_check = READ_ONCE(scx_watchdog_timestamp);
	if (unlikely(time_after(jiffies,
				last_check + READ_ONCE(root->watchdog_timeout)))) {
		u32 dur_ms = jiffies_to_msecs(jiffies - last_check);

		scx_exit(root, SCX_EXIT_ERROR_STALL, 0,
			 "watchdog failed to check in for %u.%03us",
			 dur_ms / 1000, dur_ms % 1000);
	}

	update_other_load_avgs(rq);
}

static void task_tick_scx(struct rq *rq, struct task_struct *curr, int queued)
{
	struct scx_sched *sch = scx_task_sched(curr);

	update_curr_scx(rq);

	/*
	 * While disabling, always resched and refresh core-sched timestamp as
	 * we can't trust the slice management or ops.core_sched_before().
	 */
	if (scx_bypassing(sch, cpu_of(rq))) {
		curr->scx.slice = 0;
		touch_core_sched(rq, curr);
	} else if (SCX_HAS_OP(sch, tick)) {
		SCX_CALL_OP_TASK(sch, tick, rq, curr);
	}

	if (!curr->scx.slice)
		resched_curr(rq);
}

#ifdef CONFIG_EXT_GROUP_SCHED
static struct cgroup *tg_cgrp(struct task_group *tg)
{
	/*
	 * If CGROUP_SCHED is disabled, @tg is NULL. If @tg is an autogroup,
	 * @tg->css.cgroup is NULL. In both cases, @tg can be treated as the
	 * root cgroup.
	 */
	if (tg && tg->css.cgroup)
		return tg->css.cgroup;
	else
		return &cgrp_dfl_root.cgrp;
}

#define SCX_INIT_TASK_ARGS_CGROUP(tg)		.cgroup = tg_cgrp(tg),

#else	/* CONFIG_EXT_GROUP_SCHED */

#define SCX_INIT_TASK_ARGS_CGROUP(tg)

#endif	/* CONFIG_EXT_GROUP_SCHED */

static u32 scx_get_task_state(const struct task_struct *p)
{
	return p->scx.flags & SCX_TASK_STATE_MASK;
}

static void scx_set_task_state(struct task_struct *p, u32 state)
{
	u32 prev_state = scx_get_task_state(p);
	bool warn = false;

	switch (state) {
	case SCX_TASK_NONE:
		break;
	case SCX_TASK_INIT:
		warn = prev_state != SCX_TASK_NONE;
		break;
	case SCX_TASK_READY:
		warn = prev_state == SCX_TASK_NONE;
		break;
	case SCX_TASK_ENABLED:
		warn = prev_state != SCX_TASK_READY;
		break;
	default:
		WARN_ONCE(1, "sched_ext: Invalid task state %d -> %d for %s[%d]",
			  prev_state, state, p->comm, p->pid);
		return;
	}

	WARN_ONCE(warn, "sched_ext: Invalid task state transition 0x%x -> 0x%x for %s[%d]",
		  prev_state, state, p->comm, p->pid);

	p->scx.flags &= ~SCX_TASK_STATE_MASK;
	p->scx.flags |= state;
}

static int __scx_init_task(struct scx_sched *sch, struct task_struct *p, bool fork)
{
	int ret;

	p->scx.disallow = false;

	if (SCX_HAS_OP(sch, init_task)) {
		struct scx_init_task_args args = {
			SCX_INIT_TASK_ARGS_CGROUP(task_group(p))
			.fork = fork,
		};

		ret = SCX_CALL_OP_RET(sch, init_task, NULL, p, &args);
		if (unlikely(ret)) {
			ret = ops_sanitize_err(sch, "init_task", ret);
			return ret;
		}
	}

	if (p->scx.disallow) {
		if (unlikely(scx_parent(sch))) {
			scx_error(sch, "non-root ops.init_task() set task->scx.disallow for %s[%d]",
				  p->comm, p->pid);
		} else if (unlikely(fork)) {
			scx_error(sch, "ops.init_task() set task->scx.disallow for %s[%d] during fork",
				  p->comm, p->pid);
		} else {
			struct rq *rq;
			struct rq_flags rf;

			rq = task_rq_lock(p, &rf);

			/*
			 * We're in the load path and @p->policy will be applied
			 * right after. Reverting @p->policy here and rejecting
			 * %SCHED_EXT transitions from scx_check_setscheduler()
			 * guarantees that if ops.init_task() sets @p->disallow,
			 * @p can never be in SCX.
			 */
			if (p->policy == SCHED_EXT) {
				p->policy = SCHED_NORMAL;
				atomic_long_inc(&scx_nr_rejected);
			}

			task_rq_unlock(rq, p, &rf);
		}
	}

	return 0;
}

static int scx_init_task(struct scx_sched *sch, struct task_struct *p, bool fork)
{
	int ret;

	ret = __scx_init_task(sch, p, fork);
	if (!ret) {
		/*
		 * While @p's rq is not locked. @p is not visible to the rest of
		 * SCX yet and it's safe to update the flags and state.
		 */
		p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
		scx_set_task_state(p, SCX_TASK_INIT);
	}
	return ret;
}

static void __scx_enable_task(struct scx_sched *sch, struct task_struct *p)
{
	struct rq *rq = task_rq(p);
	u32 weight;

	lockdep_assert_rq_held(rq);

	/*
	 * Verify the task is not in BPF scheduler's custody. If flag
	 * transitions are consistent, the flag should always be clear
	 * here.
	 */
	WARN_ON_ONCE(p->scx.flags & SCX_TASK_IN_CUSTODY);

	/*
	 * Set the weight before calling ops.enable() so that the scheduler
	 * doesn't see a stale value if they inspect the task struct.
	 */
	if (task_has_idle_policy(p))
		weight = WEIGHT_IDLEPRIO;
	else
		weight = sched_prio_to_weight[p->static_prio - MAX_RT_PRIO];

	p->scx.weight = sched_weight_to_cgroup(weight);

	if (SCX_HAS_OP(sch, enable))
		SCX_CALL_OP_TASK(sch, enable, rq, p);

	if (SCX_HAS_OP(sch, set_weight))
		SCX_CALL_OP_TASK(sch, set_weight, rq, p, p->scx.weight);
}

static void scx_enable_task(struct scx_sched *sch, struct task_struct *p)
{
	__scx_enable_task(sch, p);
	scx_set_task_state(p, SCX_TASK_ENABLED);
}

static void scx_disable_task(struct scx_sched *sch, struct task_struct *p)
{
	struct rq *rq = task_rq(p);

	lockdep_assert_rq_held(rq);
	WARN_ON_ONCE(scx_get_task_state(p) != SCX_TASK_ENABLED);

	clear_direct_dispatch(p);

	if (SCX_HAS_OP(sch, disable))
		SCX_CALL_OP_TASK(sch, disable, rq, p);
	scx_set_task_state(p, SCX_TASK_READY);

	/*
	 * Verify the task is not in BPF scheduler's custody. If flag
	 * transitions are consistent, the flag should always be clear
	 * here.
	 */
	WARN_ON_ONCE(p->scx.flags & SCX_TASK_IN_CUSTODY);
}

static void __scx_disable_and_exit_task(struct scx_sched *sch,
					struct task_struct *p)
{
	struct scx_exit_task_args args = {
		.cancelled = false,
	};

	lockdep_assert_held(&p->pi_lock);
	lockdep_assert_rq_held(task_rq(p));

	switch (scx_get_task_state(p)) {
	case SCX_TASK_NONE:
		return;
	case SCX_TASK_INIT:
		args.cancelled = true;
		break;
	case SCX_TASK_READY:
		break;
	case SCX_TASK_ENABLED:
		scx_disable_task(sch, p);
		break;
	default:
		WARN_ON_ONCE(true);
		return;
	}

	if (SCX_HAS_OP(sch, exit_task))
		SCX_CALL_OP_TASK(sch, exit_task, task_rq(p), p, &args);
}

/*
 * Undo a completed __scx_init_task(sch, p, false) when scx_enable_task() never
 * ran. The task state has not been transitioned, so this mirrors the
 * SCX_TASK_INIT branch in __scx_disable_and_exit_task().
 */
static void scx_sub_init_cancel_task(struct scx_sched *sch, struct task_struct *p)
{
	struct scx_exit_task_args args = { .cancelled = true };

	lockdep_assert_held(&p->pi_lock);
	lockdep_assert_rq_held(task_rq(p));

	if (SCX_HAS_OP(sch, exit_task))
		SCX_CALL_OP_TASK(sch, exit_task, task_rq(p), p, &args);
}

static void scx_disable_and_exit_task(struct scx_sched *sch,
				      struct task_struct *p)
{
	__scx_disable_and_exit_task(sch, p);

	/*
	 * If set, @p exited between __scx_init_task() and scx_enable_task() in
	 * scx_sub_enable() and is initialized for both the associated sched and
	 * its parent. Exit for the child too - scx_enable_task() never ran for
	 * it, so undo only init_task.
	 */
	if (p->scx.flags & SCX_TASK_SUB_INIT) {
		if (!WARN_ON_ONCE(!scx_enabling_sub_sched))
			scx_sub_init_cancel_task(scx_enabling_sub_sched, p);
		p->scx.flags &= ~SCX_TASK_SUB_INIT;
	}

	scx_set_task_sched(p, NULL);
	scx_set_task_state(p, SCX_TASK_NONE);
}

void init_scx_entity(struct sched_ext_entity *scx)
{
	memset(scx, 0, sizeof(*scx));
	INIT_LIST_HEAD(&scx->dsq_list.node);
	RB_CLEAR_NODE(&scx->dsq_priq);
	scx->sticky_cpu = -1;
	scx->holding_cpu = -1;
	INIT_LIST_HEAD(&scx->runnable_node);
	scx->runnable_at = jiffies;
	scx->ddsp_dsq_id = SCX_DSQ_INVALID;
	scx->slice = SCX_SLICE_DFL;
}

void scx_pre_fork(struct task_struct *p)
{
	/*
	 * BPF scheduler enable/disable paths want to be able to iterate and
	 * update all tasks which can become complex when racing forks. As
	 * enable/disable are very cold paths, let's use a percpu_rwsem to
	 * exclude forks.
	 */
	percpu_down_read(&scx_fork_rwsem);
}

int scx_fork(struct task_struct *p, struct kernel_clone_args *kargs)
{
	s32 ret;

	percpu_rwsem_assert_held(&scx_fork_rwsem);

	if (scx_init_task_enabled) {
#ifdef CONFIG_EXT_SUB_SCHED
		struct scx_sched *sch = kargs->cset->dfl_cgrp->scx_sched;
#else
		struct scx_sched *sch = scx_root;
#endif
		ret = scx_init_task(sch, p, true);
		if (!ret)
			scx_set_task_sched(p, sch);
		return ret;
	}

	return 0;
}

void scx_post_fork(struct task_struct *p)
{
	if (scx_init_task_enabled) {
		scx_set_task_state(p, SCX_TASK_READY);

		/*
		 * Enable the task immediately if it's running on sched_ext.
		 * Otherwise, it'll be enabled in switching_to_scx() if and
		 * when it's ever configured to run with a SCHED_EXT policy.
		 */
		if (p->sched_class == &ext_sched_class) {
			struct rq_flags rf;
			struct rq *rq;

			rq = task_rq_lock(p, &rf);
			scx_enable_task(scx_task_sched(p), p);
			task_rq_unlock(rq, p, &rf);
		}
	}

	raw_spin_lock_irq(&scx_tasks_lock);
	list_add_tail(&p->scx.tasks_node, &scx_tasks);
	raw_spin_unlock_irq(&scx_tasks_lock);

	percpu_up_read(&scx_fork_rwsem);
}

void scx_cancel_fork(struct task_struct *p)
{
	if (scx_enabled()) {
		struct rq *rq;
		struct rq_flags rf;

		rq = task_rq_lock(p, &rf);
		WARN_ON_ONCE(scx_get_task_state(p) >= SCX_TASK_READY);
		scx_disable_and_exit_task(scx_task_sched(p), p);
		task_rq_unlock(rq, p, &rf);
	}

	percpu_up_read(&scx_fork_rwsem);
}

/**
 * task_dead_and_done - Is a task dead and done running?
 * @p: target task
 *
 * Once sched_ext_dead() removes the dead task from scx_tasks and exits it, the
 * task no longer exists from SCX's POV. However, certain sched_class ops may be
 * invoked on these dead tasks leading to failures - e.g. sched_setscheduler()
 * may try to switch a task which finished sched_ext_dead() back into SCX
 * triggering invalid SCX task state transitions and worse.
 *
 * Once a task has finished the final switch, sched_ext_dead() is the only thing
 * that needs to happen on the task. Use this test to short-circuit sched_class
 * operations which may be called on dead tasks.
 */
static bool task_dead_and_done(struct task_struct *p)
{
	struct rq *rq = task_rq(p);

	lockdep_assert_rq_held(rq);

	/*
	 * In do_task_dead(), a dying task sets %TASK_DEAD with preemption
	 * disabled and __schedule(). If @p has %TASK_DEAD set and off CPU, @p
	 * won't ever run again.
	 */
	return unlikely(READ_ONCE(p->__state) == TASK_DEAD) &&
		!task_on_cpu(rq, p);
}

void sched_ext_dead(struct task_struct *p)
{
	unsigned long flags;

	/*
	 * By the time control reaches here, @p has %TASK_DEAD set, switched out
	 * for the last time and then dropped the rq lock - task_dead_and_done()
	 * should be returning %true nullifying the straggling sched_class ops.
	 * Remove from scx_tasks and exit @p.
	 */
	raw_spin_lock_irqsave(&scx_tasks_lock, flags);
	list_del_init(&p->scx.tasks_node);
	raw_spin_unlock_irqrestore(&scx_tasks_lock, flags);

	/*
	 * @p is off scx_tasks and wholly ours. scx_root_enable()'s READY ->
	 * ENABLED transitions can't race us. Disable ops for @p.
	 *
	 * %SCX_TASK_OFF_TASKS synchronizes against cgroup task iteration - see
	 * scx_task_iter_next_locked(). NONE tasks need no marking: cgroup
	 * iteration is only used from sub-sched paths, which require root
	 * enabled. Root enable transitions every live task to at least READY.
	 */
	if (scx_get_task_state(p) != SCX_TASK_NONE) {
		struct rq_flags rf;
		struct rq *rq;

		rq = task_rq_lock(p, &rf);
		scx_disable_and_exit_task(scx_task_sched(p), p);
		p->scx.flags |= SCX_TASK_OFF_TASKS;
		task_rq_unlock(rq, p, &rf);
	}
}

static void reweight_task_scx(struct rq *rq, struct task_struct *p,
			      const struct load_weight *lw)
{
	struct scx_sched *sch = scx_task_sched(p);

	lockdep_assert_rq_held(task_rq(p));

	if (task_dead_and_done(p))
		return;

	p->scx.weight = sched_weight_to_cgroup(scale_load_down(lw->weight));
	if (SCX_HAS_OP(sch, set_weight))
		SCX_CALL_OP_TASK(sch, set_weight, rq, p, p->scx.weight);
}

static void prio_changed_scx(struct rq *rq, struct task_struct *p, u64 oldprio)
{
}

static void switching_to_scx(struct rq *rq, struct task_struct *p)
{
	struct scx_sched *sch = scx_task_sched(p);

	if (task_dead_and_done(p))
		return;

	scx_enable_task(sch, p);

	/*
	 * set_cpus_allowed_scx() is not called while @p is associated with a
	 * different scheduler class. Keep the BPF scheduler up-to-date.
	 */
	if (SCX_HAS_OP(sch, set_cpumask))
		SCX_CALL_OP_TASK(sch, set_cpumask, rq, p, (struct cpumask *)p->cpus_ptr);
}

static void switched_from_scx(struct rq *rq, struct task_struct *p)
{
	if (task_dead_and_done(p))
		return;

	scx_disable_task(scx_task_sched(p), p);
}

static void switched_to_scx(struct rq *rq, struct task_struct *p) {}

int scx_check_setscheduler(struct task_struct *p, int policy)
{
	lockdep_assert_rq_held(task_rq(p));

	/* if disallow, reject transitioning into SCX */
	if (scx_enabled() && READ_ONCE(p->scx.disallow) &&
	    p->policy != policy && policy == SCHED_EXT)
		return -EACCES;

	return 0;
}

static void process_ddsp_deferred_locals(struct rq *rq)
{
	struct task_struct *p;

	lockdep_assert_rq_held(rq);

	/*
	 * Now that @rq can be unlocked, execute the deferred enqueueing of
	 * tasks directly dispatched to the local DSQs of other CPUs. See
	 * direct_dispatch(). Keep popping from the head instead of using
	 * list_for_each_entry_safe() as dispatch_local_dsq() may unlock @rq
	 * temporarily.
	 */
	while ((p = list_first_entry_or_null(&rq->scx.ddsp_deferred_locals,
				struct task_struct, scx.dsq_list.node))) {
		struct scx_sched *sch = scx_task_sched(p);
		struct scx_dispatch_q *dsq;
		u64 dsq_id = p->scx.ddsp_dsq_id;
		u64 enq_flags = p->scx.ddsp_enq_flags;

		list_del_init(&p->scx.dsq_list.node);
		clear_direct_dispatch(p);

		dsq = find_dsq_for_dispatch(sch, rq, dsq_id, task_cpu(p));
		if (!WARN_ON_ONCE(dsq->id != SCX_DSQ_LOCAL))
			dispatch_to_local_dsq(sch, rq, dsq, p, enq_flags);
	}
}

/*
 * Determine whether @p should be reenqueued from a local DSQ.
 *
 * @reenq_flags is mutable and accumulates state across the DSQ walk:
 *
 * - %SCX_REENQ_TSR_NOT_FIRST: Set after the first task is visited. "First"
 *   tracks position in the DSQ list, not among IMMED tasks. A non-IMMED task at
 *   the head consumes the first slot.
 *
 * - %SCX_REENQ_TSR_RQ_OPEN: Set by reenq_local() before the walk if
 *   rq_is_open() is true.
 *
 * An IMMED task is kept (returns %false) only if it's the first task in the DSQ
 * AND the current task is done — i.e. it will execute immediately. All other
 * IMMED tasks are reenqueued. This means if a non-IMMED task sits at the head,
 * every IMMED task behind it gets reenqueued.
 *
 * Reenqueued tasks go through ops.enqueue() with %SCX_ENQ_REENQ |
 * %SCX_TASK_REENQ_IMMED. If the BPF scheduler dispatches back to the same local
 * DSQ with %SCX_ENQ_IMMED while the CPU is still unavailable, this triggers
 * another reenq cycle. Repetitions are bounded by %SCX_REENQ_LOCAL_MAX_REPEAT
 * in process_deferred_reenq_locals().
 */
static bool local_task_should_reenq(struct task_struct *p, u64 *reenq_flags, u32 *reason)
{
	bool first;

	first = !(*reenq_flags & SCX_REENQ_TSR_NOT_FIRST);
	*reenq_flags |= SCX_REENQ_TSR_NOT_FIRST;

	*reason = SCX_TASK_REENQ_KFUNC;

	if ((p->scx.flags & SCX_TASK_IMMED) &&
	    (!first || !(*reenq_flags & SCX_REENQ_TSR_RQ_OPEN))) {
		__scx_add_event(scx_task_sched(p), SCX_EV_REENQ_IMMED, 1);
		*reason = SCX_TASK_REENQ_IMMED;
		return true;
	}

	return *reenq_flags & SCX_REENQ_ANY;
}

static u32 reenq_local(struct scx_sched *sch, struct rq *rq, u64 reenq_flags)
{
	LIST_HEAD(tasks);
	u32 nr_enqueued = 0;
	struct task_struct *p, *n;

	lockdep_assert_rq_held(rq);

	if (WARN_ON_ONCE(reenq_flags & __SCX_REENQ_TSR_MASK))
		reenq_flags &= ~__SCX_REENQ_TSR_MASK;
	if (rq_is_open(rq, 0))
		reenq_flags |= SCX_REENQ_TSR_RQ_OPEN;

	/*
	 * The BPF scheduler may choose to dispatch tasks back to
	 * @rq->scx.local_dsq. Move all candidate tasks off to a private list
	 * first to avoid processing the same tasks repeatedly.
	 */
	list_for_each_entry_safe(p, n, &rq->scx.local_dsq.list,
				 scx.dsq_list.node) {
		struct scx_sched *task_sch = scx_task_sched(p);
		u32 reason;

		/*
		 * If @p is being migrated, @p's current CPU may not agree with
		 * its allowed CPUs and the migration_cpu_stop is about to
		 * deactivate and re-activate @p anyway. Skip re-enqueueing.
		 *
		 * While racing sched property changes may also dequeue and
		 * re-enqueue a migrating task while its current CPU and allowed
		 * CPUs disagree, they use %ENQUEUE_RESTORE which is bypassed to
		 * the current local DSQ for running tasks and thus are not
		 * visible to the BPF scheduler.
		 */
		if (p->migration_pending)
			continue;

		if (!scx_is_descendant(task_sch, sch))
			continue;

		if (!local_task_should_reenq(p, &reenq_flags, &reason))
			continue;

		dispatch_dequeue(rq, p);

		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
		p->scx.flags |= reason;

		list_add_tail(&p->scx.dsq_list.node, &tasks);
	}

	list_for_each_entry_safe(p, n, &tasks, scx.dsq_list.node) {
		list_del_init(&p->scx.dsq_list.node);

		do_enqueue_task(rq, p, SCX_ENQ_REENQ, -1);

		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
		nr_enqueued++;
	}

	return nr_enqueued;
}

static void process_deferred_reenq_locals(struct rq *rq)
{
	u64 seq = ++rq->scx.deferred_reenq_locals_seq;

	lockdep_assert_rq_held(rq);

	while (true) {
		struct scx_sched *sch;
		u64 reenq_flags;
		bool skip = false;

		scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
			struct scx_deferred_reenq_local *drl =
				list_first_entry_or_null(&rq->scx.deferred_reenq_locals,
							 struct scx_deferred_reenq_local,
							 node);
			struct scx_sched_pcpu *sch_pcpu;

			if (!drl)
				return;

			sch_pcpu = container_of(drl, struct scx_sched_pcpu,
						deferred_reenq_local);
			sch = sch_pcpu->sch;

			reenq_flags = drl->flags;
			WRITE_ONCE(drl->flags, 0);
			list_del_init(&drl->node);

			if (likely(drl->seq != seq)) {
				drl->seq = seq;
				drl->cnt = 0;
			} else {
				if (unlikely(++drl->cnt > SCX_REENQ_LOCAL_MAX_REPEAT)) {
					scx_error(sch, "SCX_ENQ_REENQ on SCX_DSQ_LOCAL repeated %u times",
						  drl->cnt);
					skip = true;
				}

				__scx_add_event(sch, SCX_EV_REENQ_LOCAL_REPEAT, 1);
			}
		}

		if (!skip) {
			/* see schedule_dsq_reenq() */
			smp_mb();

			reenq_local(sch, rq, reenq_flags);
		}
	}
}

static bool user_task_should_reenq(struct task_struct *p, u64 reenq_flags, u32 *reason)
{
	*reason = SCX_TASK_REENQ_KFUNC;
	return reenq_flags & SCX_REENQ_ANY;
}

static void reenq_user(struct rq *rq, struct scx_dispatch_q *dsq, u64 reenq_flags)
{
	struct rq *locked_rq = rq;
	struct scx_sched *sch = dsq->sched;
	struct scx_dsq_list_node cursor = INIT_DSQ_LIST_CURSOR(cursor, dsq, 0);
	struct task_struct *p;
	s32 nr_enqueued = 0;

	lockdep_assert_rq_held(rq);

	raw_spin_lock(&dsq->lock);

	while (likely(!READ_ONCE(sch->bypass_depth))) {
		struct rq *task_rq;
		u32 reason;

		p = nldsq_cursor_next_task(&cursor, dsq);
		if (!p)
			break;

		if (!user_task_should_reenq(p, reenq_flags, &reason))
			continue;

		task_rq = task_rq(p);

		if (locked_rq != task_rq) {
			if (locked_rq)
				raw_spin_rq_unlock(locked_rq);
			if (unlikely(!raw_spin_rq_trylock(task_rq))) {
				raw_spin_unlock(&dsq->lock);
				raw_spin_rq_lock(task_rq);
				raw_spin_lock(&dsq->lock);
			}
			locked_rq = task_rq;

			/* did we lose @p while switching locks? */
			if (nldsq_cursor_lost_task(&cursor, task_rq, dsq, p))
				continue;
		}

		/* @p is on @dsq, its rq and @dsq are locked */
		dispatch_dequeue_locked(p, dsq);
		raw_spin_unlock(&dsq->lock);

		if (WARN_ON_ONCE(p->scx.flags & SCX_TASK_REENQ_REASON_MASK))
			p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;
		p->scx.flags |= reason;

		do_enqueue_task(task_rq, p, SCX_ENQ_REENQ, -1);

		p->scx.flags &= ~SCX_TASK_REENQ_REASON_MASK;

		if (!(++nr_enqueued % SCX_TASK_ITER_BATCH)) {
			raw_spin_rq_unlock(locked_rq);
			locked_rq = NULL;
			cpu_relax();
		}

		raw_spin_lock(&dsq->lock);
	}

	list_del_init(&cursor.node);
	raw_spin_unlock(&dsq->lock);

	if (locked_rq != rq) {
		if (locked_rq)
			raw_spin_rq_unlock(locked_rq);
		raw_spin_rq_lock(rq);
	}
}

static void process_deferred_reenq_users(struct rq *rq)
{
	lockdep_assert_rq_held(rq);

	while (true) {
		struct scx_dispatch_q *dsq;
		u64 reenq_flags;

		scoped_guard (raw_spinlock, &rq->scx.deferred_reenq_lock) {
			struct scx_deferred_reenq_user *dru =
				list_first_entry_or_null(&rq->scx.deferred_reenq_users,
							 struct scx_deferred_reenq_user,
							 node);
			struct scx_dsq_pcpu *dsq_pcpu;

			if (!dru)
				return;

			dsq_pcpu = container_of(dru, struct scx_dsq_pcpu,
						deferred_reenq_user);
			dsq = dsq_pcpu->dsq;
			reenq_flags = dru->flags;
			WRITE_ONCE(dru->flags, 0);
			list_del_init(&dru->node);
		}

		/* see schedule_dsq_reenq() */
		smp_mb();

		BUG_ON(dsq->id & SCX_DSQ_FLAG_BUILTIN);
		reenq_user(rq, dsq, reenq_flags);
	}
}

static void run_deferred(struct rq *rq)
{
	process_ddsp_deferred_locals(rq);

	if (!list_empty(&rq->scx.deferred_reenq_locals))
		process_deferred_reenq_locals(rq);

	if (!list_empty(&rq->scx.deferred_reenq_users))
		process_deferred_reenq_users(rq);
}

#ifdef CONFIG_NO_HZ_FULL
bool scx_can_stop_tick(struct rq *rq)
{
	struct task_struct *p = rq->curr;
	struct scx_sched *sch = scx_task_sched(p);

	if (p->sched_class != &ext_sched_class)
		return true;

	if (scx_bypassing(sch, cpu_of(rq)))
		return false;

	/*
	 * @rq can dispatch from different DSQs, so we can't tell whether it
	 * needs the tick or not by looking at nr_running. Allow stopping ticks
	 * iff the BPF scheduler indicated so. See set_next_task_scx().
	 */
	return rq->scx.flags & SCX_RQ_CAN_STOP_TICK;
}
#endif

#ifdef CONFIG_EXT_GROUP_SCHED

DEFINE_STATIC_PERCPU_RWSEM(scx_cgroup_ops_rwsem);
static bool scx_cgroup_enabled;

void scx_tg_init(struct task_group *tg)
{
	tg->scx.weight = CGROUP_WEIGHT_DFL;
	tg->scx.bw_period_us = default_bw_period_us();
	tg->scx.bw_quota_us = RUNTIME_INF;
	tg->scx.idle = false;
}

int scx_tg_online(struct task_group *tg)
{
	struct scx_sched *sch = scx_root;
	int ret = 0;

	WARN_ON_ONCE(tg->scx.flags & (SCX_TG_ONLINE | SCX_TG_INITED));

	if (scx_cgroup_enabled) {
		if (SCX_HAS_OP(sch, cgroup_init)) {
			struct scx_cgroup_init_args args =
				{ .weight = tg->scx.weight,
				  .bw_period_us = tg->scx.bw_period_us,
				  .bw_quota_us = tg->scx.bw_quota_us,
				  .bw_burst_us = tg->scx.bw_burst_us };

			ret = SCX_CALL_OP_RET(sch, cgroup_init,
					      NULL, tg->css.cgroup, &args);
			if (ret)
				ret = ops_sanitize_err(sch, "cgroup_init", ret);
		}
		if (ret == 0)
			tg->scx.flags |= SCX_TG_ONLINE | SCX_TG_INITED;
	} else {
		tg->scx.flags |= SCX_TG_ONLINE;
	}

	return ret;
}

void scx_tg_offline(struct task_group *tg)
{
	struct scx_sched *sch = scx_root;

	WARN_ON_ONCE(!(tg->scx.flags & SCX_TG_ONLINE));

	if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_exit) &&
	    (tg->scx.flags & SCX_TG_INITED))
		SCX_CALL_OP(sch, cgroup_exit, NULL, tg->css.cgroup);
	tg->scx.flags &= ~(SCX_TG_ONLINE | SCX_TG_INITED);
}

int scx_cgroup_can_attach(struct cgroup_taskset *tset)
{
	struct scx_sched *sch = scx_root;
	struct cgroup_subsys_state *css;
	struct task_struct *p;
	int ret;

	if (!scx_cgroup_enabled)
		return 0;

	cgroup_taskset_for_each(p, css, tset) {
		struct cgroup *from = tg_cgrp(task_group(p));
		struct cgroup *to = tg_cgrp(css_tg(css));

		WARN_ON_ONCE(p->scx.cgrp_moving_from);

		/*
		 * sched_move_task() omits identity migrations. Let's match the
		 * behavior so that ops.cgroup_prep_move() and ops.cgroup_move()
		 * always match one-to-one.
		 */
		if (from == to)
			continue;

		if (SCX_HAS_OP(sch, cgroup_prep_move)) {
			ret = SCX_CALL_OP_RET(sch, cgroup_prep_move, NULL,
					      p, from, css->cgroup);
			if (ret)
				goto err;
		}

		p->scx.cgrp_moving_from = from;
	}

	return 0;

err:
	cgroup_taskset_for_each(p, css, tset) {
		if (SCX_HAS_OP(sch, cgroup_cancel_move) &&
		    p->scx.cgrp_moving_from)
			SCX_CALL_OP(sch, cgroup_cancel_move, NULL,
				    p, p->scx.cgrp_moving_from, css->cgroup);
		p->scx.cgrp_moving_from = NULL;
	}

	return ops_sanitize_err(sch, "cgroup_prep_move", ret);
}

void scx_cgroup_move_task(struct task_struct *p)
{
	struct scx_sched *sch = scx_root;

	if (!scx_cgroup_enabled)
		return;

	/*
	 * @p must have ops.cgroup_prep_move() called on it and thus
	 * cgrp_moving_from set.
	 */
	if (SCX_HAS_OP(sch, cgroup_move) &&
	    !WARN_ON_ONCE(!p->scx.cgrp_moving_from))
		SCX_CALL_OP_TASK(sch, cgroup_move, task_rq(p),
				 p, p->scx.cgrp_moving_from,
				 tg_cgrp(task_group(p)));
	p->scx.cgrp_moving_from = NULL;
}

void scx_cgroup_cancel_attach(struct cgroup_taskset *tset)
{
	struct scx_sched *sch = scx_root;
	struct cgroup_subsys_state *css;
	struct task_struct *p;

	if (!scx_cgroup_enabled)
		return;

	cgroup_taskset_for_each(p, css, tset) {
		if (SCX_HAS_OP(sch, cgroup_cancel_move) &&
		    p->scx.cgrp_moving_from)
			SCX_CALL_OP(sch, cgroup_cancel_move, NULL,
				    p, p->scx.cgrp_moving_from, css->cgroup);
		p->scx.cgrp_moving_from = NULL;
	}
}

void scx_group_set_weight(struct task_group *tg, unsigned long weight)
{
	struct scx_sched *sch;

	percpu_down_read(&scx_cgroup_ops_rwsem);
	sch = scx_root;

	if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_set_weight) &&
	    tg->scx.weight != weight)
		SCX_CALL_OP(sch, cgroup_set_weight, NULL, tg_cgrp(tg), weight);

	tg->scx.weight = weight;

	percpu_up_read(&scx_cgroup_ops_rwsem);
}

void scx_group_set_idle(struct task_group *tg, bool idle)
{
	struct scx_sched *sch;

	percpu_down_read(&scx_cgroup_ops_rwsem);
	sch = scx_root;

	if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_set_idle))
		SCX_CALL_OP(sch, cgroup_set_idle, NULL, tg_cgrp(tg), idle);

	/* Update the task group's idle state */
	tg->scx.idle = idle;

	percpu_up_read(&scx_cgroup_ops_rwsem);
}

void scx_group_set_bandwidth(struct task_group *tg,
			     u64 period_us, u64 quota_us, u64 burst_us)
{
	struct scx_sched *sch;

	percpu_down_read(&scx_cgroup_ops_rwsem);
	sch = scx_root;

	if (scx_cgroup_enabled && SCX_HAS_OP(sch, cgroup_set_bandwidth) &&
	    (tg->scx.bw_period_us != period_us ||
	     tg->scx.bw_quota_us != quota_us ||
	     tg->scx.bw_burst_us != burst_us))
		SCX_CALL_OP(sch, cgroup_set_bandwidth, NULL,
			    tg_cgrp(tg), period_us, quota_us, burst_us);

	tg->scx.bw_period_us = period_us;
	tg->scx.bw_quota_us = quota_us;
	tg->scx.bw_burst_us = burst_us;

	percpu_up_read(&scx_cgroup_ops_rwsem);
}
#endif	/* CONFIG_EXT_GROUP_SCHED */

#if defined(CONFIG_EXT_GROUP_SCHED) || defined(CONFIG_EXT_SUB_SCHED)
static struct cgroup *root_cgroup(void)
{
	return &cgrp_dfl_root.cgrp;
}

static void scx_cgroup_lock(void)
{
#ifdef CONFIG_EXT_GROUP_SCHED
	percpu_down_write(&scx_cgroup_ops_rwsem);
#endif
	cgroup_lock();
}

static void scx_cgroup_unlock(void)
{
	cgroup_unlock();
#ifdef CONFIG_EXT_GROUP_SCHED
	percpu_up_write(&scx_cgroup_ops_rwsem);
#endif
}
#else	/* CONFIG_EXT_GROUP_SCHED || CONFIG_EXT_SUB_SCHED */
static struct cgroup *root_cgroup(void) { return NULL; }
static void scx_cgroup_lock(void) {}
static void scx_cgroup_unlock(void) {}
#endif	/* CONFIG_EXT_GROUP_SCHED || CONFIG_EXT_SUB_SCHED */

#ifdef CONFIG_EXT_SUB_SCHED
static struct cgroup *sch_cgroup(struct scx_sched *sch)
{
	return sch->cgrp;
}

/* for each descendant of @cgrp including self, set ->scx_sched to @sch */
static void set_cgroup_sched(struct cgroup *cgrp, struct scx_sched *sch)
{
	struct cgroup *pos;
	struct cgroup_subsys_state *css;

	cgroup_for_each_live_descendant_pre(pos, css, cgrp)
		rcu_assign_pointer(pos->scx_sched, sch);
}
#else	/* CONFIG_EXT_SUB_SCHED */
static struct cgroup *sch_cgroup(struct scx_sched *sch) { return NULL; }
static void set_cgroup_sched(struct cgroup *cgrp, struct scx_sched *sch) {}
#endif	/* CONFIG_EXT_SUB_SCHED */

/*
 * Omitted operations:
 *
 * - migrate_task_rq: Unnecessary as task to cpu mapping is transient.
 *
 * - task_fork/dead: We need fork/dead notifications for all tasks regardless of
 *   their current sched_class. Call them directly from sched core instead.
 */
DEFINE_SCHED_CLASS(ext) = {
	.enqueue_task		= enqueue_task_scx,
	.dequeue_task		= dequeue_task_scx,
	.yield_task		= yield_task_scx,
	.yield_to_task		= yield_to_task_scx,

	.wakeup_preempt		= wakeup_preempt_scx,

	.pick_task		= pick_task_scx,

	.put_prev_task		= put_prev_task_scx,
	.set_next_task		= set_next_task_scx,

	.select_task_rq		= select_task_rq_scx,
	.task_woken		= task_woken_scx,
	.set_cpus_allowed	= set_cpus_allowed_scx,

	.rq_online		= rq_online_scx,
	.rq_offline		= rq_offline_scx,

	.task_tick		= task_tick_scx,

	.switching_to		= switching_to_scx,
	.switched_from		= switched_from_scx,
	.switched_to		= switched_to_scx,
	.reweight_task		= reweight_task_scx,
	.prio_changed		= prio_changed_scx,

	.update_curr		= update_curr_scx,

#ifdef CONFIG_UCLAMP_TASK
	.uclamp_enabled		= 1,
#endif
};

static s32 init_dsq(struct scx_dispatch_q *dsq, u64 dsq_id,
		    struct scx_sched *sch)
{
	s32 cpu;

	memset(dsq, 0, sizeof(*dsq));

	raw_spin_lock_init(&dsq->lock);
	INIT_LIST_HEAD(&dsq->list);
	dsq->id = dsq_id;
	dsq->sched = sch;

	dsq->pcpu = alloc_percpu(struct scx_dsq_pcpu);
	if (!dsq->pcpu)
		return -ENOMEM;

	for_each_possible_cpu(cpu) {
		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu, cpu);

		pcpu->dsq = dsq;
		INIT_LIST_HEAD(&pcpu->deferred_reenq_user.node);
	}

	return 0;
}

static void exit_dsq(struct scx_dispatch_q *dsq)
{
	s32 cpu;

	for_each_possible_cpu(cpu) {
		struct scx_dsq_pcpu *pcpu = per_cpu_ptr(dsq->pcpu, cpu);
		struct scx_deferred_reenq_user *dru = &pcpu->deferred_reenq_user;
		struct rq *rq = cpu_rq(cpu);

		/*
		 * There must have been a RCU grace period since the last
		 * insertion and @dsq should be off the deferred list by now.
		 */
		if (WARN_ON_ONCE(!list_empty(&dru->node))) {
			guard(raw_spinlock_irqsave)(&rq->scx.deferred_reenq_lock);
			list_del_init(&dru->node);
		}
	}

	free_percpu(dsq->pcpu);
}

static void free_dsq_rcufn(struct rcu_head *rcu)
{
	struct scx_dispatch_q *dsq = container_of(rcu, struct scx_dispatch_q, rcu);

	exit_dsq(dsq);
	kfree(dsq);
}

static void free_dsq_irq_workfn(struct irq_work *irq_work)
{
	struct llist_node *to_free = llist_del_all(&dsqs_to_free);
	struct scx_dispatch_q *dsq, *tmp_dsq;

	llist_for_each_entry_safe(dsq, tmp_dsq, to_free, free_node)
		call_rcu(&dsq->rcu, free_dsq_rcufn);
}

static DEFINE_IRQ_WORK(free_dsq_irq_work, free_dsq_irq_workfn);

static void destroy_dsq(struct scx_sched *sch, u64 dsq_id)
{
	struct scx_dispatch_q *dsq;
	unsigned long flags;

	rcu_read_lock();

	dsq = find_user_dsq(sch, dsq_id);
	if (!dsq)
		goto out_unlock_rcu;

	raw_spin_lock_irqsave(&dsq->lock, flags);

	if (dsq->nr) {
		scx_error(sch, "attempting to destroy in-use dsq 0x%016llx (nr=%u)",
			  dsq->id, dsq->nr);
		goto out_unlock_dsq;
	}

	if (rhashtable_remove_fast(&sch->dsq_hash, &dsq->hash_node,
				   dsq_hash_params))
		goto out_unlock_dsq;

	/*
	 * Mark dead by invalidating ->id to prevent dispatch_enqueue() from
	 * queueing more tasks. As this function can be called from anywhere,
	 * freeing is bounced through an irq work to avoid nesting RCU
	 * operations inside scheduler locks.
	 */
	dsq->id = SCX_DSQ_INVALID;
	if (llist_add(&dsq->free_node, &dsqs_to_free))
		irq_work_queue(&free_dsq_irq_work);

out_unlock_dsq:
	raw_spin_unlock_irqrestore(&dsq->lock, flags);
out_unlock_rcu:
	rcu_read_unlock();
}

#ifdef CONFIG_EXT_GROUP_SCHED
static void scx_cgroup_exit(struct scx_sched *sch)
{
	struct cgroup_subsys_state *css;

	scx_cgroup_enabled = false;

	/*
	 * scx_tg_on/offline() are excluded through cgroup_lock(). If we walk
	 * cgroups and exit all the inited ones, all online cgroups are exited.
	 */
	css_for_each_descendant_post(css, &root_task_group.css) {
		struct task_group *tg = css_tg(css);

		if (!(tg->scx.flags & SCX_TG_INITED))
			continue;
		tg->scx.flags &= ~SCX_TG_INITED;

		if (!sch->ops.cgroup_exit)
			continue;

		SCX_CALL_OP(sch, cgroup_exit, NULL, css->cgroup);
	}
}

static int scx_cgroup_init(struct scx_sched *sch)
{
	struct cgroup_subsys_state *css;
	int ret;

	/*
	 * scx_tg_on/offline() are excluded through cgroup_lock(). If we walk
	 * cgroups and init, all online cgroups are initialized.
	 */
	css_for_each_descendant_pre(css, &root_task_group.css) {
		struct task_group *tg = css_tg(css);
		struct scx_cgroup_init_args args = {
			.weight = tg->scx.weight,
			.bw_period_us = tg->scx.bw_period_us,
			.bw_quota_us = tg->scx.bw_quota_us,
			.bw_burst_us = tg->scx.bw_burst_us,
		};

		if ((tg->scx.flags &
		     (SCX_TG_ONLINE | SCX_TG_INITED)) != SCX_TG_ONLINE)
			continue;

		if (!sch->ops.cgroup_init) {
			tg->scx.flags |= SCX_TG_INITED;
			continue;
		}

		ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL,
				      css->cgroup, &args);
		if (ret) {
			scx_error(sch, "ops.cgroup_init() failed (%d)", ret);
			return ret;
		}
		tg->scx.flags |= SCX_TG_INITED;
	}

	WARN_ON_ONCE(scx_cgroup_enabled);
	scx_cgroup_enabled = true;

	return 0;
}

#else
static void scx_cgroup_exit(struct scx_sched *sch) {}
static int scx_cgroup_init(struct scx_sched *sch) { return 0; }
#endif


/********************************************************************************
 * Sysfs interface and ops enable/disable.
 */

#define SCX_ATTR(_name)								\
	static struct kobj_attribute scx_attr_##_name = {			\
		.attr = { .name = __stringify(_name), .mode = 0444 },		\
		.show = scx_attr_##_name##_show,				\
	}

static ssize_t scx_attr_state_show(struct kobject *kobj,
				   struct kobj_attribute *ka, char *buf)
{
	return sysfs_emit(buf, "%s\n", scx_enable_state_str[scx_enable_state()]);
}
SCX_ATTR(state);

static ssize_t scx_attr_switch_all_show(struct kobject *kobj,
					struct kobj_attribute *ka, char *buf)
{
	return sysfs_emit(buf, "%d\n", READ_ONCE(scx_switching_all));
}
SCX_ATTR(switch_all);

static ssize_t scx_attr_nr_rejected_show(struct kobject *kobj,
					 struct kobj_attribute *ka, char *buf)
{
	return sysfs_emit(buf, "%ld\n", atomic_long_read(&scx_nr_rejected));
}
SCX_ATTR(nr_rejected);

static ssize_t scx_attr_hotplug_seq_show(struct kobject *kobj,
					 struct kobj_attribute *ka, char *buf)
{
	return sysfs_emit(buf, "%ld\n", atomic_long_read(&scx_hotplug_seq));
}
SCX_ATTR(hotplug_seq);

static ssize_t scx_attr_enable_seq_show(struct kobject *kobj,
					struct kobj_attribute *ka, char *buf)
{
	return sysfs_emit(buf, "%ld\n", atomic_long_read(&scx_enable_seq));
}
SCX_ATTR(enable_seq);

static struct attribute *scx_global_attrs[] = {
	&scx_attr_state.attr,
	&scx_attr_switch_all.attr,
	&scx_attr_nr_rejected.attr,
	&scx_attr_hotplug_seq.attr,
	&scx_attr_enable_seq.attr,
	NULL,
};

static const struct attribute_group scx_global_attr_group = {
	.attrs = scx_global_attrs,
};

static void free_pnode(struct scx_sched_pnode *pnode);
static void free_exit_info(struct scx_exit_info *ei);

static void scx_sched_free_rcu_work(struct work_struct *work)
{
	struct rcu_work *rcu_work = to_rcu_work(work);
	struct scx_sched *sch = container_of(rcu_work, struct scx_sched, rcu_work);
	struct rhashtable_iter rht_iter;
	struct scx_dispatch_q *dsq;
	int cpu, node;

	irq_work_sync(&sch->disable_irq_work);
	kthread_destroy_worker(sch->helper);
	timer_shutdown_sync(&sch->bypass_lb_timer);
	free_cpumask_var(sch->bypass_lb_donee_cpumask);
	free_cpumask_var(sch->bypass_lb_resched_cpumask);

#ifdef CONFIG_EXT_SUB_SCHED
	kfree(sch->cgrp_path);
	if (sch_cgroup(sch))
		cgroup_put(sch_cgroup(sch));
#endif	/* CONFIG_EXT_SUB_SCHED */

	for_each_possible_cpu(cpu) {
		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);

		/*
		 * $sch would have entered bypass mode before the RCU grace
		 * period. As that blocks new deferrals, all
		 * deferred_reenq_local_node's must be off-list by now.
		 */
		WARN_ON_ONCE(!list_empty(&pcpu->deferred_reenq_local.node));

		exit_dsq(bypass_dsq(sch, cpu));
	}

	free_percpu(sch->pcpu);

	for_each_node_state(node, N_POSSIBLE)
		free_pnode(sch->pnode[node]);
	kfree(sch->pnode);

	rhashtable_walk_enter(&sch->dsq_hash, &rht_iter);
	do {
		rhashtable_walk_start(&rht_iter);

		while (!IS_ERR_OR_NULL((dsq = rhashtable_walk_next(&rht_iter))))
			destroy_dsq(sch, dsq->id);

		rhashtable_walk_stop(&rht_iter);
	} while (dsq == ERR_PTR(-EAGAIN));
	rhashtable_walk_exit(&rht_iter);

	rhashtable_free_and_destroy(&sch->dsq_hash, NULL, NULL);
	free_exit_info(sch->exit_info);
	kfree(sch);
}

static void scx_kobj_release(struct kobject *kobj)
{
	struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);

	INIT_RCU_WORK(&sch->rcu_work, scx_sched_free_rcu_work);
	queue_rcu_work(system_dfl_wq, &sch->rcu_work);
}

static ssize_t scx_attr_ops_show(struct kobject *kobj,
				 struct kobj_attribute *ka, char *buf)
{
	struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);

	return sysfs_emit(buf, "%s\n", sch->ops.name);
}
SCX_ATTR(ops);

#define scx_attr_event_show(buf, at, events, kind) ({				\
	sysfs_emit_at(buf, at, "%s %llu\n", #kind, (events)->kind);		\
})

static ssize_t scx_attr_events_show(struct kobject *kobj,
				    struct kobj_attribute *ka, char *buf)
{
	struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);
	struct scx_event_stats events;
	int at = 0;

	scx_read_events(sch, &events);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_SELECT_CPU_FALLBACK);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_DISPATCH_KEEP_LAST);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_ENQ_SKIP_EXITING);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_REENQ_IMMED);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_REENQ_LOCAL_REPEAT);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_REFILL_SLICE_DFL);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_BYPASS_DURATION);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_BYPASS_DISPATCH);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_BYPASS_ACTIVATE);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_INSERT_NOT_OWNED);
	at += scx_attr_event_show(buf, at, &events, SCX_EV_SUB_BYPASS_DISPATCH);
	return at;
}
SCX_ATTR(events);

static struct attribute *scx_sched_attrs[] = {
	&scx_attr_ops.attr,
	&scx_attr_events.attr,
	NULL,
};
ATTRIBUTE_GROUPS(scx_sched);

static const struct kobj_type scx_ktype = {
	.release = scx_kobj_release,
	.sysfs_ops = &kobj_sysfs_ops,
	.default_groups = scx_sched_groups,
};

static int scx_uevent(const struct kobject *kobj, struct kobj_uevent_env *env)
{
	const struct scx_sched *sch;

	/*
	 * scx_uevent() can be reached by both scx_sched kobjects (scx_ktype)
	 * and sub-scheduler kset kobjects (kset_ktype) through the parent
	 * chain walk. Filter out the latter to avoid invalid casts.
	 */
	if (kobj->ktype != &scx_ktype)
		return 0;

	sch = container_of(kobj, struct scx_sched, kobj);

	return add_uevent_var(env, "SCXOPS=%s", sch->ops.name);
}

static const struct kset_uevent_ops scx_uevent_ops = {
	.uevent = scx_uevent,
};

/*
 * Used by sched_fork() and __setscheduler_prio() to pick the matching
 * sched_class. dl/rt are already handled.
 */
bool task_should_scx(int policy)
{
	if (!scx_enabled() || unlikely(scx_enable_state() == SCX_DISABLING))
		return false;
	if (READ_ONCE(scx_switching_all))
		return true;
	return policy == SCHED_EXT;
}

bool scx_allow_ttwu_queue(const struct task_struct *p)
{
	struct scx_sched *sch;

	if (!scx_enabled())
		return true;

	sch = scx_task_sched(p);
	if (unlikely(!sch))
		return true;

	if (sch->ops.flags & SCX_OPS_ALLOW_QUEUED_WAKEUP)
		return true;

	if (unlikely(p->sched_class != &ext_sched_class))
		return true;

	return false;
}

/**
 * handle_lockup - sched_ext common lockup handler
 * @fmt: format string
 *
 * Called on system stall or lockup condition and initiates abort of sched_ext
 * if enabled, which may resolve the reported lockup.
 *
 * Returns %true if sched_ext is enabled and abort was initiated, which may
 * resolve the lockup. %false if sched_ext is not enabled or abort was already
 * initiated by someone else.
 */
static __printf(1, 2) bool handle_lockup(const char *fmt, ...)
{
	struct scx_sched *sch;
	va_list args;
	bool ret;

	guard(rcu)();

	sch = rcu_dereference(scx_root);
	if (unlikely(!sch))
		return false;

	switch (scx_enable_state()) {
	case SCX_ENABLING:
	case SCX_ENABLED:
		va_start(args, fmt);
		ret = scx_verror(sch, fmt, args);
		va_end(args);
		return ret;
	default:
		return false;
	}
}

/**
 * scx_rcu_cpu_stall - sched_ext RCU CPU stall handler
 *
 * While there are various reasons why RCU CPU stalls can occur on a system
 * that may not be caused by the current BPF scheduler, try kicking out the
 * current scheduler in an attempt to recover the system to a good state before
 * issuing panics.
 *
 * Returns %true if sched_ext is enabled and abort was initiated, which may
 * resolve the reported RCU stall. %false if sched_ext is not enabled or someone
 * else already initiated abort.
 */
bool scx_rcu_cpu_stall(void)
{
	return handle_lockup("RCU CPU stall detected!");
}

/**
 * scx_softlockup - sched_ext softlockup handler
 * @dur_s: number of seconds of CPU stuck due to soft lockup
 *
 * On some multi-socket setups (e.g. 2x Intel 8480c), the BPF scheduler can
 * live-lock the system by making many CPUs target the same DSQ to the point
 * where soft-lockup detection triggers. This function is called from
 * soft-lockup watchdog when the triggering point is close and tries to unjam
 * the system and aborting the BPF scheduler.
 */
void scx_softlockup(u32 dur_s)
{
	if (!handle_lockup("soft lockup - CPU %d stuck for %us", smp_processor_id(), dur_s))
		return;

	printk_deferred(KERN_ERR "sched_ext: Soft lockup - CPU %d stuck for %us, disabling BPF scheduler\n",
			smp_processor_id(), dur_s);
}

/*
 * scx_hardlockup() runs from NMI and eventually calls scx_claim_exit(),
 * which takes scx_sched_lock. scx_sched_lock isn't NMI-safe and grabbing
 * it from NMI context can lead to deadlocks. Defer via irq_work; the
 * disable path runs off irq_work anyway.
 */
static atomic_t scx_hardlockup_cpu = ATOMIC_INIT(-1);

static void scx_hardlockup_irq_workfn(struct irq_work *work)
{
	int cpu = atomic_xchg(&scx_hardlockup_cpu, -1);

	if (cpu >= 0 && handle_lockup("hard lockup - CPU %d", cpu))
		printk_deferred(KERN_ERR "sched_ext: Hard lockup - CPU %d, disabling BPF scheduler\n",
				cpu);
}

static DEFINE_IRQ_WORK(scx_hardlockup_irq_work, scx_hardlockup_irq_workfn);

/**
 * scx_hardlockup - sched_ext hardlockup handler
 *
 * A poorly behaving BPF scheduler can trigger hard lockup by e.g. putting
 * numerous affinitized tasks in a single queue and directing all CPUs at it.
 * Try kicking out the current scheduler in an attempt to recover the system to
 * a good state before taking more drastic actions.
 *
 * Queues an irq_work; the handle_lockup() call happens in IRQ context (see
 * scx_hardlockup_irq_workfn).
 *
 * Returns %true if sched_ext is enabled and the work was queued, %false
 * otherwise.
 */
bool scx_hardlockup(int cpu)
{
	if (!rcu_access_pointer(scx_root))
		return false;

	atomic_cmpxchg(&scx_hardlockup_cpu, -1, cpu);
	irq_work_queue(&scx_hardlockup_irq_work);
	return true;
}

static u32 bypass_lb_cpu(struct scx_sched *sch, s32 donor,
			 struct cpumask *donee_mask, struct cpumask *resched_mask,
			 u32 nr_donor_target, u32 nr_donee_target)
{
	struct rq *donor_rq = cpu_rq(donor);
	struct scx_dispatch_q *donor_dsq = bypass_dsq(sch, donor);
	struct task_struct *p, *n;
	struct scx_dsq_list_node cursor = INIT_DSQ_LIST_CURSOR(cursor, donor_dsq, 0);
	s32 delta = READ_ONCE(donor_dsq->nr) - nr_donor_target;
	u32 nr_balanced = 0, min_delta_us;

	/*
	 * All we want to guarantee is reasonable forward progress. No reason to
	 * fine tune. Assuming every task on @donor_dsq runs their full slice,
	 * consider offloading iff the total queued duration is over the
	 * threshold.
	 */
	min_delta_us = READ_ONCE(scx_bypass_lb_intv_us) / SCX_BYPASS_LB_MIN_DELTA_DIV;
	if (delta < DIV_ROUND_UP(min_delta_us, READ_ONCE(scx_slice_bypass_us)))
		return 0;

	raw_spin_rq_lock_irq(donor_rq);
	raw_spin_lock(&donor_dsq->lock);
	list_add(&cursor.node, &donor_dsq->list);
resume:
	n = container_of(&cursor, struct task_struct, scx.dsq_list);
	n = nldsq_next_task(donor_dsq, n, false);

	while ((p = n)) {
		struct scx_dispatch_q *donee_dsq;
		int donee;

		n = nldsq_next_task(donor_dsq, n, false);

		if (donor_dsq->nr <= nr_donor_target)
			break;

		if (cpumask_empty(donee_mask))
			break;

		/*
		 * If an earlier pass placed @p on @donor_dsq from a different
		 * CPU and the donee hasn't consumed it yet, @p is still on the
		 * previous CPU and task_rq(@p) != @donor_rq. @p can't be moved
		 * without its rq locked. Skip.
		 */
		if (task_rq(p) != donor_rq)
			continue;

		donee = cpumask_any_and_distribute(donee_mask, p->cpus_ptr);
		if (donee >= nr_cpu_ids)
			continue;

		donee_dsq = bypass_dsq(sch, donee);

		/*
		 * $p's rq is not locked but $p's DSQ lock protects its
		 * scheduling properties making this test safe.
		 */
		if (!task_can_run_on_remote_rq(sch, p, cpu_rq(donee), false))
			continue;

		/*
		 * Moving $p from one non-local DSQ to another. The source rq
		 * and DSQ are already locked. Do an abbreviated dequeue and
		 * then perform enqueue without unlocking $donor_dsq.
		 *
		 * We don't want to drop and reacquire the lock on each
		 * iteration as @donor_dsq can be very long and potentially
		 * highly contended. Donee DSQs are less likely to be contended.
		 * The nested locking is safe as only this LB moves tasks
		 * between bypass DSQs.
		 */
		dispatch_dequeue_locked(p, donor_dsq);
		dispatch_enqueue(sch, cpu_rq(donee), donee_dsq, p, SCX_ENQ_NESTED);

		/*
		 * $donee might have been idle and need to be woken up. No need
		 * to be clever. Kick every CPU that receives tasks.
		 */
		cpumask_set_cpu(donee, resched_mask);

		if (READ_ONCE(donee_dsq->nr) >= nr_donee_target)
			cpumask_clear_cpu(donee, donee_mask);

		nr_balanced++;
		if (!(nr_balanced % SCX_BYPASS_LB_BATCH) && n) {
			list_move_tail(&cursor.node, &n->scx.dsq_list.node);
			raw_spin_unlock(&donor_dsq->lock);
			raw_spin_rq_unlock_irq(donor_rq);
			cpu_relax();
			raw_spin_rq_lock_irq(donor_rq);
			raw_spin_lock(&donor_dsq->lock);
			goto resume;
		}
	}

	list_del_init(&cursor.node);
	raw_spin_unlock(&donor_dsq->lock);
	raw_spin_rq_unlock_irq(donor_rq);

	return nr_balanced;
}

static void bypass_lb_node(struct scx_sched *sch, int node)
{
	const struct cpumask *node_mask = cpumask_of_node(node);
	struct cpumask *donee_mask = sch->bypass_lb_donee_cpumask;
	struct cpumask *resched_mask = sch->bypass_lb_resched_cpumask;
	u32 nr_tasks = 0, nr_cpus = 0, nr_balanced = 0;
	u32 nr_target, nr_donor_target;
	u32 before_min = U32_MAX, before_max = 0;
	u32 after_min = U32_MAX, after_max = 0;
	int cpu;

	/* count the target tasks and CPUs */
	for_each_cpu_and(cpu, cpu_online_mask, node_mask) {
		u32 nr = READ_ONCE(bypass_dsq(sch, cpu)->nr);

		nr_tasks += nr;
		nr_cpus++;

		before_min = min(nr, before_min);
		before_max = max(nr, before_max);
	}

	if (!nr_cpus)
		return;

	/*
	 * We don't want CPUs to have more than $nr_donor_target tasks and
	 * balancing to fill donee CPUs upto $nr_target. Once targets are
	 * calculated, find the donee CPUs.
	 */
	nr_target = DIV_ROUND_UP(nr_tasks, nr_cpus);
	nr_donor_target = DIV_ROUND_UP(nr_target * SCX_BYPASS_LB_DONOR_PCT, 100);

	cpumask_clear(donee_mask);
	for_each_cpu_and(cpu, cpu_online_mask, node_mask) {
		if (READ_ONCE(bypass_dsq(sch, cpu)->nr) < nr_target)
			cpumask_set_cpu(cpu, donee_mask);
	}

	/* iterate !donee CPUs and see if they should be offloaded */
	cpumask_clear(resched_mask);
	for_each_cpu_and(cpu, cpu_online_mask, node_mask) {
		if (cpumask_empty(donee_mask))
			break;
		if (cpumask_test_cpu(cpu, donee_mask))
			continue;
		if (READ_ONCE(bypass_dsq(sch, cpu)->nr) <= nr_donor_target)
			continue;

		nr_balanced += bypass_lb_cpu(sch, cpu, donee_mask, resched_mask,
					     nr_donor_target, nr_target);
	}

	for_each_cpu(cpu, resched_mask)
		resched_cpu(cpu);

	for_each_cpu_and(cpu, cpu_online_mask, node_mask) {
		u32 nr = READ_ONCE(bypass_dsq(sch, cpu)->nr);

		after_min = min(nr, after_min);
		after_max = max(nr, after_max);

	}

	trace_sched_ext_bypass_lb(node, nr_cpus, nr_tasks, nr_balanced,
				  before_min, before_max, after_min, after_max);
}

/*
 * In bypass mode, all tasks are put on the per-CPU bypass DSQs. If the machine
 * is over-saturated and the BPF scheduler skewed tasks into few CPUs, some
 * bypass DSQs can be overloaded. If there are enough tasks to saturate other
 * lightly loaded CPUs, such imbalance can lead to very high execution latency
 * on the overloaded CPUs and thus to hung tasks and RCU stalls. To avoid such
 * outcomes, a simple load balancing mechanism is implemented by the following
 * timer which runs periodically while bypass mode is in effect.
 */
static void scx_bypass_lb_timerfn(struct timer_list *timer)
{
	struct scx_sched *sch = container_of(timer, struct scx_sched, bypass_lb_timer);
	int node;
	u32 intv_us;

	if (!bypass_dsp_enabled(sch))
		return;

	for_each_node_with_cpus(node)
		bypass_lb_node(sch, node);

	intv_us = READ_ONCE(scx_bypass_lb_intv_us);
	if (intv_us)
		mod_timer(timer, jiffies + usecs_to_jiffies(intv_us));
}

static bool inc_bypass_depth(struct scx_sched *sch)
{
	lockdep_assert_held(&scx_bypass_lock);

	WARN_ON_ONCE(sch->bypass_depth < 0);
	WRITE_ONCE(sch->bypass_depth, sch->bypass_depth + 1);
	if (sch->bypass_depth != 1)
		return false;

	WRITE_ONCE(sch->slice_dfl, READ_ONCE(scx_slice_bypass_us) * NSEC_PER_USEC);
	sch->bypass_timestamp = ktime_get_ns();
	scx_add_event(sch, SCX_EV_BYPASS_ACTIVATE, 1);
	return true;
}

static bool dec_bypass_depth(struct scx_sched *sch)
{
	lockdep_assert_held(&scx_bypass_lock);

	WARN_ON_ONCE(sch->bypass_depth < 1);
	WRITE_ONCE(sch->bypass_depth, sch->bypass_depth - 1);
	if (sch->bypass_depth != 0)
		return false;

	WRITE_ONCE(sch->slice_dfl, SCX_SLICE_DFL);
	scx_add_event(sch, SCX_EV_BYPASS_DURATION,
		      ktime_get_ns() - sch->bypass_timestamp);
	return true;
}

static void enable_bypass_dsp(struct scx_sched *sch)
{
	struct scx_sched *host = scx_parent(sch) ?: sch;
	u32 intv_us = READ_ONCE(scx_bypass_lb_intv_us);
	s32 ret;

	/*
	 * @sch->bypass_depth transitioning from 0 to 1 triggers enabling.
	 * Shouldn't stagger.
	 */
	if (WARN_ON_ONCE(test_and_set_bit(0, &sch->bypass_dsp_claim)))
		return;

	/*
	 * When a sub-sched bypasses, its tasks are queued on the bypass DSQs of
	 * the nearest non-bypassing ancestor or root. As enable_bypass_dsp() is
	 * called iff @sch is not already bypassed due to an ancestor bypassing,
	 * we can assume that the parent is not bypassing and thus will be the
	 * host of the bypass DSQs.
	 *
	 * While the situation may change in the future, the following
	 * guarantees that the nearest non-bypassing ancestor or root has bypass
	 * dispatch enabled while a descendant is bypassing, which is all that's
	 * required.
	 *
	 * bypass_dsp_enabled() test is used to determine whether to enter the
	 * bypass dispatch handling path from both bypassing and hosting scheds.
	 * Bump enable depth on both @sch and bypass dispatch host.
	 */
	ret = atomic_inc_return(&sch->bypass_dsp_enable_depth);
	WARN_ON_ONCE(ret <= 0);

	if (host != sch) {
		ret = atomic_inc_return(&host->bypass_dsp_enable_depth);
		WARN_ON_ONCE(ret <= 0);
	}

	/*
	 * The LB timer will stop running if bypass dispatch is disabled. Start
	 * after enabling bypass dispatch.
	 */
	if (intv_us && !timer_pending(&host->bypass_lb_timer))
		mod_timer(&host->bypass_lb_timer,
			  jiffies + usecs_to_jiffies(intv_us));
}

/* may be called without holding scx_bypass_lock */
static void disable_bypass_dsp(struct scx_sched *sch)
{
	s32 ret;

	if (!test_and_clear_bit(0, &sch->bypass_dsp_claim))
		return;

	ret = atomic_dec_return(&sch->bypass_dsp_enable_depth);
	WARN_ON_ONCE(ret < 0);

	if (scx_parent(sch)) {
		ret = atomic_dec_return(&scx_parent(sch)->bypass_dsp_enable_depth);
		WARN_ON_ONCE(ret < 0);
	}
}

/**
 * scx_bypass - [Un]bypass scx_ops and guarantee forward progress
 * @sch: sched to bypass
 * @bypass: true for bypass, false for unbypass
 *
 * Bypassing guarantees that all runnable tasks make forward progress without
 * trusting the BPF scheduler. We can't grab any mutexes or rwsems as they might
 * be held by tasks that the BPF scheduler is forgetting to run, which
 * unfortunately also excludes toggling the static branches.
 *
 * Let's work around by overriding a couple ops and modifying behaviors based on
 * the DISABLING state and then cycling the queued tasks through dequeue/enqueue
 * to force global FIFO scheduling.
 *
 * - ops.select_cpu() is ignored and the default select_cpu() is used.
 *
 * - ops.enqueue() is ignored and tasks are queued in simple global FIFO order.
 *   %SCX_OPS_ENQ_LAST is also ignored.
 *
 * - ops.dispatch() is ignored.
 *
 * - balance_one() does not set %SCX_RQ_BAL_KEEP on non-zero slice as slice
 *   can't be trusted. Whenever a tick triggers, the running task is rotated to
 *   the tail of the queue with core_sched_at touched.
 *
 * - pick_next_task() suppresses zero slice warning.
 *
 * - scx_kick_cpu() is disabled to avoid irq_work malfunction during PM
 *   operations.
 *
 * - scx_prio_less() reverts to the default core_sched_at order.
 */
static void scx_bypass(struct scx_sched *sch, bool bypass)
{
	struct scx_sched *pos;
	unsigned long flags;
	int cpu;

	raw_spin_lock_irqsave(&scx_bypass_lock, flags);

	if (bypass) {
		if (!inc_bypass_depth(sch))
			goto unlock;

		enable_bypass_dsp(sch);
	} else {
		if (!dec_bypass_depth(sch))
			goto unlock;
	}

	/*
	 * Bypass state is propagated to all descendants - an scx_sched bypasses
	 * if itself or any of its ancestors are in bypass mode.
	 */
	raw_spin_lock(&scx_sched_lock);
	scx_for_each_descendant_pre(pos, sch) {
		if (pos == sch)
			continue;
		if (bypass)
			inc_bypass_depth(pos);
		else
			dec_bypass_depth(pos);
	}
	raw_spin_unlock(&scx_sched_lock);

	/*
	 * No task property is changing. We just need to make sure all currently
	 * queued tasks are re-queued according to the new scx_bypassing()
	 * state. As an optimization, walk each rq's runnable_list instead of
	 * the scx_tasks list.
	 *
	 * This function can't trust the scheduler and thus can't use
	 * cpus_read_lock(). Walk all possible CPUs instead of online.
	 */
	for_each_possible_cpu(cpu) {
		struct rq *rq = cpu_rq(cpu);
		struct task_struct *p, *n;

		raw_spin_rq_lock(rq);
		raw_spin_lock(&scx_sched_lock);

		scx_for_each_descendant_pre(pos, sch) {
			struct scx_sched_pcpu *pcpu = per_cpu_ptr(pos->pcpu, cpu);

			if (pos->bypass_depth)
				pcpu->flags |= SCX_SCHED_PCPU_BYPASSING;
			else
				pcpu->flags &= ~SCX_SCHED_PCPU_BYPASSING;
		}

		raw_spin_unlock(&scx_sched_lock);

		/*
		 * We need to guarantee that no tasks are on the BPF scheduler
		 * while bypassing. Either we see enabled or the enable path
		 * sees scx_bypassing() before moving tasks to SCX.
		 */
		if (!scx_enabled()) {
			raw_spin_rq_unlock(rq);
			continue;
		}

		/*
		 * The use of list_for_each_entry_safe_reverse() is required
		 * because each task is going to be removed from and added back
		 * to the runnable_list during iteration. Because they're added
		 * to the tail of the list, safe reverse iteration can still
		 * visit all nodes.
		 */
		list_for_each_entry_safe_reverse(p, n, &rq->scx.runnable_list,
						 scx.runnable_node) {
			if (!scx_is_descendant(scx_task_sched(p), sch))
				continue;

			/* cycling deq/enq is enough, see the function comment */
			scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
				/* nothing */ ;
			}
		}

		/* resched to restore ticks and idle state */
		if (cpu_online(cpu) || cpu == smp_processor_id())
			resched_curr(rq);

		raw_spin_rq_unlock(rq);
	}

	/* disarming must come after moving all tasks out of the bypass DSQs */
	if (!bypass)
		disable_bypass_dsp(sch);
unlock:
	raw_spin_unlock_irqrestore(&scx_bypass_lock, flags);
}

static void free_exit_info(struct scx_exit_info *ei)
{
	kvfree(ei->dump);
	kfree(ei->msg);
	kfree(ei->bt);
	kfree(ei);
}

static struct scx_exit_info *alloc_exit_info(size_t exit_dump_len)
{
	struct scx_exit_info *ei;

	ei = kzalloc_obj(*ei);
	if (!ei)
		return NULL;

	ei->bt = kzalloc_objs(ei->bt[0], SCX_EXIT_BT_LEN);
	ei->msg = kzalloc(SCX_EXIT_MSG_LEN, GFP_KERNEL);
	ei->dump = kvzalloc(exit_dump_len, GFP_KERNEL);

	if (!ei->bt || !ei->msg || !ei->dump) {
		free_exit_info(ei);
		return NULL;
	}

	return ei;
}

static const char *scx_exit_reason(enum scx_exit_kind kind)
{
	switch (kind) {
	case SCX_EXIT_UNREG:
		return "unregistered from user space";
	case SCX_EXIT_UNREG_BPF:
		return "unregistered from BPF";
	case SCX_EXIT_UNREG_KERN:
		return "unregistered from the main kernel";
	case SCX_EXIT_SYSRQ:
		return "disabled by sysrq-S";
	case SCX_EXIT_PARENT:
		return "parent exiting";
	case SCX_EXIT_ERROR:
		return "runtime error";
	case SCX_EXIT_ERROR_BPF:
		return "scx_bpf_error";
	case SCX_EXIT_ERROR_STALL:
		return "runnable task stall";
	default:
		return "<UNKNOWN>";
	}
}

static void free_kick_syncs(void)
{
	int cpu;

	for_each_possible_cpu(cpu) {
		struct scx_kick_syncs **ksyncs = per_cpu_ptr(&scx_kick_syncs, cpu);
		struct scx_kick_syncs *to_free;

		to_free = rcu_replace_pointer(*ksyncs, NULL, true);
		if (to_free)
			kvfree_rcu(to_free, rcu);
	}
}

static void refresh_watchdog(void)
{
	struct scx_sched *sch;
	unsigned long intv = ULONG_MAX;

	/* take the shortest timeout and use its half for watchdog interval */
	rcu_read_lock();
	list_for_each_entry_rcu(sch, &scx_sched_all, all)
		intv = max(min(intv, sch->watchdog_timeout / 2), 1);
	rcu_read_unlock();

	WRITE_ONCE(scx_watchdog_timestamp, jiffies);
	WRITE_ONCE(scx_watchdog_interval, intv);

	if (intv < ULONG_MAX)
		mod_delayed_work(system_dfl_wq, &scx_watchdog_work, intv);
	else
		cancel_delayed_work_sync(&scx_watchdog_work);
}

static s32 scx_link_sched(struct scx_sched *sch)
{
	scoped_guard(raw_spinlock_irq, &scx_sched_lock) {
#ifdef CONFIG_EXT_SUB_SCHED
		struct scx_sched *parent = scx_parent(sch);
		s32 ret;

		if (parent) {
			/*
			 * scx_claim_exit() propagates exit_kind transition to
			 * its sub-scheds while holding scx_sched_lock - either
			 * we can see the parent's non-NONE exit_kind or the
			 * parent can shoot us down.
			 */
			if (atomic_read(&parent->exit_kind) != SCX_EXIT_NONE) {
				scx_error(sch, "parent disabled");
				return -ENOENT;
			}

			ret = rhashtable_lookup_insert_fast(&scx_sched_hash,
					&sch->hash_node, scx_sched_hash_params);
			if (ret) {
				scx_error(sch, "failed to insert into scx_sched_hash (%d)", ret);
				return ret;
			}

			list_add_tail(&sch->sibling, &parent->children);
		}
#endif	/* CONFIG_EXT_SUB_SCHED */

		list_add_tail_rcu(&sch->all, &scx_sched_all);
	}

	refresh_watchdog();
	return 0;
}

static void scx_unlink_sched(struct scx_sched *sch)
{
	scoped_guard(raw_spinlock_irq, &scx_sched_lock) {
#ifdef CONFIG_EXT_SUB_SCHED
		if (scx_parent(sch)) {
			rhashtable_remove_fast(&scx_sched_hash, &sch->hash_node,
					       scx_sched_hash_params);
			list_del_init(&sch->sibling);
		}
#endif	/* CONFIG_EXT_SUB_SCHED */
		list_del_rcu(&sch->all);
	}

	refresh_watchdog();
}

/*
 * Called to disable future dumps and wait for in-progress one while disabling
 * @sch. Once @sch becomes empty during disable, there's no point in dumping it.
 * This prevents calling dump ops on a dead sch.
 */
static void scx_disable_dump(struct scx_sched *sch)
{
	guard(raw_spinlock_irqsave)(&scx_dump_lock);
	sch->dump_disabled = true;
}

#ifdef CONFIG_EXT_SUB_SCHED
static DECLARE_WAIT_QUEUE_HEAD(scx_unlink_waitq);

static void drain_descendants(struct scx_sched *sch)
{
	/*
	 * Child scheds that finished the critical part of disabling will take
	 * themselves off @sch->children. Wait for it to drain. As propagation
	 * is recursive, empty @sch->children means that all proper descendant
	 * scheds reached unlinking stage.
	 */
	wait_event(scx_unlink_waitq, list_empty(&sch->children));
}

static void scx_fail_parent(struct scx_sched *sch,
			    struct task_struct *failed, s32 fail_code)
{
	struct scx_sched *parent = scx_parent(sch);
	struct scx_task_iter sti;
	struct task_struct *p;

	scx_error(parent, "ops.init_task() failed (%d) for %s[%d] while disabling a sub-scheduler",
		  fail_code, failed->comm, failed->pid);

	/*
	 * Once $parent is bypassed, it's safe to put SCX_TASK_NONE tasks into
	 * it. This may cause downstream failures on the BPF side but $parent is
	 * dying anyway.
	 */
	scx_bypass(parent, true);

	scx_task_iter_start(&sti, sch->cgrp);
	while ((p = scx_task_iter_next_locked(&sti))) {
		if (scx_task_on_sched(parent, p))
			continue;

		scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
			scx_disable_and_exit_task(sch, p);
			rcu_assign_pointer(p->scx.sched, parent);
		}
	}
	scx_task_iter_stop(&sti);
}

static void scx_sub_disable(struct scx_sched *sch)
{
	struct scx_sched *parent = scx_parent(sch);
	struct scx_task_iter sti;
	struct task_struct *p;
	int ret;

	/*
	 * Guarantee forward progress and wait for descendants to be disabled.
	 * To limit disruptions, $parent is not bypassed. Tasks are fully
	 * prepped and then inserted back into $parent.
	 */
	scx_bypass(sch, true);
	drain_descendants(sch);

	/*
	 * Here, every runnable task is guaranteed to make forward progress and
	 * we can safely use blocking synchronization constructs. Actually
	 * disable ops.
	 */
	mutex_lock(&scx_enable_mutex);
	percpu_down_write(&scx_fork_rwsem);
	scx_cgroup_lock();

	set_cgroup_sched(sch_cgroup(sch), parent);

	scx_task_iter_start(&sti, sch->cgrp);
	while ((p = scx_task_iter_next_locked(&sti))) {
		struct rq *rq;
		struct rq_flags rf;

		/* filter out duplicate visits */
		if (scx_task_on_sched(parent, p))
			continue;

		/*
		 * By the time control reaches here, all descendant schedulers
		 * should already have been disabled.
		 */
		WARN_ON_ONCE(!scx_task_on_sched(sch, p));

		/*
		 * If $p is about to be freed, nothing prevents $sch from
		 * unloading before $p reaches sched_ext_free(). Disable and
		 * exit $p right away.
		 */
		if (!tryget_task_struct(p)) {
			scx_disable_and_exit_task(sch, p);
			continue;
		}

		scx_task_iter_unlock(&sti);

		/*
		 * $p is READY or ENABLED on @sch. Initialize for $parent,
		 * disable and exit from @sch, and then switch over to $parent.
		 *
		 * If a task fails to initialize for $parent, the only available
		 * action is disabling $parent too. While this allows disabling
		 * of a child sched to cause the parent scheduler to fail, the
		 * failure can only originate from ops.init_task() of the
		 * parent. A child can't directly affect the parent through its
		 * own failures.
		 */
		ret = __scx_init_task(parent, p, false);
		if (ret) {
			scx_fail_parent(sch, p, ret);
			put_task_struct(p);
			break;
		}

		rq = task_rq_lock(p, &rf);
		scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
			/*
			 * $p is initialized for $parent and still attached to
			 * @sch. Disable and exit for @sch, switch over to
			 * $parent, override the state to READY to account for
			 * $p having already been initialized, and then enable.
			 */
			scx_disable_and_exit_task(sch, p);
			scx_set_task_state(p, SCX_TASK_INIT);
			rcu_assign_pointer(p->scx.sched, parent);
			scx_set_task_state(p, SCX_TASK_READY);
			scx_enable_task(parent, p);
		}
		task_rq_unlock(rq, p, &rf);

		put_task_struct(p);
	}
	scx_task_iter_stop(&sti);

	scx_disable_dump(sch);

	scx_cgroup_unlock();
	percpu_up_write(&scx_fork_rwsem);

	/*
	 * All tasks are moved off of @sch but there may still be on-going
	 * operations (e.g. ops.select_cpu()). Drain them by flushing RCU. Use
	 * the expedited version as ancestors may be waiting in bypass mode.
	 * Also, tell the parent that there is no need to keep running bypass
	 * DSQs for us.
	 */
	synchronize_rcu_expedited();
	disable_bypass_dsp(sch);

	scx_unlink_sched(sch);

	mutex_unlock(&scx_enable_mutex);

	/*
	 * @sch is now unlinked from the parent's children list. Notify and call
	 * ops.sub_detach/exit(). Note that ops.sub_detach/exit() must be called
	 * after unlinking and releasing all locks. See scx_claim_exit().
	 */
	wake_up_all(&scx_unlink_waitq);

	if (parent->ops.sub_detach && sch->sub_attached) {
		struct scx_sub_detach_args sub_detach_args = {
			.ops = &sch->ops,
			.cgroup_path = sch->cgrp_path,
		};
		SCX_CALL_OP(parent, sub_detach, NULL,
			    &sub_detach_args);
	}

	if (sch->ops.exit)
		SCX_CALL_OP(sch, exit, NULL, sch->exit_info);
	if (sch->sub_kset)
		kset_unregister(sch->sub_kset);
	kobject_del(&sch->kobj);
}
#else	/* CONFIG_EXT_SUB_SCHED */
static void drain_descendants(struct scx_sched *sch) { }
static void scx_sub_disable(struct scx_sched *sch) { }
#endif	/* CONFIG_EXT_SUB_SCHED */

static void scx_root_disable(struct scx_sched *sch)
{
	struct scx_exit_info *ei = sch->exit_info;
	struct scx_task_iter sti;
	struct task_struct *p;
	int cpu;

	/* guarantee forward progress and wait for descendants to be disabled */
	scx_bypass(sch, true);
	drain_descendants(sch);

	switch (scx_set_enable_state(SCX_DISABLING)) {
	case SCX_DISABLING:
		WARN_ONCE(true, "sched_ext: duplicate disabling instance?");
		break;
	case SCX_DISABLED:
		pr_warn("sched_ext: ops error detected without ops (%s)\n",
			sch->exit_info->msg);
		WARN_ON_ONCE(scx_set_enable_state(SCX_DISABLED) != SCX_DISABLING);
		goto done;
	default:
		break;
	}

	/*
	 * Here, every runnable task is guaranteed to make forward progress and
	 * we can safely use blocking synchronization constructs. Actually
	 * disable ops.
	 */
	mutex_lock(&scx_enable_mutex);

	static_branch_disable(&__scx_switched_all);
	WRITE_ONCE(scx_switching_all, false);

	/*
	 * Shut down cgroup support before tasks so that the cgroup attach path
	 * doesn't race against scx_disable_and_exit_task().
	 */
	scx_cgroup_lock();
	scx_cgroup_exit(sch);
	scx_cgroup_unlock();

	/*
	 * The BPF scheduler is going away. All tasks including %TASK_DEAD ones
	 * must be switched out and exited synchronously.
	 */
	percpu_down_write(&scx_fork_rwsem);

	scx_init_task_enabled = false;

	scx_task_iter_start(&sti, NULL);
	while ((p = scx_task_iter_next_locked(&sti))) {
		unsigned int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE | DEQUEUE_NOCLOCK;
		const struct sched_class *old_class = p->sched_class;
		const struct sched_class *new_class = scx_setscheduler_class(p);

		update_rq_clock(task_rq(p));

		if (old_class != new_class)
			queue_flags |= DEQUEUE_CLASS;

		scoped_guard (sched_change, p, queue_flags) {
			p->sched_class = new_class;
		}

		scx_disable_and_exit_task(scx_task_sched(p), p);
	}
	scx_task_iter_stop(&sti);

	scx_disable_dump(sch);

	scx_cgroup_lock();
	set_cgroup_sched(sch_cgroup(sch), NULL);
	scx_cgroup_unlock();

	percpu_up_write(&scx_fork_rwsem);

	/*
	 * Invalidate all the rq clocks to prevent getting outdated
	 * rq clocks from a previous scx scheduler.
	 */
	for_each_possible_cpu(cpu) {
		struct rq *rq = cpu_rq(cpu);
		scx_rq_clock_invalidate(rq);
	}

	/* no task is on scx, turn off all the switches and flush in-progress calls */
	static_branch_disable(&__scx_enabled);
	bitmap_zero(sch->has_op, SCX_OPI_END);
	scx_idle_disable();
	synchronize_rcu();

	if (ei->kind >= SCX_EXIT_ERROR) {
		pr_err("sched_ext: BPF scheduler \"%s\" disabled (%s)\n",
		       sch->ops.name, ei->reason);

		if (ei->msg[0] != '\0')
			pr_err("sched_ext: %s: %s\n", sch->ops.name, ei->msg);
#ifdef CONFIG_STACKTRACE
		stack_trace_print(ei->bt, ei->bt_len, 2);
#endif
	} else {
		pr_info("sched_ext: BPF scheduler \"%s\" disabled (%s)\n",
			sch->ops.name, ei->reason);
	}

	if (sch->ops.exit)
		SCX_CALL_OP(sch, exit, NULL, ei);

	scx_unlink_sched(sch);

	/*
	 * scx_root clearing must be inside cpus_read_lock(). See
	 * handle_hotplug().
	 */
	cpus_read_lock();
	RCU_INIT_POINTER(scx_root, NULL);
	cpus_read_unlock();

	/*
	 * Delete the kobject from the hierarchy synchronously. Otherwise, sysfs
	 * could observe an object of the same name still in the hierarchy when
	 * the next scheduler is loaded.
	 */
#ifdef CONFIG_EXT_SUB_SCHED
	if (sch->sub_kset)
		kset_unregister(sch->sub_kset);
#endif
	kobject_del(&sch->kobj);

	free_kick_syncs();

	mutex_unlock(&scx_enable_mutex);

	WARN_ON_ONCE(scx_set_enable_state(SCX_DISABLED) != SCX_DISABLING);
done:
	scx_bypass(sch, false);
}

/*
 * Claim the exit on @sch. The caller must ensure that the helper kthread work
 * is kicked before the current task can be preempted. Once exit_kind is
 * claimed, scx_error() can no longer trigger, so if the current task gets
 * preempted and the BPF scheduler fails to schedule it back, the helper work
 * will never be kicked and the whole system can wedge.
 */
static bool scx_claim_exit(struct scx_sched *sch, enum scx_exit_kind kind)
{
	int none = SCX_EXIT_NONE;

	lockdep_assert_preemption_disabled();

	if (WARN_ON_ONCE(kind == SCX_EXIT_NONE || kind == SCX_EXIT_DONE))
		kind = SCX_EXIT_ERROR;

	if (!atomic_try_cmpxchg(&sch->exit_kind, &none, kind))
		return false;

	/*
	 * Some CPUs may be trapped in the dispatch paths. Set the aborting
	 * flag to break potential live-lock scenarios, ensuring we can
	 * successfully reach scx_bypass().
	 */
	WRITE_ONCE(sch->aborting, true);

	/*
	 * Propagate exits to descendants immediately. Each has a dedicated
	 * helper kthread and can run in parallel. While most of disabling is
	 * serialized, running them in separate threads allows parallelizing
	 * ops.exit(), which can take arbitrarily long prolonging bypass mode.
	 *
	 * To guarantee forward progress, this propagation must be in-line so
	 * that ->aborting is synchronously asserted for all sub-scheds. The
	 * propagation is also the interlocking point against sub-sched
	 * attachment. See scx_link_sched().
	 *
	 * This doesn't cause recursions as propagation only takes place for
	 * non-propagation exits.
	 */
	if (kind != SCX_EXIT_PARENT) {
		scoped_guard (raw_spinlock_irqsave, &scx_sched_lock) {
			struct scx_sched *pos;
			scx_for_each_descendant_pre(pos, sch)
				scx_disable(pos, SCX_EXIT_PARENT);
		}
	}

	return true;
}

static void scx_disable_workfn(struct kthread_work *work)
{
	struct scx_sched *sch = container_of(work, struct scx_sched, disable_work);
	struct scx_exit_info *ei = sch->exit_info;
	int kind;

	kind = atomic_read(&sch->exit_kind);
	while (true) {
		if (kind == SCX_EXIT_DONE)	/* already disabled? */
			return;
		WARN_ON_ONCE(kind == SCX_EXIT_NONE);
		if (atomic_try_cmpxchg(&sch->exit_kind, &kind, SCX_EXIT_DONE))
			break;
	}
	ei->kind = kind;
	ei->reason = scx_exit_reason(ei->kind);

	if (scx_parent(sch))
		scx_sub_disable(sch);
	else
		scx_root_disable(sch);
}

static void scx_disable(struct scx_sched *sch, enum scx_exit_kind kind)
{
	guard(preempt)();
	if (scx_claim_exit(sch, kind))
		irq_work_queue(&sch->disable_irq_work);
}

/**
 * scx_flush_disable_work - flush the disable work and wait for it to finish
 * @sch: the scheduler
 *
 * sch->disable_work might still not queued, causing kthread_flush_work()
 * as a noop. Syncing the irq_work first is required to guarantee the
 * kthread work has been queued before waiting for it.
 */
static void scx_flush_disable_work(struct scx_sched *sch)
{
	int kind;

	do {
		irq_work_sync(&sch->disable_irq_work);
		kthread_flush_work(&sch->disable_work);
		kind = atomic_read(&sch->exit_kind);
	} while (kind != SCX_EXIT_NONE && kind != SCX_EXIT_DONE);
}

static void dump_newline(struct seq_buf *s)
{
	trace_sched_ext_dump("");

	/* @s may be zero sized and seq_buf triggers WARN if so */
	if (s->size)
		seq_buf_putc(s, '\n');
}

static __printf(2, 3) void dump_line(struct seq_buf *s, const char *fmt, ...)
{
	va_list args;

#ifdef CONFIG_TRACEPOINTS
	if (trace_sched_ext_dump_enabled()) {
		/* protected by scx_dump_lock */
		static char line_buf[SCX_EXIT_MSG_LEN];

		va_start(args, fmt);
		vscnprintf(line_buf, sizeof(line_buf), fmt, args);
		va_end(args);

		trace_call__sched_ext_dump(line_buf);
	}
#endif
	/* @s may be zero sized and seq_buf triggers WARN if so */
	if (s->size) {
		va_start(args, fmt);
		seq_buf_vprintf(s, fmt, args);
		va_end(args);

		seq_buf_putc(s, '\n');
	}
}

static void dump_stack_trace(struct seq_buf *s, const char *prefix,
			     const unsigned long *bt, unsigned int len)
{
	unsigned int i;

	for (i = 0; i < len; i++)
		dump_line(s, "%s%pS", prefix, (void *)bt[i]);
}

static void ops_dump_init(struct seq_buf *s, const char *prefix)
{
	struct scx_dump_data *dd = &scx_dump_data;

	lockdep_assert_irqs_disabled();

	dd->cpu = smp_processor_id();		/* allow scx_bpf_dump() */
	dd->first = true;
	dd->cursor = 0;
	dd->s = s;
	dd->prefix = prefix;
}

static void ops_dump_flush(void)
{
	struct scx_dump_data *dd = &scx_dump_data;
	char *line = dd->buf.line;

	if (!dd->cursor)
		return;

	/*
	 * There's something to flush and this is the first line. Insert a blank
	 * line to distinguish ops dump.
	 */
	if (dd->first) {
		dump_newline(dd->s);
		dd->first = false;
	}

	/*
	 * There may be multiple lines in $line. Scan and emit each line
	 * separately.
	 */
	while (true) {
		char *end = line;
		char c;

		while (*end != '\n' && *end != '\0')
			end++;

		/*
		 * If $line overflowed, it may not have newline at the end.
		 * Always emit with a newline.
		 */
		c = *end;
		*end = '\0';
		dump_line(dd->s, "%s%s", dd->prefix, line);
		if (c == '\0')
			break;

		/* move to the next line */
		end++;
		if (*end == '\0')
			break;
		line = end;
	}

	dd->cursor = 0;
}

static void ops_dump_exit(void)
{
	ops_dump_flush();
	scx_dump_data.cpu = -1;
}

static void scx_dump_task(struct scx_sched *sch, struct seq_buf *s, struct scx_dump_ctx *dctx,
			  struct rq *rq, struct task_struct *p, char marker)
{
	static unsigned long bt[SCX_EXIT_BT_LEN];
	struct scx_sched *task_sch = scx_task_sched(p);
	const char *own_marker;
	char sch_id_buf[32];
	char dsq_id_buf[19] = "(n/a)";
	unsigned long ops_state = atomic_long_read(&p->scx.ops_state);
	unsigned int bt_len = 0;

	own_marker = task_sch == sch ? "*" : "";

	if (task_sch->level == 0)
		scnprintf(sch_id_buf, sizeof(sch_id_buf), "root");
	else
		scnprintf(sch_id_buf, sizeof(sch_id_buf), "sub%d-%llu",
			  task_sch->level, task_sch->ops.sub_cgroup_id);

	if (p->scx.dsq)
		scnprintf(dsq_id_buf, sizeof(dsq_id_buf), "0x%llx",
			  (unsigned long long)p->scx.dsq->id);

	dump_newline(s);
	dump_line(s, " %c%c %s[%d] %s%s %+ldms",
		  marker, task_state_to_char(p), p->comm, p->pid,
		  own_marker, sch_id_buf,
		  jiffies_delta_msecs(p->scx.runnable_at, dctx->at_jiffies));
	dump_line(s, "      scx_state/flags=%u/0x%x dsq_flags=0x%x ops_state/qseq=%lu/%lu",
		  scx_get_task_state(p) >> SCX_TASK_STATE_SHIFT,
		  p->scx.flags & ~SCX_TASK_STATE_MASK,
		  p->scx.dsq_flags, ops_state & SCX_OPSS_STATE_MASK,
		  ops_state >> SCX_OPSS_QSEQ_SHIFT);
	dump_line(s, "      sticky/holding_cpu=%d/%d dsq_id=%s",
		  p->scx.sticky_cpu, p->scx.holding_cpu, dsq_id_buf);
	dump_line(s, "      dsq_vtime=%llu slice=%llu weight=%u",
		  p->scx.dsq_vtime, p->scx.slice, p->scx.weight);
	dump_line(s, "      cpus=%*pb no_mig=%u", cpumask_pr_args(p->cpus_ptr),
		  p->migration_disabled);

	if (SCX_HAS_OP(sch, dump_task)) {
		ops_dump_init(s, "    ");
		SCX_CALL_OP(sch, dump_task, rq, dctx, p);
		ops_dump_exit();
	}

#ifdef CONFIG_STACKTRACE
	bt_len = stack_trace_save_tsk(p, bt, SCX_EXIT_BT_LEN, 1);
#endif
	if (bt_len) {
		dump_newline(s);
		dump_stack_trace(s, "    ", bt, bt_len);
	}
}

/*
 * Dump scheduler state. If @dump_all_tasks is true, dump all tasks regardless
 * of which scheduler they belong to. If false, only dump tasks owned by @sch.
 * For SysRq-D dumps, @dump_all_tasks=false since all schedulers are dumped
 * separately. For error dumps, @dump_all_tasks=true since only the failing
 * scheduler is dumped.
 */
static void scx_dump_state(struct scx_sched *sch, struct scx_exit_info *ei,
			   size_t dump_len, bool dump_all_tasks)
{
	static const char trunc_marker[] = "\n\n~~~~ TRUNCATED ~~~~\n";
	struct scx_dump_ctx dctx = {
		.kind = ei->kind,
		.exit_code = ei->exit_code,
		.reason = ei->reason,
		.at_ns = ktime_get_ns(),
		.at_jiffies = jiffies,
	};
	struct seq_buf s;
	struct scx_event_stats events;
	char *buf;
	int cpu;

	guard(raw_spinlock_irqsave)(&scx_dump_lock);

	if (sch->dump_disabled)
		return;

	seq_buf_init(&s, ei->dump, dump_len);

#ifdef CONFIG_EXT_SUB_SCHED
	if (sch->level == 0)
		dump_line(&s, "%s: root", sch->ops.name);
	else
		dump_line(&s, "%s: sub%d-%llu %s",
			  sch->ops.name, sch->level, sch->ops.sub_cgroup_id,
			  sch->cgrp_path);
#endif
	if (ei->kind == SCX_EXIT_NONE) {
		dump_line(&s, "Debug dump triggered by %s", ei->reason);
	} else {
		dump_line(&s, "%s[%d] triggered exit kind %d:",
			  current->comm, current->pid, ei->kind);
		dump_line(&s, "  %s (%s)", ei->reason, ei->msg);
		dump_newline(&s);
		dump_line(&s, "Backtrace:");
		dump_stack_trace(&s, "  ", ei->bt, ei->bt_len);
	}

	if (SCX_HAS_OP(sch, dump)) {
		ops_dump_init(&s, "");
		SCX_CALL_OP(sch, dump, NULL, &dctx);
		ops_dump_exit();
	}

	dump_newline(&s);
	dump_line(&s, "CPU states");
	dump_line(&s, "----------");

	for_each_possible_cpu(cpu) {
		struct rq *rq = cpu_rq(cpu);
		struct rq_flags rf;
		struct task_struct *p;
		struct seq_buf ns;
		size_t avail, used;
		bool idle;

		rq_lock_irqsave(rq, &rf);

		idle = list_empty(&rq->scx.runnable_list) &&
			rq->curr->sched_class == &idle_sched_class;

		if (idle && !SCX_HAS_OP(sch, dump_cpu))
			goto next;

		/*
		 * We don't yet know whether ops.dump_cpu() will produce output
		 * and we may want to skip the default CPU dump if it doesn't.
		 * Use a nested seq_buf to generate the standard dump so that we
		 * can decide whether to commit later.
		 */
		avail = seq_buf_get_buf(&s, &buf);
		seq_buf_init(&ns, buf, avail);

		dump_newline(&ns);
		dump_line(&ns, "CPU %-4d: nr_run=%u flags=0x%x cpu_rel=%d ops_qseq=%lu ksync=%lu",
			  cpu, rq->scx.nr_running, rq->scx.flags,
			  rq->scx.cpu_released, rq->scx.ops_qseq,
			  rq->scx.kick_sync);
		dump_line(&ns, "          curr=%s[%d] class=%ps",
			  rq->curr->comm, rq->curr->pid,
			  rq->curr->sched_class);
		if (!cpumask_empty(rq->scx.cpus_to_kick))
			dump_line(&ns, "  cpus_to_kick   : %*pb",
				  cpumask_pr_args(rq->scx.cpus_to_kick));
		if (!cpumask_empty(rq->scx.cpus_to_kick_if_idle))
			dump_line(&ns, "  idle_to_kick   : %*pb",
				  cpumask_pr_args(rq->scx.cpus_to_kick_if_idle));
		if (!cpumask_empty(rq->scx.cpus_to_preempt))
			dump_line(&ns, "  cpus_to_preempt: %*pb",
				  cpumask_pr_args(rq->scx.cpus_to_preempt));
		if (!cpumask_empty(rq->scx.cpus_to_wait))
			dump_line(&ns, "  cpus_to_wait   : %*pb",
				  cpumask_pr_args(rq->scx.cpus_to_wait));
		if (!cpumask_empty(rq->scx.cpus_to_sync))
			dump_line(&ns, "  cpus_to_sync   : %*pb",
				  cpumask_pr_args(rq->scx.cpus_to_sync));

		used = seq_buf_used(&ns);
		if (SCX_HAS_OP(sch, dump_cpu)) {
			ops_dump_init(&ns, "  ");
			SCX_CALL_OP(sch, dump_cpu, rq, &dctx, cpu, idle);
			ops_dump_exit();
		}

		/*
		 * If idle && nothing generated by ops.dump_cpu(), there's
		 * nothing interesting. Skip.
		 */
		if (idle && used == seq_buf_used(&ns))
			goto next;

		/*
		 * $s may already have overflowed when $ns was created. If so,
		 * calling commit on it will trigger BUG.
		 */
		if (avail) {
			seq_buf_commit(&s, seq_buf_used(&ns));
			if (seq_buf_has_overflowed(&ns))
				seq_buf_set_overflow(&s);
		}

		if (rq->curr->sched_class == &ext_sched_class &&
		    (dump_all_tasks || scx_task_on_sched(sch, rq->curr)))
			scx_dump_task(sch, &s, &dctx, rq, rq->curr, '*');

		list_for_each_entry(p, &rq->scx.runnable_list, scx.runnable_node)
			if (dump_all_tasks || scx_task_on_sched(sch, p))
				scx_dump_task(sch, &s, &dctx, rq, p, ' ');
	next:
		rq_unlock_irqrestore(rq, &rf);
	}

	dump_newline(&s);
	dump_line(&s, "Event counters");
	dump_line(&s, "--------------");

	scx_read_events(sch, &events);
	scx_dump_event(s, &events, SCX_EV_SELECT_CPU_FALLBACK);
	scx_dump_event(s, &events, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
	scx_dump_event(s, &events, SCX_EV_DISPATCH_KEEP_LAST);
	scx_dump_event(s, &events, SCX_EV_ENQ_SKIP_EXITING);
	scx_dump_event(s, &events, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED);
	scx_dump_event(s, &events, SCX_EV_REENQ_IMMED);
	scx_dump_event(s, &events, SCX_EV_REENQ_LOCAL_REPEAT);
	scx_dump_event(s, &events, SCX_EV_REFILL_SLICE_DFL);
	scx_dump_event(s, &events, SCX_EV_BYPASS_DURATION);
	scx_dump_event(s, &events, SCX_EV_BYPASS_DISPATCH);
	scx_dump_event(s, &events, SCX_EV_BYPASS_ACTIVATE);
	scx_dump_event(s, &events, SCX_EV_INSERT_NOT_OWNED);
	scx_dump_event(s, &events, SCX_EV_SUB_BYPASS_DISPATCH);

	if (seq_buf_has_overflowed(&s) && dump_len >= sizeof(trunc_marker))
		memcpy(ei->dump + dump_len - sizeof(trunc_marker),
		       trunc_marker, sizeof(trunc_marker));
}

static void scx_disable_irq_workfn(struct irq_work *irq_work)
{
	struct scx_sched *sch = container_of(irq_work, struct scx_sched, disable_irq_work);
	struct scx_exit_info *ei = sch->exit_info;

	if (ei->kind >= SCX_EXIT_ERROR)
		scx_dump_state(sch, ei, sch->ops.exit_dump_len, true);

	kthread_queue_work(sch->helper, &sch->disable_work);
}

static bool scx_vexit(struct scx_sched *sch,
		      enum scx_exit_kind kind, s64 exit_code,
		      const char *fmt, va_list args)
{
	struct scx_exit_info *ei = sch->exit_info;

	guard(preempt)();

	if (!scx_claim_exit(sch, kind))
		return false;

	ei->exit_code = exit_code;
#ifdef CONFIG_STACKTRACE
	if (kind >= SCX_EXIT_ERROR)
		ei->bt_len = stack_trace_save(ei->bt, SCX_EXIT_BT_LEN, 1);
#endif
	vscnprintf(ei->msg, SCX_EXIT_MSG_LEN, fmt, args);

	/*
	 * Set ei->kind and ->reason for scx_dump_state(). They'll be set again
	 * in scx_disable_workfn().
	 */
	ei->kind = kind;
	ei->reason = scx_exit_reason(ei->kind);

	irq_work_queue(&sch->disable_irq_work);
	return true;
}

static int alloc_kick_syncs(void)
{
	int cpu;

	/*
	 * Allocate per-CPU arrays sized by nr_cpu_ids. Use kvzalloc as size
	 * can exceed percpu allocator limits on large machines.
	 */
	for_each_possible_cpu(cpu) {
		struct scx_kick_syncs **ksyncs = per_cpu_ptr(&scx_kick_syncs, cpu);
		struct scx_kick_syncs *new_ksyncs;

		WARN_ON_ONCE(rcu_access_pointer(*ksyncs));

		new_ksyncs = kvzalloc_node(struct_size(new_ksyncs, syncs, nr_cpu_ids),
					   GFP_KERNEL, cpu_to_node(cpu));
		if (!new_ksyncs) {
			free_kick_syncs();
			return -ENOMEM;
		}

		rcu_assign_pointer(*ksyncs, new_ksyncs);
	}

	return 0;
}

static void free_pnode(struct scx_sched_pnode *pnode)
{
	if (!pnode)
		return;
	exit_dsq(&pnode->global_dsq);
	kfree(pnode);
}

static struct scx_sched_pnode *alloc_pnode(struct scx_sched *sch, int node)
{
	struct scx_sched_pnode *pnode;

	pnode = kzalloc_node(sizeof(*pnode), GFP_KERNEL, node);
	if (!pnode)
		return NULL;

	if (init_dsq(&pnode->global_dsq, SCX_DSQ_GLOBAL, sch)) {
		kfree(pnode);
		return NULL;
	}

	return pnode;
}

/*
 * Allocate and initialize a new scx_sched. @cgrp's reference is always
 * consumed whether the function succeeds or fails.
 */
static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops,
						 struct cgroup *cgrp,
						 struct scx_sched *parent)
{
	struct scx_sched *sch;
	s32 level = parent ? parent->level + 1 : 0;
	s32 node, cpu, ret, bypass_fail_cpu = nr_cpu_ids;

	sch = kzalloc_flex(*sch, ancestors, level + 1);
	if (!sch) {
		ret = -ENOMEM;
		goto err_put_cgrp;
	}

	sch->exit_info = alloc_exit_info(ops->exit_dump_len);
	if (!sch->exit_info) {
		ret = -ENOMEM;
		goto err_free_sch;
	}

	ret = rhashtable_init(&sch->dsq_hash, &dsq_hash_params);
	if (ret < 0)
		goto err_free_ei;

	sch->pnode = kzalloc_objs(sch->pnode[0], nr_node_ids);
	if (!sch->pnode) {
		ret = -ENOMEM;
		goto err_free_hash;
	}

	for_each_node_state(node, N_POSSIBLE) {
		sch->pnode[node] = alloc_pnode(sch, node);
		if (!sch->pnode[node]) {
			ret = -ENOMEM;
			goto err_free_pnode;
		}
	}

	sch->dsp_max_batch = ops->dispatch_max_batch ?: SCX_DSP_DFL_MAX_BATCH;
	sch->pcpu = __alloc_percpu(struct_size_t(struct scx_sched_pcpu,
						 dsp_ctx.buf, sch->dsp_max_batch),
				   __alignof__(struct scx_sched_pcpu));
	if (!sch->pcpu) {
		ret = -ENOMEM;
		goto err_free_pnode;
	}

	for_each_possible_cpu(cpu) {
		ret = init_dsq(bypass_dsq(sch, cpu), SCX_DSQ_BYPASS, sch);
		if (ret) {
			bypass_fail_cpu = cpu;
			goto err_free_pcpu;
		}
	}

	for_each_possible_cpu(cpu) {
		struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu);

		pcpu->sch = sch;
		INIT_LIST_HEAD(&pcpu->deferred_reenq_local.node);
	}

	sch->helper = kthread_run_worker(0, "sched_ext_helper");
	if (IS_ERR(sch->helper)) {
		ret = PTR_ERR(sch->helper);
		goto err_free_pcpu;
	}

	sched_set_fifo(sch->helper->task);

	if (parent)
		memcpy(sch->ancestors, parent->ancestors,
		       level * sizeof(parent->ancestors[0]));
	sch->ancestors[level] = sch;
	sch->level = level;

	if (ops->timeout_ms)
		sch->watchdog_timeout = msecs_to_jiffies(ops->timeout_ms);
	else
		sch->watchdog_timeout = SCX_WATCHDOG_MAX_TIMEOUT;

	sch->slice_dfl = SCX_SLICE_DFL;
	atomic_set(&sch->exit_kind, SCX_EXIT_NONE);
	init_irq_work(&sch->disable_irq_work, scx_disable_irq_workfn);
	kthread_init_work(&sch->disable_work, scx_disable_workfn);
	timer_setup(&sch->bypass_lb_timer, scx_bypass_lb_timerfn, 0);

	if (!alloc_cpumask_var(&sch->bypass_lb_donee_cpumask, GFP_KERNEL)) {
		ret = -ENOMEM;
		goto err_stop_helper;
	}
	if (!alloc_cpumask_var(&sch->bypass_lb_resched_cpumask, GFP_KERNEL)) {
		ret = -ENOMEM;
		goto err_free_lb_cpumask;
	}
	sch->ops = *ops;
	rcu_assign_pointer(ops->priv, sch);

	sch->kobj.kset = scx_kset;

#ifdef CONFIG_EXT_SUB_SCHED
	char *buf = kzalloc(PATH_MAX, GFP_KERNEL);
	if (!buf) {
		ret = -ENOMEM;
		goto err_free_lb_resched;
	}
	cgroup_path(cgrp, buf, PATH_MAX);
	sch->cgrp_path = kstrdup(buf, GFP_KERNEL);
	kfree(buf);
	if (!sch->cgrp_path) {
		ret = -ENOMEM;
		goto err_free_lb_resched;
	}

	sch->cgrp = cgrp;
	INIT_LIST_HEAD(&sch->children);
	INIT_LIST_HEAD(&sch->sibling);

	if (parent)
		ret = kobject_init_and_add(&sch->kobj, &scx_ktype,
					   &parent->sub_kset->kobj,
					   "sub-%llu", cgroup_id(cgrp));
	else
		ret = kobject_init_and_add(&sch->kobj, &scx_ktype, NULL, "root");

	if (ret < 0) {
		kobject_put(&sch->kobj);
		return ERR_PTR(ret);
	}

	if (ops->sub_attach) {
		sch->sub_kset = kset_create_and_add("sub", NULL, &sch->kobj);
		if (!sch->sub_kset) {
			kobject_put(&sch->kobj);
			return ERR_PTR(-ENOMEM);
		}
	}
#else	/* CONFIG_EXT_SUB_SCHED */
	ret = kobject_init_and_add(&sch->kobj, &scx_ktype, NULL, "root");
	if (ret < 0) {
		kobject_put(&sch->kobj);
		return ERR_PTR(ret);
	}
#endif	/* CONFIG_EXT_SUB_SCHED */
	return sch;

err_free_lb_resched:
	free_cpumask_var(sch->bypass_lb_resched_cpumask);
err_free_lb_cpumask:
	free_cpumask_var(sch->bypass_lb_donee_cpumask);
err_stop_helper:
	kthread_destroy_worker(sch->helper);
err_free_pcpu:
	for_each_possible_cpu(cpu) {
		if (cpu == bypass_fail_cpu)
			break;
		exit_dsq(bypass_dsq(sch, cpu));
	}
	free_percpu(sch->pcpu);
err_free_pnode:
	for_each_node_state(node, N_POSSIBLE)
		free_pnode(sch->pnode[node]);
	kfree(sch->pnode);
err_free_hash:
	rhashtable_free_and_destroy(&sch->dsq_hash, NULL, NULL);
err_free_ei:
	free_exit_info(sch->exit_info);
err_free_sch:
	kfree(sch);
err_put_cgrp:
#ifdef CONFIG_EXT_SUB_SCHED
	cgroup_put(cgrp);
#endif
	return ERR_PTR(ret);
}

static int check_hotplug_seq(struct scx_sched *sch,
			      const struct sched_ext_ops *ops)
{
	unsigned long long global_hotplug_seq;

	/*
	 * If a hotplug event has occurred between when a scheduler was
	 * initialized, and when we were able to attach, exit and notify user
	 * space about it.
	 */
	if (ops->hotplug_seq) {
		global_hotplug_seq = atomic_long_read(&scx_hotplug_seq);
		if (ops->hotplug_seq != global_hotplug_seq) {
			scx_exit(sch, SCX_EXIT_UNREG_KERN,
				 SCX_ECODE_ACT_RESTART | SCX_ECODE_RSN_HOTPLUG,
				 "expected hotplug seq %llu did not match actual %llu",
				 ops->hotplug_seq, global_hotplug_seq);
			return -EBUSY;
		}
	}

	return 0;
}

static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
{
	/*
	 * It doesn't make sense to specify the SCX_OPS_ENQ_LAST flag if the
	 * ops.enqueue() callback isn't implemented.
	 */
	if ((ops->flags & SCX_OPS_ENQ_LAST) && !ops->enqueue) {
		scx_error(sch, "SCX_OPS_ENQ_LAST requires ops.enqueue() to be implemented");
		return -EINVAL;
	}

	/*
	 * SCX_OPS_BUILTIN_IDLE_PER_NODE requires built-in CPU idle
	 * selection policy to be enabled.
	 */
	if ((ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE) &&
	    (ops->update_idle && !(ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE))) {
		scx_error(sch, "SCX_OPS_BUILTIN_IDLE_PER_NODE requires CPU idle selection enabled");
		return -EINVAL;
	}

	if (ops->cpu_acquire || ops->cpu_release)
		pr_warn("ops->cpu_acquire/release() are deprecated, use sched_switch TP instead\n");

	return 0;
}

/*
 * scx_enable() is offloaded to a dedicated system-wide RT kthread to avoid
 * starvation. During the READY -> ENABLED task switching loop, the calling
 * thread's sched_class gets switched from fair to ext. As fair has higher
 * priority than ext, the calling thread can be indefinitely starved under
 * fair-class saturation, leading to a system hang.
 */
struct scx_enable_cmd {
	struct kthread_work	work;
	struct sched_ext_ops	*ops;
	int			ret;
};

static void scx_root_enable_workfn(struct kthread_work *work)
{
	struct scx_enable_cmd *cmd = container_of(work, struct scx_enable_cmd, work);
	struct sched_ext_ops *ops = cmd->ops;
	struct cgroup *cgrp = root_cgroup();
	struct scx_sched *sch;
	struct scx_task_iter sti;
	struct task_struct *p;
	int i, cpu, ret;

	mutex_lock(&scx_enable_mutex);

	if (scx_enable_state() != SCX_DISABLED) {
		ret = -EBUSY;
		goto err_unlock;
	}

	ret = alloc_kick_syncs();
	if (ret)
		goto err_unlock;

#ifdef CONFIG_EXT_SUB_SCHED
	cgroup_get(cgrp);
#endif
	sch = scx_alloc_and_add_sched(ops, cgrp, NULL);
	if (IS_ERR(sch)) {
		ret = PTR_ERR(sch);
		goto err_free_ksyncs;
	}

	/*
	 * Transition to ENABLING and clear exit info to arm the disable path.
	 * Failure triggers full disabling from here on.
	 */
	WARN_ON_ONCE(scx_set_enable_state(SCX_ENABLING) != SCX_DISABLED);
	WARN_ON_ONCE(scx_root);

	atomic_long_set(&scx_nr_rejected, 0);

	for_each_possible_cpu(cpu) {
		struct rq *rq = cpu_rq(cpu);

		rq->scx.local_dsq.sched = sch;
		rq->scx.cpuperf_target = SCX_CPUPERF_ONE;
	}

	/*
	 * Keep CPUs stable during enable so that the BPF scheduler can track
	 * online CPUs by watching ->on/offline_cpu() after ->init().
	 */
	cpus_read_lock();

	/*
	 * Make the scheduler instance visible. Must be inside cpus_read_lock().
	 * See handle_hotplug().
	 */
	rcu_assign_pointer(scx_root, sch);

	ret = scx_link_sched(sch);
	if (ret) {
		cpus_read_unlock();
		goto err_disable;
	}

	scx_idle_enable(ops);

	if (sch->ops.init) {
		ret = SCX_CALL_OP_RET(sch, init, NULL);
		if (ret) {
			ret = ops_sanitize_err(sch, "init", ret);
			cpus_read_unlock();
			scx_error(sch, "ops.init() failed (%d)", ret);
			goto err_disable;
		}
		sch->exit_info->flags |= SCX_EFLAG_INITIALIZED;
	}

	for (i = SCX_OPI_CPU_HOTPLUG_BEGIN; i < SCX_OPI_CPU_HOTPLUG_END; i++)
		if (((void (**)(void))ops)[i])
			set_bit(i, sch->has_op);

	ret = check_hotplug_seq(sch, ops);
	if (ret) {
		cpus_read_unlock();
		goto err_disable;
	}
	scx_idle_update_selcpu_topology(ops);

	cpus_read_unlock();

	ret = validate_ops(sch, ops);
	if (ret)
		goto err_disable;

	/*
	 * Once __scx_enabled is set, %current can be switched to SCX anytime.
	 * This can lead to stalls as some BPF schedulers (e.g. userspace
	 * scheduling) may not function correctly before all tasks are switched.
	 * Init in bypass mode to guarantee forward progress.
	 */
	scx_bypass(sch, true);

	for (i = SCX_OPI_NORMAL_BEGIN; i < SCX_OPI_NORMAL_END; i++)
		if (((void (**)(void))ops)[i])
			set_bit(i, sch->has_op);

	if (sch->ops.cpu_acquire || sch->ops.cpu_release)
		sch->ops.flags |= SCX_OPS_HAS_CPU_PREEMPT;

	/*
	 * Lock out forks, cgroup on/offlining and moves before opening the
	 * floodgate so that they don't wander into the operations prematurely.
	 */
	percpu_down_write(&scx_fork_rwsem);

	WARN_ON_ONCE(scx_init_task_enabled);
	scx_init_task_enabled = true;

	/*
	 * Enable ops for every task. Fork is excluded by scx_fork_rwsem
	 * preventing new tasks from being added. No need to exclude tasks
	 * leaving as sched_ext_free() can handle both prepped and enabled
	 * tasks. Prep all tasks first and then enable them with preemption
	 * disabled.
	 *
	 * All cgroups should be initialized before scx_init_task() so that the
	 * BPF scheduler can reliably track each task's cgroup membership from
	 * scx_init_task(). Lock out cgroup on/offlining and task migrations
	 * while tasks are being initialized so that scx_cgroup_can_attach()
	 * never sees uninitialized tasks.
	 */
	scx_cgroup_lock();
	set_cgroup_sched(sch_cgroup(sch), sch);
	ret = scx_cgroup_init(sch);
	if (ret)
		goto err_disable_unlock_all;

	scx_task_iter_start(&sti, NULL);
	while ((p = scx_task_iter_next_locked(&sti))) {
		/*
		 * @p may already be dead, have lost all its usages counts and
		 * be waiting for RCU grace period before being freed. @p can't
		 * be initialized for SCX in such cases and should be ignored.
		 */
		if (!tryget_task_struct(p))
			continue;

		scx_task_iter_unlock(&sti);

		ret = scx_init_task(sch, p, false);
		if (ret) {
			put_task_struct(p);
			scx_task_iter_stop(&sti);
			scx_error(sch, "ops.init_task() failed (%d) for %s[%d]",
				  ret, p->comm, p->pid);
			goto err_disable_unlock_all;
		}

		scx_set_task_sched(p, sch);
		scx_set_task_state(p, SCX_TASK_READY);

		put_task_struct(p);
	}
	scx_task_iter_stop(&sti);
	scx_cgroup_unlock();
	percpu_up_write(&scx_fork_rwsem);

	/*
	 * All tasks are READY. It's safe to turn on scx_enabled() and switch
	 * all eligible tasks.
	 */
	WRITE_ONCE(scx_switching_all, !(ops->flags & SCX_OPS_SWITCH_PARTIAL));
	static_branch_enable(&__scx_enabled);

	/*
	 * We're fully committed and can't fail. The task READY -> ENABLED
	 * transitions here are synchronized against sched_ext_free() through
	 * scx_tasks_lock.
	 */
	percpu_down_write(&scx_fork_rwsem);
	scx_task_iter_start(&sti, NULL);
	while ((p = scx_task_iter_next_locked(&sti))) {
		unsigned int queue_flags = DEQUEUE_SAVE | DEQUEUE_MOVE;
		const struct sched_class *old_class = p->sched_class;
		const struct sched_class *new_class = scx_setscheduler_class(p);

		if (scx_get_task_state(p) != SCX_TASK_READY)
			continue;

		if (old_class != new_class)
			queue_flags |= DEQUEUE_CLASS;

		scoped_guard (sched_change, p, queue_flags) {
			p->scx.slice = READ_ONCE(sch->slice_dfl);
			p->sched_class = new_class;
		}
	}
	scx_task_iter_stop(&sti);
	percpu_up_write(&scx_fork_rwsem);

	scx_bypass(sch, false);

	if (!scx_tryset_enable_state(SCX_ENABLED, SCX_ENABLING)) {
		WARN_ON_ONCE(atomic_read(&sch->exit_kind) == SCX_EXIT_NONE);
		goto err_disable;
	}

	if (!(ops->flags & SCX_OPS_SWITCH_PARTIAL))
		static_branch_enable(&__scx_switched_all);

	pr_info("sched_ext: BPF scheduler \"%s\" enabled%s\n",
		sch->ops.name, scx_switched_all() ? "" : " (partial)");
	kobject_uevent(&sch->kobj, KOBJ_ADD);
	mutex_unlock(&scx_enable_mutex);

	atomic_long_inc(&scx_enable_seq);

	cmd->ret = 0;
	return;

err_free_ksyncs:
	free_kick_syncs();
err_unlock:
	mutex_unlock(&scx_enable_mutex);
	cmd->ret = ret;
	return;

err_disable_unlock_all:
	scx_cgroup_unlock();
	percpu_up_write(&scx_fork_rwsem);
	/* we'll soon enter disable path, keep bypass on */
err_disable:
	mutex_unlock(&scx_enable_mutex);
	/*
	 * Returning an error code here would not pass all the error information
	 * to userspace. Record errno using scx_error() for cases scx_error()
	 * wasn't already invoked and exit indicating success so that the error
	 * is notified through ops.exit() with all the details.
	 *
	 * Flush scx_disable_work to ensure that error is reported before init
	 * completion. sch's base reference will be put by bpf_scx_unreg().
	 */
	scx_error(sch, "scx_root_enable() failed (%d)", ret);
	scx_flush_disable_work(sch);
	cmd->ret = 0;
}

#ifdef CONFIG_EXT_SUB_SCHED
/* verify that a scheduler can be attached to @cgrp and return the parent */
static struct scx_sched *find_parent_sched(struct cgroup *cgrp)
{
	struct scx_sched *parent = cgrp->scx_sched;
	struct scx_sched *pos;

	lockdep_assert_held(&scx_sched_lock);

	/* can't attach twice to the same cgroup */
	if (parent->cgrp == cgrp)
		return ERR_PTR(-EBUSY);

	/* does $parent allow sub-scheds? */
	if (!parent->ops.sub_attach)
		return ERR_PTR(-EOPNOTSUPP);

	/* can't insert between $parent and its exiting children */
	list_for_each_entry(pos, &parent->children, sibling)
		if (cgroup_is_descendant(pos->cgrp, cgrp))
			return ERR_PTR(-EBUSY);

	return parent;
}

static bool assert_task_ready_or_enabled(struct task_struct *p)
{
	u32 state = scx_get_task_state(p);

	switch (state) {
	case SCX_TASK_READY:
	case SCX_TASK_ENABLED:
		return true;
	default:
		WARN_ONCE(true, "sched_ext: Invalid task state %d for %s[%d] during enabling sub sched",
			  state, p->comm, p->pid);
		return false;
	}
}

static void scx_sub_enable_workfn(struct kthread_work *work)
{
	struct scx_enable_cmd *cmd = container_of(work, struct scx_enable_cmd, work);
	struct sched_ext_ops *ops = cmd->ops;
	struct cgroup *cgrp;
	struct scx_sched *parent, *sch;
	struct scx_task_iter sti;
	struct task_struct *p;
	s32 i, ret;

	mutex_lock(&scx_enable_mutex);

	if (!scx_enabled()) {
		ret = -ENODEV;
		goto out_unlock;
	}

	cgrp = cgroup_get_from_id(ops->sub_cgroup_id);
	if (IS_ERR(cgrp)) {
		ret = PTR_ERR(cgrp);
		goto out_unlock;
	}

	raw_spin_lock_irq(&scx_sched_lock);
	parent = find_parent_sched(cgrp);
	if (IS_ERR(parent)) {
		raw_spin_unlock_irq(&scx_sched_lock);
		ret = PTR_ERR(parent);
		goto out_put_cgrp;
	}
	kobject_get(&parent->kobj);
	raw_spin_unlock_irq(&scx_sched_lock);

	/* scx_alloc_and_add_sched() consumes @cgrp whether it succeeds or not */
	sch = scx_alloc_and_add_sched(ops, cgrp, parent);
	kobject_put(&parent->kobj);
	if (IS_ERR(sch)) {
		ret = PTR_ERR(sch);
		goto out_unlock;
	}

	ret = scx_link_sched(sch);
	if (ret)
		goto err_disable;

	if (sch->level >= SCX_SUB_MAX_DEPTH) {
		scx_error(sch, "max nesting depth %d violated",
			  SCX_SUB_MAX_DEPTH);
		goto err_disable;
	}

	if (sch->ops.init) {
		ret = SCX_CALL_OP_RET(sch, init, NULL);
		if (ret) {
			ret = ops_sanitize_err(sch, "init", ret);
			scx_error(sch, "ops.init() failed (%d)", ret);
			goto err_disable;
		}
		sch->exit_info->flags |= SCX_EFLAG_INITIALIZED;
	}

	if (validate_ops(sch, ops))
		goto err_disable;

	struct scx_sub_attach_args sub_attach_args = {
		.ops = &sch->ops,
		.cgroup_path = sch->cgrp_path,
	};

	ret = SCX_CALL_OP_RET(parent, sub_attach, NULL,
			      &sub_attach_args);
	if (ret) {
		ret = ops_sanitize_err(sch, "sub_attach", ret);
		scx_error(sch, "parent rejected (%d)", ret);
		goto err_disable;
	}
	sch->sub_attached = true;

	scx_bypass(sch, true);

	for (i = SCX_OPI_BEGIN; i < SCX_OPI_END; i++)
		if (((void (**)(void))ops)[i])
			set_bit(i, sch->has_op);

	percpu_down_write(&scx_fork_rwsem);
	scx_cgroup_lock();

	/*
	 * Set cgroup->scx_sched's and check CSS_ONLINE. Either we see
	 * !CSS_ONLINE or scx_cgroup_lifetime_notify() sees and shoots us down.
	 */
	set_cgroup_sched(sch_cgroup(sch), sch);
	if (!(cgrp->self.flags & CSS_ONLINE)) {
		scx_error(sch, "cgroup is not online");
		goto err_unlock_and_disable;
	}

	/*
	 * Initialize tasks for the new child $sch without exiting them for
	 * $parent so that the tasks can always be reverted back to $parent
	 * sched on child init failure.
	 */
	WARN_ON_ONCE(scx_enabling_sub_sched);
	scx_enabling_sub_sched = sch;

	scx_task_iter_start(&sti, sch->cgrp);
	while ((p = scx_task_iter_next_locked(&sti))) {
		struct rq *rq;
		struct rq_flags rf;

		/*
		 * Task iteration may visit the same task twice when racing
		 * against exiting. Use %SCX_TASK_SUB_INIT to mark tasks which
		 * finished __scx_init_task() and skip if set.
		 *
		 * A task may exit and get freed between __scx_init_task()
		 * completion and scx_enable_task(). In such cases,
		 * scx_disable_and_exit_task() must exit the task for both the
		 * parent and child scheds.
		 */
		if (p->scx.flags & SCX_TASK_SUB_INIT)
			continue;

		/* see scx_root_enable() */
		if (!tryget_task_struct(p))
			continue;

		if (!assert_task_ready_or_enabled(p)) {
			ret = -EINVAL;
			goto abort;
		}

		scx_task_iter_unlock(&sti);

		/*
		 * As $p is still on $parent, it can't be transitioned to INIT.
		 * Let's worry about task state later. Use __scx_init_task().
		 */
		ret = __scx_init_task(sch, p, false);
		if (ret)
			goto abort;

		rq = task_rq_lock(p, &rf);
		p->scx.flags |= SCX_TASK_SUB_INIT;
		task_rq_unlock(rq, p, &rf);

		put_task_struct(p);
	}
	scx_task_iter_stop(&sti);

	/*
	 * All tasks are prepped. Disable/exit tasks for $parent and enable for
	 * the new @sch.
	 */
	scx_task_iter_start(&sti, sch->cgrp);
	while ((p = scx_task_iter_next_locked(&sti))) {
		/*
		 * Use clearing of %SCX_TASK_SUB_INIT to detect and skip
		 * duplicate iterations.
		 */
		if (!(p->scx.flags & SCX_TASK_SUB_INIT))
			continue;

		scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
			/*
			 * $p must be either READY or ENABLED. If ENABLED,
			 * __scx_disabled_and_exit_task() first disables and
			 * makes it READY. However, after exiting $p, it will
			 * leave $p as READY.
			 */
			assert_task_ready_or_enabled(p);
			__scx_disable_and_exit_task(parent, p);

			/*
			 * $p is now only initialized for @sch and READY, which
			 * is what we want. Assign it to @sch and enable.
			 */
			rcu_assign_pointer(p->scx.sched, sch);
			scx_enable_task(sch, p);

			p->scx.flags &= ~SCX_TASK_SUB_INIT;
		}
	}
	scx_task_iter_stop(&sti);

	scx_enabling_sub_sched = NULL;

	scx_cgroup_unlock();
	percpu_up_write(&scx_fork_rwsem);

	scx_bypass(sch, false);

	pr_info("sched_ext: BPF sub-scheduler \"%s\" enabled\n", sch->ops.name);
	kobject_uevent(&sch->kobj, KOBJ_ADD);
	ret = 0;
	goto out_unlock;

out_put_cgrp:
	cgroup_put(cgrp);
out_unlock:
	mutex_unlock(&scx_enable_mutex);
	cmd->ret = ret;
	return;

abort:
	put_task_struct(p);
	scx_task_iter_stop(&sti);

	/*
	 * Undo __scx_init_task() for tasks we marked. scx_enable_task() never
	 * ran for @sch on them, so calling scx_disable_task() here would invoke
	 * ops.disable() without a matching ops.enable(). scx_enabling_sub_sched
	 * must stay set until SUB_INIT is cleared from every marked task -
	 * scx_disable_and_exit_task() reads it when a task exits concurrently.
	 */
	scx_task_iter_start(&sti, sch->cgrp);
	while ((p = scx_task_iter_next_locked(&sti))) {
		if (p->scx.flags & SCX_TASK_SUB_INIT) {
			scx_sub_init_cancel_task(sch, p);
			p->scx.flags &= ~SCX_TASK_SUB_INIT;
		}
	}
	scx_task_iter_stop(&sti);
	scx_enabling_sub_sched = NULL;
err_unlock_and_disable:
	/* we'll soon enter disable path, keep bypass on */
	scx_cgroup_unlock();
	percpu_up_write(&scx_fork_rwsem);
err_disable:
	mutex_unlock(&scx_enable_mutex);
	scx_flush_disable_work(sch);
	cmd->ret = 0;
}

static s32 scx_cgroup_lifetime_notify(struct notifier_block *nb,
				      unsigned long action, void *data)
{
	struct cgroup *cgrp = data;
	struct cgroup *parent = cgroup_parent(cgrp);

	if (!cgroup_on_dfl(cgrp))
		return NOTIFY_OK;

	switch (action) {
	case CGROUP_LIFETIME_ONLINE:
		/* inherit ->scx_sched from $parent */
		if (parent)
			rcu_assign_pointer(cgrp->scx_sched, parent->scx_sched);
		break;
	case CGROUP_LIFETIME_OFFLINE:
		/* if there is a sched attached, shoot it down */
		if (cgrp->scx_sched && cgrp->scx_sched->cgrp == cgrp)
			scx_exit(cgrp->scx_sched, SCX_EXIT_UNREG_KERN,
				 SCX_ECODE_RSN_CGROUP_OFFLINE,
				 "cgroup %llu going offline", cgroup_id(cgrp));
		break;
	}

	return NOTIFY_OK;
}

static struct notifier_block scx_cgroup_lifetime_nb = {
	.notifier_call = scx_cgroup_lifetime_notify,
};

static s32 __init scx_cgroup_lifetime_notifier_init(void)
{
	return blocking_notifier_chain_register(&cgroup_lifetime_notifier,
						&scx_cgroup_lifetime_nb);
}
core_initcall(scx_cgroup_lifetime_notifier_init);
#endif	/* CONFIG_EXT_SUB_SCHED */

static s32 scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
{
	static struct kthread_worker *helper;
	static DEFINE_MUTEX(helper_mutex);
	struct scx_enable_cmd cmd;

	if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN),
			   cpu_possible_mask)) {
		pr_err("sched_ext: Not compatible with \"isolcpus=\" domain isolation\n");
		return -EINVAL;
	}

	if (!READ_ONCE(helper)) {
		mutex_lock(&helper_mutex);
		if (!helper) {
			struct kthread_worker *w =
				kthread_run_worker(0, "scx_enable_helper");
			if (IS_ERR_OR_NULL(w)) {
				mutex_unlock(&helper_mutex);
				return -ENOMEM;
			}
			sched_set_fifo(w->task);
			WRITE_ONCE(helper, w);
		}
		mutex_unlock(&helper_mutex);
	}

#ifdef CONFIG_EXT_SUB_SCHED
	if (ops->sub_cgroup_id > 1)
		kthread_init_work(&cmd.work, scx_sub_enable_workfn);
	else
#endif	/* CONFIG_EXT_SUB_SCHED */
		kthread_init_work(&cmd.work, scx_root_enable_workfn);
	cmd.ops = ops;

	kthread_queue_work(READ_ONCE(helper), &cmd.work);
	kthread_flush_work(&cmd.work);
	return cmd.ret;
}


/********************************************************************************
 * bpf_struct_ops plumbing.
 */
#include <linux/bpf_verifier.h>
#include <linux/bpf.h>
#include <linux/btf.h>

static const struct btf_type *task_struct_type;

static bool bpf_scx_is_valid_access(int off, int size,
				    enum bpf_access_type type,
				    const struct bpf_prog *prog,
				    struct bpf_insn_access_aux *info)
{
	if (type != BPF_READ)
		return false;
	if (off < 0 || off >= sizeof(__u64) * MAX_BPF_FUNC_ARGS)
		return false;
	if (off % size != 0)
		return false;

	return btf_ctx_access(off, size, type, prog, info);
}

static int bpf_scx_btf_struct_access(struct bpf_verifier_log *log,
				     const struct bpf_reg_state *reg, int off,
				     int size)
{
	const struct btf_type *t;

	t = btf_type_by_id(reg->btf, reg->btf_id);
	if (t == task_struct_type) {
		/*
		 * COMPAT: Will be removed in v6.23.
		 */
		if ((off >= offsetof(struct task_struct, scx.slice) &&
		     off + size <= offsetofend(struct task_struct, scx.slice)) ||
		    (off >= offsetof(struct task_struct, scx.dsq_vtime) &&
		     off + size <= offsetofend(struct task_struct, scx.dsq_vtime))) {
			pr_warn("sched_ext: Writing directly to p->scx.slice/dsq_vtime is deprecated, use scx_bpf_task_set_slice/dsq_vtime()");
			return SCALAR_VALUE;
		}

		if (off >= offsetof(struct task_struct, scx.disallow) &&
		    off + size <= offsetofend(struct task_struct, scx.disallow))
			return SCALAR_VALUE;
	}

	return -EACCES;
}

static const struct bpf_verifier_ops bpf_scx_verifier_ops = {
	.get_func_proto = bpf_base_func_proto,
	.is_valid_access = bpf_scx_is_valid_access,
	.btf_struct_access = bpf_scx_btf_struct_access,
};

static int bpf_scx_init_member(const struct btf_type *t,
			       const struct btf_member *member,
			       void *kdata, const void *udata)
{
	const struct sched_ext_ops *uops = udata;
	struct sched_ext_ops *ops = kdata;
	u32 moff = __btf_member_bit_offset(t, member) / 8;
	int ret;

	switch (moff) {
	case offsetof(struct sched_ext_ops, dispatch_max_batch):
		if (*(u32 *)(udata + moff) > INT_MAX)
			return -E2BIG;
		ops->dispatch_max_batch = *(u32 *)(udata + moff);
		return 1;
	case offsetof(struct sched_ext_ops, flags):
		if (*(u64 *)(udata + moff) & ~SCX_OPS_ALL_FLAGS)
			return -EINVAL;
		ops->flags = *(u64 *)(udata + moff);
		return 1;
	case offsetof(struct sched_ext_ops, name):
		ret = bpf_obj_name_cpy(ops->name, uops->name,
				       sizeof(ops->name));
		if (ret < 0)
			return ret;
		if (ret == 0)
			return -EINVAL;
		return 1;
	case offsetof(struct sched_ext_ops, timeout_ms):
		if (msecs_to_jiffies(*(u32 *)(udata + moff)) >
		    SCX_WATCHDOG_MAX_TIMEOUT)
			return -E2BIG;
		ops->timeout_ms = *(u32 *)(udata + moff);
		return 1;
	case offsetof(struct sched_ext_ops, exit_dump_len):
		ops->exit_dump_len =
			*(u32 *)(udata + moff) ?: SCX_EXIT_DUMP_DFL_LEN;
		return 1;
	case offsetof(struct sched_ext_ops, hotplug_seq):
		ops->hotplug_seq = *(u64 *)(udata + moff);
		return 1;
#ifdef CONFIG_EXT_SUB_SCHED
	case offsetof(struct sched_ext_ops, sub_cgroup_id):
		ops->sub_cgroup_id = *(u64 *)(udata + moff);
		return 1;
#endif	/* CONFIG_EXT_SUB_SCHED */
	}

	return 0;
}

#ifdef CONFIG_EXT_SUB_SCHED
static void scx_pstack_recursion_on_dispatch(struct bpf_prog *prog)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(prog->aux);
	if (unlikely(!sch))
		return;

	scx_error(sch, "dispatch recursion detected");
}
#endif	/* CONFIG_EXT_SUB_SCHED */

static int bpf_scx_check_member(const struct btf_type *t,
				const struct btf_member *member,
				const struct bpf_prog *prog)
{
	u32 moff = __btf_member_bit_offset(t, member) / 8;

	switch (moff) {
	case offsetof(struct sched_ext_ops, init_task):
#ifdef CONFIG_EXT_GROUP_SCHED
	case offsetof(struct sched_ext_ops, cgroup_init):
	case offsetof(struct sched_ext_ops, cgroup_exit):
	case offsetof(struct sched_ext_ops, cgroup_prep_move):
#endif
	case offsetof(struct sched_ext_ops, cpu_online):
	case offsetof(struct sched_ext_ops, cpu_offline):
	case offsetof(struct sched_ext_ops, init):
	case offsetof(struct sched_ext_ops, exit):
	case offsetof(struct sched_ext_ops, sub_attach):
	case offsetof(struct sched_ext_ops, sub_detach):
		break;
	default:
		if (prog->sleepable)
			return -EINVAL;
	}

#ifdef CONFIG_EXT_SUB_SCHED
	/*
	 * Enable private stack for operations that can nest along the
	 * hierarchy.
	 *
	 * XXX - Ideally, we should only do this for scheds that allow
	 * sub-scheds and sub-scheds themselves but I don't know how to access
	 * struct_ops from here.
	 */
	switch (moff) {
	case offsetof(struct sched_ext_ops, dispatch):
		prog->aux->priv_stack_requested = true;
		prog->aux->recursion_detected = scx_pstack_recursion_on_dispatch;
	}
#endif	/* CONFIG_EXT_SUB_SCHED */

	return 0;
}

static int bpf_scx_reg(void *kdata, struct bpf_link *link)
{
	return scx_enable(kdata, link);
}

static void bpf_scx_unreg(void *kdata, struct bpf_link *link)
{
	struct sched_ext_ops *ops = kdata;
	struct scx_sched *sch = rcu_dereference_protected(ops->priv, true);

	scx_disable(sch, SCX_EXIT_UNREG);
	scx_flush_disable_work(sch);
	RCU_INIT_POINTER(ops->priv, NULL);
	kobject_put(&sch->kobj);
}

static int bpf_scx_init(struct btf *btf)
{
	task_struct_type = btf_type_by_id(btf, btf_tracing_ids[BTF_TRACING_TYPE_TASK]);

	return 0;
}

static int bpf_scx_update(void *kdata, void *old_kdata, struct bpf_link *link)
{
	/*
	 * sched_ext does not support updating the actively-loaded BPF
	 * scheduler, as registering a BPF scheduler can always fail if the
	 * scheduler returns an error code for e.g. ops.init(), ops.init_task(),
	 * etc. Similarly, we can always race with unregistration happening
	 * elsewhere, such as with sysrq.
	 */
	return -EOPNOTSUPP;
}

static int bpf_scx_validate(void *kdata)
{
	return 0;
}

static s32 sched_ext_ops__select_cpu(struct task_struct *p, s32 prev_cpu, u64 wake_flags) { return -EINVAL; }
static void sched_ext_ops__enqueue(struct task_struct *p, u64 enq_flags) {}
static void sched_ext_ops__dequeue(struct task_struct *p, u64 enq_flags) {}
static void sched_ext_ops__dispatch(s32 prev_cpu, struct task_struct *prev__nullable) {}
static void sched_ext_ops__tick(struct task_struct *p) {}
static void sched_ext_ops__runnable(struct task_struct *p, u64 enq_flags) {}
static void sched_ext_ops__running(struct task_struct *p) {}
static void sched_ext_ops__stopping(struct task_struct *p, bool runnable) {}
static void sched_ext_ops__quiescent(struct task_struct *p, u64 deq_flags) {}
static bool sched_ext_ops__yield(struct task_struct *from, struct task_struct *to__nullable) { return false; }
static bool sched_ext_ops__core_sched_before(struct task_struct *a, struct task_struct *b) { return false; }
static void sched_ext_ops__set_weight(struct task_struct *p, u32 weight) {}
static void sched_ext_ops__set_cpumask(struct task_struct *p, const struct cpumask *mask) {}
static void sched_ext_ops__update_idle(s32 cpu, bool idle) {}
static void sched_ext_ops__cpu_acquire(s32 cpu, struct scx_cpu_acquire_args *args) {}
static void sched_ext_ops__cpu_release(s32 cpu, struct scx_cpu_release_args *args) {}
static s32 sched_ext_ops__init_task(struct task_struct *p, struct scx_init_task_args *args) { return -EINVAL; }
static void sched_ext_ops__exit_task(struct task_struct *p, struct scx_exit_task_args *args) {}
static void sched_ext_ops__enable(struct task_struct *p) {}
static void sched_ext_ops__disable(struct task_struct *p) {}
#ifdef CONFIG_EXT_GROUP_SCHED
static s32 sched_ext_ops__cgroup_init(struct cgroup *cgrp, struct scx_cgroup_init_args *args) { return -EINVAL; }
static void sched_ext_ops__cgroup_exit(struct cgroup *cgrp) {}
static s32 sched_ext_ops__cgroup_prep_move(struct task_struct *p, struct cgroup *from, struct cgroup *to) { return -EINVAL; }
static void sched_ext_ops__cgroup_move(struct task_struct *p, struct cgroup *from, struct cgroup *to) {}
static void sched_ext_ops__cgroup_cancel_move(struct task_struct *p, struct cgroup *from, struct cgroup *to) {}
static void sched_ext_ops__cgroup_set_weight(struct cgroup *cgrp, u32 weight) {}
static void sched_ext_ops__cgroup_set_bandwidth(struct cgroup *cgrp, u64 period_us, u64 quota_us, u64 burst_us) {}
static void sched_ext_ops__cgroup_set_idle(struct cgroup *cgrp, bool idle) {}
#endif	/* CONFIG_EXT_GROUP_SCHED */
static s32 sched_ext_ops__sub_attach(struct scx_sub_attach_args *args) { return -EINVAL; }
static void sched_ext_ops__sub_detach(struct scx_sub_detach_args *args) {}
static void sched_ext_ops__cpu_online(s32 cpu) {}
static void sched_ext_ops__cpu_offline(s32 cpu) {}
static s32 sched_ext_ops__init(void) { return -EINVAL; }
static void sched_ext_ops__exit(struct scx_exit_info *info) {}
static void sched_ext_ops__dump(struct scx_dump_ctx *ctx) {}
static void sched_ext_ops__dump_cpu(struct scx_dump_ctx *ctx, s32 cpu, bool idle) {}
static void sched_ext_ops__dump_task(struct scx_dump_ctx *ctx, struct task_struct *p) {}

static struct sched_ext_ops __bpf_ops_sched_ext_ops = {
	.select_cpu		= sched_ext_ops__select_cpu,
	.enqueue		= sched_ext_ops__enqueue,
	.dequeue		= sched_ext_ops__dequeue,
	.dispatch		= sched_ext_ops__dispatch,
	.tick			= sched_ext_ops__tick,
	.runnable		= sched_ext_ops__runnable,
	.running		= sched_ext_ops__running,
	.stopping		= sched_ext_ops__stopping,
	.quiescent		= sched_ext_ops__quiescent,
	.yield			= sched_ext_ops__yield,
	.core_sched_before	= sched_ext_ops__core_sched_before,
	.set_weight		= sched_ext_ops__set_weight,
	.set_cpumask		= sched_ext_ops__set_cpumask,
	.update_idle		= sched_ext_ops__update_idle,
	.cpu_acquire		= sched_ext_ops__cpu_acquire,
	.cpu_release		= sched_ext_ops__cpu_release,
	.init_task		= sched_ext_ops__init_task,
	.exit_task		= sched_ext_ops__exit_task,
	.enable			= sched_ext_ops__enable,
	.disable		= sched_ext_ops__disable,
#ifdef CONFIG_EXT_GROUP_SCHED
	.cgroup_init		= sched_ext_ops__cgroup_init,
	.cgroup_exit		= sched_ext_ops__cgroup_exit,
	.cgroup_prep_move	= sched_ext_ops__cgroup_prep_move,
	.cgroup_move		= sched_ext_ops__cgroup_move,
	.cgroup_cancel_move	= sched_ext_ops__cgroup_cancel_move,
	.cgroup_set_weight	= sched_ext_ops__cgroup_set_weight,
	.cgroup_set_bandwidth	= sched_ext_ops__cgroup_set_bandwidth,
	.cgroup_set_idle	= sched_ext_ops__cgroup_set_idle,
#endif
	.sub_attach		= sched_ext_ops__sub_attach,
	.sub_detach		= sched_ext_ops__sub_detach,
	.cpu_online		= sched_ext_ops__cpu_online,
	.cpu_offline		= sched_ext_ops__cpu_offline,
	.init			= sched_ext_ops__init,
	.exit			= sched_ext_ops__exit,
	.dump			= sched_ext_ops__dump,
	.dump_cpu		= sched_ext_ops__dump_cpu,
	.dump_task		= sched_ext_ops__dump_task,
};

static struct bpf_struct_ops bpf_sched_ext_ops = {
	.verifier_ops = &bpf_scx_verifier_ops,
	.reg = bpf_scx_reg,
	.unreg = bpf_scx_unreg,
	.check_member = bpf_scx_check_member,
	.init_member = bpf_scx_init_member,
	.init = bpf_scx_init,
	.update = bpf_scx_update,
	.validate = bpf_scx_validate,
	.name = "sched_ext_ops",
	.owner = THIS_MODULE,
	.cfi_stubs = &__bpf_ops_sched_ext_ops
};


/********************************************************************************
 * System integration and init.
 */

static void sysrq_handle_sched_ext_reset(u8 key)
{
	struct scx_sched *sch;

	rcu_read_lock();
	sch = rcu_dereference(scx_root);
	if (likely(sch))
		scx_disable(sch, SCX_EXIT_SYSRQ);
	else
		pr_info("sched_ext: BPF schedulers not loaded\n");
	rcu_read_unlock();
}

static const struct sysrq_key_op sysrq_sched_ext_reset_op = {
	.handler	= sysrq_handle_sched_ext_reset,
	.help_msg	= "reset-sched-ext(S)",
	.action_msg	= "Disable sched_ext and revert all tasks to CFS",
	.enable_mask	= SYSRQ_ENABLE_RTNICE,
};

static void sysrq_handle_sched_ext_dump(u8 key)
{
	struct scx_exit_info ei = { .kind = SCX_EXIT_NONE, .reason = "SysRq-D" };
	struct scx_sched *sch;

	list_for_each_entry_rcu(sch, &scx_sched_all, all)
		scx_dump_state(sch, &ei, 0, false);
}

static const struct sysrq_key_op sysrq_sched_ext_dump_op = {
	.handler	= sysrq_handle_sched_ext_dump,
	.help_msg	= "dump-sched-ext(D)",
	.action_msg	= "Trigger sched_ext debug dump",
	.enable_mask	= SYSRQ_ENABLE_RTNICE,
};

static bool can_skip_idle_kick(struct rq *rq)
{
	lockdep_assert_rq_held(rq);

	/*
	 * We can skip idle kicking if @rq is going to go through at least one
	 * full SCX scheduling cycle before going idle. Just checking whether
	 * curr is not idle is insufficient because we could be racing
	 * balance_one() trying to pull the next task from a remote rq, which
	 * may fail, and @rq may become idle afterwards.
	 *
	 * The race window is small and we don't and can't guarantee that @rq is
	 * only kicked while idle anyway. Skip only when sure.
	 */
	return !is_idle_task(rq->curr) && !(rq->scx.flags & SCX_RQ_IN_BALANCE);
}

static bool kick_one_cpu(s32 cpu, struct rq *this_rq, unsigned long *ksyncs)
{
	struct rq *rq = cpu_rq(cpu);
	struct scx_rq *this_scx = &this_rq->scx;
	const struct sched_class *cur_class;
	bool should_wait = false;
	unsigned long flags;

	raw_spin_rq_lock_irqsave(rq, flags);
	cur_class = rq->curr->sched_class;

	/*
	 * During CPU hotplug, a CPU may depend on kicking itself to make
	 * forward progress. Allow kicking self regardless of online state. If
	 * @cpu is running a higher class task, we have no control over @cpu.
	 * Skip kicking.
	 */
	if ((cpu_online(cpu) || cpu == cpu_of(this_rq)) &&
	    !sched_class_above(cur_class, &ext_sched_class)) {
		if (cpumask_test_cpu(cpu, this_scx->cpus_to_preempt)) {
			if (cur_class == &ext_sched_class)
				rq->curr->scx.slice = 0;
			cpumask_clear_cpu(cpu, this_scx->cpus_to_preempt);
		}

		if (cpumask_test_cpu(cpu, this_scx->cpus_to_wait)) {
			if (cur_class == &ext_sched_class) {
				cpumask_set_cpu(cpu, this_scx->cpus_to_sync);
				ksyncs[cpu] = rq->scx.kick_sync;
				should_wait = true;
			}
			cpumask_clear_cpu(cpu, this_scx->cpus_to_wait);
		}

		resched_curr(rq);
	} else {
		cpumask_clear_cpu(cpu, this_scx->cpus_to_preempt);
		cpumask_clear_cpu(cpu, this_scx->cpus_to_wait);
	}

	raw_spin_rq_unlock_irqrestore(rq, flags);

	return should_wait;
}

static void kick_one_cpu_if_idle(s32 cpu, struct rq *this_rq)
{
	struct rq *rq = cpu_rq(cpu);
	unsigned long flags;

	raw_spin_rq_lock_irqsave(rq, flags);

	if (!can_skip_idle_kick(rq) &&
	    (cpu_online(cpu) || cpu == cpu_of(this_rq)))
		resched_curr(rq);

	raw_spin_rq_unlock_irqrestore(rq, flags);
}

static void kick_cpus_irq_workfn(struct irq_work *irq_work)
{
	struct rq *this_rq = this_rq();
	struct scx_rq *this_scx = &this_rq->scx;
	struct scx_kick_syncs __rcu *ksyncs_pcpu = __this_cpu_read(scx_kick_syncs);
	bool should_wait = false;
	unsigned long *ksyncs;
	s32 cpu;

	/* can race with free_kick_syncs() during scheduler disable */
	if (unlikely(!ksyncs_pcpu))
		return;

	ksyncs = rcu_dereference_bh(ksyncs_pcpu)->syncs;

	for_each_cpu(cpu, this_scx->cpus_to_kick) {
		should_wait |= kick_one_cpu(cpu, this_rq, ksyncs);
		cpumask_clear_cpu(cpu, this_scx->cpus_to_kick);
		cpumask_clear_cpu(cpu, this_scx->cpus_to_kick_if_idle);
	}

	for_each_cpu(cpu, this_scx->cpus_to_kick_if_idle) {
		kick_one_cpu_if_idle(cpu, this_rq);
		cpumask_clear_cpu(cpu, this_scx->cpus_to_kick_if_idle);
	}

	/*
	 * Can't wait in hardirq — kick_sync can't advance, deadlocking if
	 * CPUs wait for each other. Defer to kick_sync_wait_bal_cb().
	 */
	if (should_wait) {
		raw_spin_rq_lock(this_rq);
		this_scx->kick_sync_pending = true;
		resched_curr(this_rq);
		raw_spin_rq_unlock(this_rq);
	}
}

/**
 * print_scx_info - print out sched_ext scheduler state
 * @log_lvl: the log level to use when printing
 * @p: target task
 *
 * If a sched_ext scheduler is enabled, print the name and state of the
 * scheduler. If @p is on sched_ext, print further information about the task.
 *
 * This function can be safely called on any task as long as the task_struct
 * itself is accessible. While safe, this function isn't synchronized and may
 * print out mixups or garbages of limited length.
 */
void print_scx_info(const char *log_lvl, struct task_struct *p)
{
	struct scx_sched *sch;
	enum scx_enable_state state = scx_enable_state();
	const char *all = READ_ONCE(scx_switching_all) ? "+all" : "";
	char runnable_at_buf[22] = "?";
	struct sched_class *class;
	unsigned long runnable_at;

	guard(rcu)();

	sch = scx_task_sched_rcu(p);

	if (!sch)
		return;

	/*
	 * Carefully check if the task was running on sched_ext, and then
	 * carefully copy the time it's been runnable, and its state.
	 */
	if (copy_from_kernel_nofault(&class, &p->sched_class, sizeof(class)) ||
	    class != &ext_sched_class) {
		printk("%sSched_ext: %s (%s%s)", log_lvl, sch->ops.name,
		       scx_enable_state_str[state], all);
		return;
	}

	if (!copy_from_kernel_nofault(&runnable_at, &p->scx.runnable_at,
				      sizeof(runnable_at)))
		scnprintf(runnable_at_buf, sizeof(runnable_at_buf), "%+ldms",
			  jiffies_delta_msecs(runnable_at, jiffies));

	/* print everything onto one line to conserve console space */
	printk("%sSched_ext: %s (%s%s), task: runnable_at=%s",
	       log_lvl, sch->ops.name, scx_enable_state_str[state], all,
	       runnable_at_buf);
}

static int scx_pm_handler(struct notifier_block *nb, unsigned long event, void *ptr)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = rcu_dereference(scx_root);
	if (!sch)
		return NOTIFY_OK;

	/*
	 * SCX schedulers often have userspace components which are sometimes
	 * involved in critial scheduling paths. PM operations involve freezing
	 * userspace which can lead to scheduling misbehaviors including stalls.
	 * Let's bypass while PM operations are in progress.
	 */
	switch (event) {
	case PM_HIBERNATION_PREPARE:
	case PM_SUSPEND_PREPARE:
	case PM_RESTORE_PREPARE:
		scx_bypass(sch, true);
		break;
	case PM_POST_HIBERNATION:
	case PM_POST_SUSPEND:
	case PM_POST_RESTORE:
		scx_bypass(sch, false);
		break;
	}

	return NOTIFY_OK;
}

static struct notifier_block scx_pm_notifier = {
	.notifier_call = scx_pm_handler,
};

void __init init_sched_ext_class(void)
{
	s32 cpu, v;

	/*
	 * The following is to prevent the compiler from optimizing out the enum
	 * definitions so that BPF scheduler implementations can use them
	 * through the generated vmlinux.h.
	 */
	WRITE_ONCE(v, SCX_ENQ_WAKEUP | SCX_DEQ_SLEEP | SCX_KICK_PREEMPT |
		   SCX_TG_ONLINE);

	scx_idle_init_masks();

	for_each_possible_cpu(cpu) {
		struct rq *rq = cpu_rq(cpu);
		int  n = cpu_to_node(cpu);

		/* local_dsq's sch will be set during scx_root_enable() */
		BUG_ON(init_dsq(&rq->scx.local_dsq, SCX_DSQ_LOCAL, NULL));

		INIT_LIST_HEAD(&rq->scx.runnable_list);
		INIT_LIST_HEAD(&rq->scx.ddsp_deferred_locals);

		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_kick, GFP_KERNEL, n));
		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_kick_if_idle, GFP_KERNEL, n));
		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_preempt, GFP_KERNEL, n));
		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_wait, GFP_KERNEL, n));
		BUG_ON(!zalloc_cpumask_var_node(&rq->scx.cpus_to_sync, GFP_KERNEL, n));
		raw_spin_lock_init(&rq->scx.deferred_reenq_lock);
		INIT_LIST_HEAD(&rq->scx.deferred_reenq_locals);
		INIT_LIST_HEAD(&rq->scx.deferred_reenq_users);
		rq->scx.deferred_irq_work = IRQ_WORK_INIT_HARD(deferred_irq_workfn);
		rq->scx.kick_cpus_irq_work = IRQ_WORK_INIT_HARD(kick_cpus_irq_workfn);

		if (cpu_online(cpu))
			cpu_rq(cpu)->scx.flags |= SCX_RQ_ONLINE;
	}

	register_sysrq_key('S', &sysrq_sched_ext_reset_op);
	register_sysrq_key('D', &sysrq_sched_ext_dump_op);
	INIT_DELAYED_WORK(&scx_watchdog_work, scx_watchdog_workfn);

#ifdef CONFIG_EXT_SUB_SCHED
	BUG_ON(rhashtable_init(&scx_sched_hash, &scx_sched_hash_params));
#endif	/* CONFIG_EXT_SUB_SCHED */
}


/********************************************************************************
 * Helpers that can be called from the BPF scheduler.
 */
static bool scx_vet_enq_flags(struct scx_sched *sch, u64 dsq_id, u64 *enq_flags)
{
	bool is_local = dsq_id == SCX_DSQ_LOCAL ||
		(dsq_id & SCX_DSQ_LOCAL_ON) == SCX_DSQ_LOCAL_ON;

	if (*enq_flags & SCX_ENQ_IMMED) {
		if (unlikely(!is_local)) {
			scx_error(sch, "SCX_ENQ_IMMED on a non-local DSQ 0x%llx", dsq_id);
			return false;
		}
	} else if ((sch->ops.flags & SCX_OPS_ALWAYS_ENQ_IMMED) && is_local) {
		*enq_flags |= SCX_ENQ_IMMED;
	}

	return true;
}

static bool scx_dsq_insert_preamble(struct scx_sched *sch, struct task_struct *p,
				    u64 dsq_id, u64 *enq_flags)
{
	lockdep_assert_irqs_disabled();

	if (unlikely(!p)) {
		scx_error(sch, "called with NULL task");
		return false;
	}

	if (unlikely(*enq_flags & __SCX_ENQ_INTERNAL_MASK)) {
		scx_error(sch, "invalid enq_flags 0x%llx", *enq_flags);
		return false;
	}

	/* see SCX_EV_INSERT_NOT_OWNED definition */
	if (unlikely(!scx_task_on_sched(sch, p))) {
		__scx_add_event(sch, SCX_EV_INSERT_NOT_OWNED, 1);
		return false;
	}

	if (!scx_vet_enq_flags(sch, dsq_id, enq_flags))
		return false;

	return true;
}

static void scx_dsq_insert_commit(struct scx_sched *sch, struct task_struct *p,
				  u64 dsq_id, u64 enq_flags)
{
	struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;
	struct task_struct *ddsp_task;

	ddsp_task = __this_cpu_read(direct_dispatch_task);
	if (ddsp_task) {
		mark_direct_dispatch(sch, ddsp_task, p, dsq_id, enq_flags);
		return;
	}

	if (unlikely(dspc->cursor >= sch->dsp_max_batch)) {
		scx_error(sch, "dispatch buffer overflow");
		return;
	}

	dspc->buf[dspc->cursor++] = (struct scx_dsp_buf_ent){
		.task = p,
		.qseq = atomic_long_read(&p->scx.ops_state) & SCX_OPSS_QSEQ_MASK,
		.dsq_id = dsq_id,
		.enq_flags = enq_flags,
	};
}

__bpf_kfunc_start_defs();

/**
 * scx_bpf_dsq_insert - Insert a task into the FIFO queue of a DSQ
 * @p: task_struct to insert
 * @dsq_id: DSQ to insert into
 * @slice: duration @p can run for in nsecs, 0 to keep the current value
 * @enq_flags: SCX_ENQ_*
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Insert @p into the FIFO queue of the DSQ identified by @dsq_id. It is safe to
 * call this function spuriously. Can be called from ops.enqueue(),
 * ops.select_cpu(), and ops.dispatch().
 *
 * When called from ops.select_cpu() or ops.enqueue(), it's for direct dispatch
 * and @p must match the task being enqueued.
 *
 * When called from ops.select_cpu(), @enq_flags and @dsp_id are stored, and @p
 * will be directly inserted into the corresponding dispatch queue after
 * ops.select_cpu() returns. If @p is inserted into SCX_DSQ_LOCAL, it will be
 * inserted into the local DSQ of the CPU returned by ops.select_cpu().
 * @enq_flags are OR'd with the enqueue flags on the enqueue path before the
 * task is inserted.
 *
 * When called from ops.dispatch(), there are no restrictions on @p or @dsq_id
 * and this function can be called upto ops.dispatch_max_batch times to insert
 * multiple tasks. scx_bpf_dispatch_nr_slots() returns the number of the
 * remaining slots. scx_bpf_dsq_move_to_local() flushes the batch and resets the
 * counter.
 *
 * This function doesn't have any locking restrictions and may be called under
 * BPF locks (in the future when BPF introduces more flexible locking).
 *
 * @p is allowed to run for @slice. The scheduling path is triggered on slice
 * exhaustion. If zero, the current residual slice is maintained. If
 * %SCX_SLICE_INF, @p never expires and the BPF scheduler must kick the CPU with
 * scx_bpf_kick_cpu() to trigger scheduling.
 *
 * Returns %true on successful insertion, %false on failure. On the root
 * scheduler, %false return triggers scheduler abort and the caller doesn't need
 * to check the return value.
 */
__bpf_kfunc bool scx_bpf_dsq_insert___v2(struct task_struct *p, u64 dsq_id,
					 u64 slice, u64 enq_flags,
					 const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return false;

	if (!scx_dsq_insert_preamble(sch, p, dsq_id, &enq_flags))
		return false;

	if (slice)
		p->scx.slice = slice;
	else
		p->scx.slice = p->scx.slice ?: 1;

	scx_dsq_insert_commit(sch, p, dsq_id, enq_flags);

	return true;
}

/*
 * COMPAT: Will be removed in v6.23 along with the ___v2 suffix.
 */
__bpf_kfunc void scx_bpf_dsq_insert(struct task_struct *p, u64 dsq_id,
				    u64 slice, u64 enq_flags,
				    const struct bpf_prog_aux *aux)
{
	scx_bpf_dsq_insert___v2(p, dsq_id, slice, enq_flags, aux);
}

static bool scx_dsq_insert_vtime(struct scx_sched *sch, struct task_struct *p,
				 u64 dsq_id, u64 slice, u64 vtime, u64 enq_flags)
{
	if (!scx_dsq_insert_preamble(sch, p, dsq_id, &enq_flags))
		return false;

	if (slice)
		p->scx.slice = slice;
	else
		p->scx.slice = p->scx.slice ?: 1;

	p->scx.dsq_vtime = vtime;

	scx_dsq_insert_commit(sch, p, dsq_id, enq_flags | SCX_ENQ_DSQ_PRIQ);

	return true;
}

struct scx_bpf_dsq_insert_vtime_args {
	/* @p can't be packed together as KF_RCU is not transitive */
	u64			dsq_id;
	u64			slice;
	u64			vtime;
	u64			enq_flags;
};

/**
 * __scx_bpf_dsq_insert_vtime - Arg-wrapped vtime DSQ insertion
 * @p: task_struct to insert
 * @args: struct containing the rest of the arguments
 *       @args->dsq_id: DSQ to insert into
 *       @args->slice: duration @p can run for in nsecs, 0 to keep the current value
 *       @args->vtime: @p's ordering inside the vtime-sorted queue of the target DSQ
 *       @args->enq_flags: SCX_ENQ_*
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Wrapper kfunc that takes arguments via struct to work around BPF's 5 argument
 * limit. BPF programs should use scx_bpf_dsq_insert_vtime() which is provided
 * as an inline wrapper in common.bpf.h.
 *
 * Insert @p into the vtime priority queue of the DSQ identified by
 * @args->dsq_id. Tasks queued into the priority queue are ordered by
 * @args->vtime. All other aspects are identical to scx_bpf_dsq_insert().
 *
 * @args->vtime ordering is according to time_before64() which considers
 * wrapping. A numerically larger vtime may indicate an earlier position in the
 * ordering and vice-versa.
 *
 * A DSQ can only be used as a FIFO or priority queue at any given time and this
 * function must not be called on a DSQ which already has one or more FIFO tasks
 * queued and vice-versa. Also, the built-in DSQs (SCX_DSQ_LOCAL and
 * SCX_DSQ_GLOBAL) cannot be used as priority queues.
 *
 * Returns %true on successful insertion, %false on failure. On the root
 * scheduler, %false return triggers scheduler abort and the caller doesn't need
 * to check the return value.
 */
__bpf_kfunc bool
__scx_bpf_dsq_insert_vtime(struct task_struct *p,
			   struct scx_bpf_dsq_insert_vtime_args *args,
			   const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return false;

	return scx_dsq_insert_vtime(sch, p, args->dsq_id, args->slice,
				    args->vtime, args->enq_flags);
}

/*
 * COMPAT: Will be removed in v6.23.
 */
__bpf_kfunc void scx_bpf_dsq_insert_vtime(struct task_struct *p, u64 dsq_id,
					  u64 slice, u64 vtime, u64 enq_flags)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = rcu_dereference(scx_root);
	if (unlikely(!sch))
		return;

#ifdef CONFIG_EXT_SUB_SCHED
	/*
	 * Disallow if any sub-scheds are attached. There is no way to tell
	 * which scheduler called us, just error out @p's scheduler.
	 */
	if (unlikely(!list_empty(&sch->children))) {
		scx_error(scx_task_sched(p), "__scx_bpf_dsq_insert_vtime() must be used");
		return;
	}
#endif

	scx_dsq_insert_vtime(sch, p, dsq_id, slice, vtime, enq_flags);
}

__bpf_kfunc_end_defs();

BTF_KFUNCS_START(scx_kfunc_ids_enqueue_dispatch)
BTF_ID_FLAGS(func, scx_bpf_dsq_insert, KF_IMPLICIT_ARGS | KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_insert___v2, KF_IMPLICIT_ARGS | KF_RCU)
BTF_ID_FLAGS(func, __scx_bpf_dsq_insert_vtime, KF_IMPLICIT_ARGS | KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_insert_vtime, KF_RCU)
BTF_KFUNCS_END(scx_kfunc_ids_enqueue_dispatch)

static const struct btf_kfunc_id_set scx_kfunc_set_enqueue_dispatch = {
	.owner			= THIS_MODULE,
	.set			= &scx_kfunc_ids_enqueue_dispatch,
	.filter			= scx_kfunc_context_filter,
};

static bool scx_dsq_move(struct bpf_iter_scx_dsq_kern *kit,
			 struct task_struct *p, u64 dsq_id, u64 enq_flags)
{
	struct scx_dispatch_q *src_dsq = kit->dsq, *dst_dsq;
	struct scx_sched *sch;
	struct rq *this_rq, *src_rq, *locked_rq;
	bool dispatched = false;
	bool in_balance;
	unsigned long flags;

	/*
	 * The verifier considers an iterator slot initialized on any
	 * KF_ITER_NEW return, so a BPF program may legally reach here after
	 * bpf_iter_scx_dsq_new() failed and left @kit->dsq NULL.
	 */
	if (unlikely(!src_dsq))
		return false;

	sch = src_dsq->sched;

	if (!scx_vet_enq_flags(sch, dsq_id, &enq_flags))
		return false;

	/*
	 * If the BPF scheduler keeps calling this function repeatedly, it can
	 * cause similar live-lock conditions as consume_dispatch_q().
	 */
	if (unlikely(READ_ONCE(sch->aborting)))
		return false;

	if (unlikely(!scx_task_on_sched(sch, p))) {
		scx_error(sch, "scx_bpf_dsq_move[_vtime]() on %s[%d] but the task belongs to a different scheduler",
			  p->comm, p->pid);
		return false;
	}

	/*
	 * Can be called from either ops.dispatch() locking this_rq() or any
	 * context where no rq lock is held. If latter, lock @p's task_rq which
	 * we'll likely need anyway.
	 */
	src_rq = task_rq(p);

	local_irq_save(flags);
	this_rq = this_rq();
	in_balance = this_rq->scx.flags & SCX_RQ_IN_BALANCE;

	if (in_balance) {
		if (this_rq != src_rq) {
			raw_spin_rq_unlock(this_rq);
			raw_spin_rq_lock(src_rq);
		}
	} else {
		raw_spin_rq_lock(src_rq);
	}

	locked_rq = src_rq;
	raw_spin_lock(&src_dsq->lock);

	/* did someone else get to it while we dropped the locks? */
	if (nldsq_cursor_lost_task(&kit->cursor, src_rq, src_dsq, p)) {
		raw_spin_unlock(&src_dsq->lock);
		goto out;
	}

	/* @p is still on $src_dsq and stable, determine the destination */
	dst_dsq = find_dsq_for_dispatch(sch, this_rq, dsq_id, task_cpu(p));

	/*
	 * Apply vtime and slice updates before moving so that the new time is
	 * visible before inserting into $dst_dsq. @p is still on $src_dsq but
	 * this is safe as we're locking it.
	 */
	if (kit->cursor.flags & __SCX_DSQ_ITER_HAS_VTIME)
		p->scx.dsq_vtime = kit->vtime;
	if (kit->cursor.flags & __SCX_DSQ_ITER_HAS_SLICE)
		p->scx.slice = kit->slice;

	/* execute move */
	locked_rq = move_task_between_dsqs(sch, p, enq_flags, src_dsq, dst_dsq);
	dispatched = true;
out:
	if (in_balance) {
		if (this_rq != locked_rq) {
			raw_spin_rq_unlock(locked_rq);
			raw_spin_rq_lock(this_rq);
		}
	} else {
		raw_spin_rq_unlock_irqrestore(locked_rq, flags);
	}

	kit->cursor.flags &= ~(__SCX_DSQ_ITER_HAS_SLICE |
			       __SCX_DSQ_ITER_HAS_VTIME);
	return dispatched;
}

__bpf_kfunc_start_defs();

/**
 * scx_bpf_dispatch_nr_slots - Return the number of remaining dispatch slots
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Can only be called from ops.dispatch().
 */
__bpf_kfunc u32 scx_bpf_dispatch_nr_slots(const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return 0;

	return sch->dsp_max_batch - __this_cpu_read(sch->pcpu->dsp_ctx.cursor);
}

/**
 * scx_bpf_dispatch_cancel - Cancel the latest dispatch
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Cancel the latest dispatch. Can be called multiple times to cancel further
 * dispatches. Can only be called from ops.dispatch().
 */
__bpf_kfunc void scx_bpf_dispatch_cancel(const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct scx_dsp_ctx *dspc;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return;

	dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;

	if (dspc->cursor > 0)
		dspc->cursor--;
	else
		scx_error(sch, "dispatch buffer underflow");
}

/**
 * scx_bpf_dsq_move_to_local - move a task from a DSQ to the current CPU's local DSQ
 * @dsq_id: DSQ to move task from. Must be a user-created DSQ
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 * @enq_flags: %SCX_ENQ_*
 *
 * Move a task from the non-local DSQ identified by @dsq_id to the current CPU's
 * local DSQ for execution with @enq_flags applied. Can only be called from
 * ops.dispatch().
 *
 * Built-in DSQs (%SCX_DSQ_GLOBAL and %SCX_DSQ_LOCAL*) are not supported as
 * sources. Local DSQs support reenqueueing (a task can be picked up for
 * execution, dequeued for property changes, or reenqueued), but the BPF
 * scheduler cannot directly iterate or move tasks from them. %SCX_DSQ_GLOBAL
 * is similar but also doesn't support reenqueueing, as it maps to multiple
 * per-node DSQs making the scope difficult to define; this may change in the
 * future.
 *
 * This function flushes the in-flight dispatches from scx_bpf_dsq_insert()
 * before trying to move from the specified DSQ. It may also grab rq locks and
 * thus can't be called under any BPF locks.
 *
 * Returns %true if a task has been moved, %false if there isn't any task to
 * move.
 */
__bpf_kfunc bool scx_bpf_dsq_move_to_local___v2(u64 dsq_id, u64 enq_flags,
						const struct bpf_prog_aux *aux)
{
	struct scx_dispatch_q *dsq;
	struct scx_sched *sch;
	struct scx_dsp_ctx *dspc;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return false;

	if (!scx_vet_enq_flags(sch, SCX_DSQ_LOCAL, &enq_flags))
		return false;

	dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx;

	flush_dispatch_buf(sch, dspc->rq);

	dsq = find_user_dsq(sch, dsq_id);
	if (unlikely(!dsq)) {
		scx_error(sch, "invalid DSQ ID 0x%016llx", dsq_id);
		return false;
	}

	if (consume_dispatch_q(sch, dspc->rq, dsq, enq_flags)) {
		/*
		 * A successfully consumed task can be dequeued before it starts
		 * running while the CPU is trying to migrate other dispatched
		 * tasks. Bump nr_tasks to tell balance_one() to retry on empty
		 * local DSQ.
		 */
		dspc->nr_tasks++;
		return true;
	} else {
		return false;
	}
}

/*
 * COMPAT: ___v2 was introduced in v7.1. Remove this and ___v2 tag in the future.
 */
__bpf_kfunc bool scx_bpf_dsq_move_to_local(u64 dsq_id, const struct bpf_prog_aux *aux)
{
	return scx_bpf_dsq_move_to_local___v2(dsq_id, 0, aux);
}

/**
 * scx_bpf_dsq_move_set_slice - Override slice when moving between DSQs
 * @it__iter: DSQ iterator in progress
 * @slice: duration the moved task can run for in nsecs
 *
 * Override the slice of the next task that will be moved from @it__iter using
 * scx_bpf_dsq_move[_vtime](). If this function is not called, the previous
 * slice duration is kept.
 */
__bpf_kfunc void scx_bpf_dsq_move_set_slice(struct bpf_iter_scx_dsq *it__iter,
					    u64 slice)
{
	struct bpf_iter_scx_dsq_kern *kit = (void *)it__iter;

	kit->slice = slice;
	kit->cursor.flags |= __SCX_DSQ_ITER_HAS_SLICE;
}

/**
 * scx_bpf_dsq_move_set_vtime - Override vtime when moving between DSQs
 * @it__iter: DSQ iterator in progress
 * @vtime: task's ordering inside the vtime-sorted queue of the target DSQ
 *
 * Override the vtime of the next task that will be moved from @it__iter using
 * scx_bpf_dsq_move_vtime(). If this function is not called, the previous slice
 * vtime is kept. If scx_bpf_dsq_move() is used to dispatch the next task, the
 * override is ignored and cleared.
 */
__bpf_kfunc void scx_bpf_dsq_move_set_vtime(struct bpf_iter_scx_dsq *it__iter,
					    u64 vtime)
{
	struct bpf_iter_scx_dsq_kern *kit = (void *)it__iter;

	kit->vtime = vtime;
	kit->cursor.flags |= __SCX_DSQ_ITER_HAS_VTIME;
}

/**
 * scx_bpf_dsq_move - Move a task from DSQ iteration to a DSQ
 * @it__iter: DSQ iterator in progress
 * @p: task to transfer
 * @dsq_id: DSQ to move @p to
 * @enq_flags: SCX_ENQ_*
 *
 * Transfer @p which is on the DSQ currently iterated by @it__iter to the DSQ
 * specified by @dsq_id. All DSQs - local DSQs, global DSQ and user DSQs - can
 * be the destination.
 *
 * For the transfer to be successful, @p must still be on the DSQ and have been
 * queued before the DSQ iteration started. This function doesn't care whether
 * @p was obtained from the DSQ iteration. @p just has to be on the DSQ and have
 * been queued before the iteration started.
 *
 * @p's slice is kept by default. Use scx_bpf_dsq_move_set_slice() to update.
 *
 * Can be called from ops.dispatch() or any BPF context which doesn't hold a rq
 * lock (e.g. BPF timers or SYSCALL programs).
 *
 * Returns %true if @p has been consumed, %false if @p had already been
 * consumed, dequeued, or, for sub-scheds, @dsq_id points to a disallowed local
 * DSQ.
 */
__bpf_kfunc bool scx_bpf_dsq_move(struct bpf_iter_scx_dsq *it__iter,
				  struct task_struct *p, u64 dsq_id,
				  u64 enq_flags)
{
	return scx_dsq_move((struct bpf_iter_scx_dsq_kern *)it__iter,
			    p, dsq_id, enq_flags);
}

/**
 * scx_bpf_dsq_move_vtime - Move a task from DSQ iteration to a PRIQ DSQ
 * @it__iter: DSQ iterator in progress
 * @p: task to transfer
 * @dsq_id: DSQ to move @p to
 * @enq_flags: SCX_ENQ_*
 *
 * Transfer @p which is on the DSQ currently iterated by @it__iter to the
 * priority queue of the DSQ specified by @dsq_id. The destination must be a
 * user DSQ as only user DSQs support priority queue.
 *
 * @p's slice and vtime are kept by default. Use scx_bpf_dsq_move_set_slice()
 * and scx_bpf_dsq_move_set_vtime() to update.
 *
 * All other aspects are identical to scx_bpf_dsq_move(). See
 * scx_bpf_dsq_insert_vtime() for more information on @vtime.
 */
__bpf_kfunc bool scx_bpf_dsq_move_vtime(struct bpf_iter_scx_dsq *it__iter,
					struct task_struct *p, u64 dsq_id,
					u64 enq_flags)
{
	return scx_dsq_move((struct bpf_iter_scx_dsq_kern *)it__iter,
			    p, dsq_id, enq_flags | SCX_ENQ_DSQ_PRIQ);
}

#ifdef CONFIG_EXT_SUB_SCHED
/**
 * scx_bpf_sub_dispatch - Trigger dispatching on a child scheduler
 * @cgroup_id: cgroup ID of the child scheduler to dispatch
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Allows a parent scheduler to trigger dispatching on one of its direct
 * child schedulers. The child scheduler runs its dispatch operation to
 * move tasks from dispatch queues to the local runqueue.
 *
 * Returns: true on success, false if cgroup_id is invalid, not a direct
 * child, or caller lacks dispatch permission.
 */
__bpf_kfunc bool scx_bpf_sub_dispatch(u64 cgroup_id, const struct bpf_prog_aux *aux)
{
	struct rq *this_rq = this_rq();
	struct scx_sched *parent, *child;

	guard(rcu)();
	parent = scx_prog_sched(aux);
	if (unlikely(!parent))
		return false;

	child = scx_find_sub_sched(cgroup_id);

	if (unlikely(!child))
		return false;

	if (unlikely(scx_parent(child) != parent)) {
		scx_error(parent, "trying to dispatch a distant sub-sched on cgroup %llu",
			  cgroup_id);
		return false;
	}

	return scx_dispatch_sched(child, this_rq, this_rq->scx.sub_dispatch_prev,
				  true);
}
#endif	/* CONFIG_EXT_SUB_SCHED */

__bpf_kfunc_end_defs();

BTF_KFUNCS_START(scx_kfunc_ids_dispatch)
BTF_ID_FLAGS(func, scx_bpf_dispatch_nr_slots, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dispatch_cancel, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_to_local___v2, KF_IMPLICIT_ARGS)
/* scx_bpf_dsq_move*() also in scx_kfunc_ids_unlocked: callable from unlocked contexts */
BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_vtime, KF_RCU)
#ifdef CONFIG_EXT_SUB_SCHED
BTF_ID_FLAGS(func, scx_bpf_sub_dispatch, KF_IMPLICIT_ARGS)
#endif
BTF_KFUNCS_END(scx_kfunc_ids_dispatch)

static const struct btf_kfunc_id_set scx_kfunc_set_dispatch = {
	.owner			= THIS_MODULE,
	.set			= &scx_kfunc_ids_dispatch,
	.filter			= scx_kfunc_context_filter,
};

__bpf_kfunc_start_defs();

/**
 * scx_bpf_reenqueue_local - Re-enqueue tasks on a local DSQ
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Iterate over all of the tasks currently enqueued on the local DSQ of the
 * caller's CPU, and re-enqueue them in the BPF scheduler. Returns the number of
 * processed tasks. Can only be called from ops.cpu_release().
 */
__bpf_kfunc u32 scx_bpf_reenqueue_local(const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct rq *rq;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return 0;

	rq = cpu_rq(smp_processor_id());
	lockdep_assert_rq_held(rq);

	return reenq_local(sch, rq, SCX_REENQ_ANY);
}

__bpf_kfunc_end_defs();

BTF_KFUNCS_START(scx_kfunc_ids_cpu_release)
BTF_ID_FLAGS(func, scx_bpf_reenqueue_local, KF_IMPLICIT_ARGS)
BTF_KFUNCS_END(scx_kfunc_ids_cpu_release)

static const struct btf_kfunc_id_set scx_kfunc_set_cpu_release = {
	.owner			= THIS_MODULE,
	.set			= &scx_kfunc_ids_cpu_release,
	.filter			= scx_kfunc_context_filter,
};

__bpf_kfunc_start_defs();

/**
 * scx_bpf_create_dsq - Create a custom DSQ
 * @dsq_id: DSQ to create
 * @node: NUMA node to allocate from
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Create a custom DSQ identified by @dsq_id. Can be called from any sleepable
 * scx callback, and any BPF_PROG_TYPE_SYSCALL prog.
 */
__bpf_kfunc s32 scx_bpf_create_dsq(u64 dsq_id, s32 node, const struct bpf_prog_aux *aux)
{
	struct scx_dispatch_q *dsq;
	struct scx_sched *sch;
	s32 ret;

	if (unlikely(node >= (int)nr_node_ids ||
		     (node < 0 && node != NUMA_NO_NODE)))
		return -EINVAL;

	if (unlikely(dsq_id & SCX_DSQ_FLAG_BUILTIN))
		return -EINVAL;

	dsq = kmalloc_node(sizeof(*dsq), GFP_KERNEL, node);
	if (!dsq)
		return -ENOMEM;

	/*
	 * init_dsq() must be called in GFP_KERNEL context. Init it with NULL
	 * @sch and update afterwards.
	 */
	ret = init_dsq(dsq, dsq_id, NULL);
	if (ret) {
		kfree(dsq);
		return ret;
	}

	rcu_read_lock();

	sch = scx_prog_sched(aux);
	if (sch) {
		dsq->sched = sch;
		ret = rhashtable_lookup_insert_fast(&sch->dsq_hash, &dsq->hash_node,
						    dsq_hash_params);
	} else {
		ret = -ENODEV;
	}

	rcu_read_unlock();
	if (ret) {
		exit_dsq(dsq);
		kfree(dsq);
	}
	return ret;
}

__bpf_kfunc_end_defs();

BTF_KFUNCS_START(scx_kfunc_ids_unlocked)
BTF_ID_FLAGS(func, scx_bpf_create_dsq, KF_IMPLICIT_ARGS | KF_SLEEPABLE)
/* also in scx_kfunc_ids_dispatch: also callable from ops.dispatch() */
BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_slice, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_set_vtime, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_dsq_move_vtime, KF_RCU)
/* also in scx_kfunc_ids_select_cpu: also callable from ops.select_cpu()/ops.enqueue() */
BTF_ID_FLAGS(func, __scx_bpf_select_cpu_and, KF_IMPLICIT_ARGS | KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_select_cpu_and, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_select_cpu_dfl, KF_IMPLICIT_ARGS | KF_RCU)
BTF_KFUNCS_END(scx_kfunc_ids_unlocked)

static const struct btf_kfunc_id_set scx_kfunc_set_unlocked = {
	.owner			= THIS_MODULE,
	.set			= &scx_kfunc_ids_unlocked,
	.filter			= scx_kfunc_context_filter,
};

__bpf_kfunc_start_defs();

/**
 * scx_bpf_task_set_slice - Set task's time slice
 * @p: task of interest
 * @slice: time slice to set in nsecs
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Set @p's time slice to @slice. Returns %true on success, %false if the
 * calling scheduler doesn't have authority over @p.
 */
__bpf_kfunc bool scx_bpf_task_set_slice(struct task_struct *p, u64 slice,
					const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (unlikely(!sch || !scx_task_on_sched(sch, p)))
		return false;

	p->scx.slice = slice;
	return true;
}

/**
 * scx_bpf_task_set_dsq_vtime - Set task's virtual time for DSQ ordering
 * @p: task of interest
 * @vtime: virtual time to set
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Set @p's virtual time to @vtime. Returns %true on success, %false if the
 * calling scheduler doesn't have authority over @p.
 */
__bpf_kfunc bool scx_bpf_task_set_dsq_vtime(struct task_struct *p, u64 vtime,
					    const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (unlikely(!sch || !scx_task_on_sched(sch, p)))
		return false;

	p->scx.dsq_vtime = vtime;
	return true;
}

static void scx_kick_cpu(struct scx_sched *sch, s32 cpu, u64 flags)
{
	struct rq *this_rq;
	unsigned long irq_flags;

	if (!ops_cpu_valid(sch, cpu, NULL))
		return;

	local_irq_save(irq_flags);

	this_rq = this_rq();

	/*
	 * While bypassing for PM ops, IRQ handling may not be online which can
	 * lead to irq_work_queue() malfunction such as infinite busy wait for
	 * IRQ status update. Suppress kicking.
	 */
	if (scx_bypassing(sch, cpu_of(this_rq)))
		goto out;

	/*
	 * Actual kicking is bounced to kick_cpus_irq_workfn() to avoid nesting
	 * rq locks. We can probably be smarter and avoid bouncing if called
	 * from ops which don't hold a rq lock.
	 */
	if (flags & SCX_KICK_IDLE) {
		struct rq *target_rq = cpu_rq(cpu);

		if (unlikely(flags & (SCX_KICK_PREEMPT | SCX_KICK_WAIT)))
			scx_error(sch, "PREEMPT/WAIT cannot be used with SCX_KICK_IDLE");

		if (raw_spin_rq_trylock(target_rq)) {
			if (can_skip_idle_kick(target_rq)) {
				raw_spin_rq_unlock(target_rq);
				goto out;
			}
			raw_spin_rq_unlock(target_rq);
		}
		cpumask_set_cpu(cpu, this_rq->scx.cpus_to_kick_if_idle);
	} else {
		cpumask_set_cpu(cpu, this_rq->scx.cpus_to_kick);

		if (flags & SCX_KICK_PREEMPT)
			cpumask_set_cpu(cpu, this_rq->scx.cpus_to_preempt);
		if (flags & SCX_KICK_WAIT)
			cpumask_set_cpu(cpu, this_rq->scx.cpus_to_wait);
	}

	irq_work_queue(&this_rq->scx.kick_cpus_irq_work);
out:
	local_irq_restore(irq_flags);
}

/**
 * scx_bpf_kick_cpu - Trigger reschedule on a CPU
 * @cpu: cpu to kick
 * @flags: %SCX_KICK_* flags
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Kick @cpu into rescheduling. This can be used to wake up an idle CPU or
 * trigger rescheduling on a busy CPU. This can be called from any online
 * scx_ops operation and the actual kicking is performed asynchronously through
 * an irq work.
 */
__bpf_kfunc void scx_bpf_kick_cpu(s32 cpu, u64 flags, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (likely(sch))
		scx_kick_cpu(sch, cpu, flags);
}

/**
 * scx_bpf_dsq_nr_queued - Return the number of queued tasks
 * @dsq_id: id of the DSQ
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Return the number of tasks in the DSQ matching @dsq_id. If not found,
 * -%ENOENT is returned.
 */
__bpf_kfunc s32 scx_bpf_dsq_nr_queued(u64 dsq_id, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct scx_dispatch_q *dsq;
	s32 ret;

	preempt_disable();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch)) {
		ret = -ENODEV;
		goto out;
	}

	if (dsq_id == SCX_DSQ_LOCAL) {
		ret = READ_ONCE(this_rq()->scx.local_dsq.nr);
		goto out;
	} else if ((dsq_id & SCX_DSQ_LOCAL_ON) == SCX_DSQ_LOCAL_ON) {
		s32 cpu = dsq_id & SCX_DSQ_LOCAL_CPU_MASK;

		if (ops_cpu_valid(sch, cpu, NULL)) {
			ret = READ_ONCE(cpu_rq(cpu)->scx.local_dsq.nr);
			goto out;
		}
	} else {
		dsq = find_user_dsq(sch, dsq_id);
		if (dsq) {
			ret = READ_ONCE(dsq->nr);
			goto out;
		}
	}
	ret = -ENOENT;
out:
	preempt_enable();
	return ret;
}

/**
 * scx_bpf_destroy_dsq - Destroy a custom DSQ
 * @dsq_id: DSQ to destroy
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Destroy the custom DSQ identified by @dsq_id. Only DSQs created with
 * scx_bpf_create_dsq() can be destroyed. The caller must ensure that the DSQ is
 * empty and no further tasks are dispatched to it. Ignored if called on a DSQ
 * which doesn't exist. Can be called from any online scx_ops operations.
 */
__bpf_kfunc void scx_bpf_destroy_dsq(u64 dsq_id, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();
	sch = scx_prog_sched(aux);
	if (sch)
		destroy_dsq(sch, dsq_id);
}

/**
 * bpf_iter_scx_dsq_new - Create a DSQ iterator
 * @it: iterator to initialize
 * @dsq_id: DSQ to iterate
 * @flags: %SCX_DSQ_ITER_*
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Initialize BPF iterator @it which can be used with bpf_for_each() to walk
 * tasks in the DSQ specified by @dsq_id. Iteration using @it only includes
 * tasks which are already queued when this function is invoked.
 */
__bpf_kfunc int bpf_iter_scx_dsq_new(struct bpf_iter_scx_dsq *it, u64 dsq_id,
				     u64 flags, const struct bpf_prog_aux *aux)
{
	struct bpf_iter_scx_dsq_kern *kit = (void *)it;
	struct scx_sched *sch;

	BUILD_BUG_ON(sizeof(struct bpf_iter_scx_dsq_kern) >
		     sizeof(struct bpf_iter_scx_dsq));
	BUILD_BUG_ON(__alignof__(struct bpf_iter_scx_dsq_kern) !=
		     __alignof__(struct bpf_iter_scx_dsq));
	BUILD_BUG_ON(__SCX_DSQ_ITER_ALL_FLAGS &
		     ((1U << __SCX_DSQ_LNODE_PRIV_SHIFT) - 1));

	/*
	 * next() and destroy() will be called regardless of the return value.
	 * Always clear $kit->dsq.
	 */
	kit->dsq = NULL;

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return -ENODEV;

	if (flags & ~__SCX_DSQ_ITER_USER_FLAGS)
		return -EINVAL;

	kit->dsq = find_user_dsq(sch, dsq_id);
	if (!kit->dsq)
		return -ENOENT;

	kit->cursor = INIT_DSQ_LIST_CURSOR(kit->cursor, kit->dsq, flags);

	return 0;
}

/**
 * bpf_iter_scx_dsq_next - Progress a DSQ iterator
 * @it: iterator to progress
 *
 * Return the next task. See bpf_iter_scx_dsq_new().
 */
__bpf_kfunc struct task_struct *bpf_iter_scx_dsq_next(struct bpf_iter_scx_dsq *it)
{
	struct bpf_iter_scx_dsq_kern *kit = (void *)it;

	if (!kit->dsq)
		return NULL;

	guard(raw_spinlock_irqsave)(&kit->dsq->lock);

	return nldsq_cursor_next_task(&kit->cursor, kit->dsq);
}

/**
 * bpf_iter_scx_dsq_destroy - Destroy a DSQ iterator
 * @it: iterator to destroy
 *
 * Undo scx_iter_scx_dsq_new().
 */
__bpf_kfunc void bpf_iter_scx_dsq_destroy(struct bpf_iter_scx_dsq *it)
{
	struct bpf_iter_scx_dsq_kern *kit = (void *)it;

	if (!kit->dsq)
		return;

	if (!list_empty(&kit->cursor.node)) {
		unsigned long flags;

		raw_spin_lock_irqsave(&kit->dsq->lock, flags);
		list_del_init(&kit->cursor.node);
		raw_spin_unlock_irqrestore(&kit->dsq->lock, flags);
	}
	kit->dsq = NULL;
}

/**
 * scx_bpf_dsq_peek - Lockless peek at the first element.
 * @dsq_id: DSQ to examine.
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Read the first element in the DSQ. This is semantically equivalent to using
 * the DSQ iterator, but is lockfree. Of course, like any lockless operation,
 * this provides only a point-in-time snapshot, and the contents may change
 * by the time any subsequent locking operation reads the queue.
 *
 * Returns the pointer, or NULL indicates an empty queue OR internal error.
 */
__bpf_kfunc struct task_struct *scx_bpf_dsq_peek(u64 dsq_id,
						 const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct scx_dispatch_q *dsq;

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return NULL;

	if (unlikely(dsq_id & SCX_DSQ_FLAG_BUILTIN)) {
		scx_error(sch, "peek disallowed on builtin DSQ 0x%llx", dsq_id);
		return NULL;
	}

	dsq = find_user_dsq(sch, dsq_id);
	if (unlikely(!dsq)) {
		scx_error(sch, "peek on non-existent DSQ 0x%llx", dsq_id);
		return NULL;
	}

	return rcu_dereference(dsq->first_task);
}

/**
 * scx_bpf_dsq_reenq - Re-enqueue tasks on a DSQ
 * @dsq_id: DSQ to re-enqueue
 * @reenq_flags: %SCX_RENQ_*
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Iterate over all of the tasks currently enqueued on the DSQ identified by
 * @dsq_id, and re-enqueue them in the BPF scheduler. The following DSQs are
 * supported:
 *
 * - Local DSQs (%SCX_DSQ_LOCAL or %SCX_DSQ_LOCAL_ON | $cpu)
 * - User DSQs
 *
 * Re-enqueues are performed asynchronously. Can be called from anywhere.
 */
__bpf_kfunc void scx_bpf_dsq_reenq(u64 dsq_id, u64 reenq_flags,
				   const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct scx_dispatch_q *dsq;

	guard(preempt)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return;

	if (unlikely(reenq_flags & ~__SCX_REENQ_USER_MASK)) {
		scx_error(sch, "invalid SCX_REENQ flags 0x%llx", reenq_flags);
		return;
	}

	/* not specifying any filter bits is the same as %SCX_REENQ_ANY */
	if (!(reenq_flags & __SCX_REENQ_FILTER_MASK))
		reenq_flags |= SCX_REENQ_ANY;

	dsq = find_dsq_for_dispatch(sch, this_rq(), dsq_id, smp_processor_id());
	schedule_dsq_reenq(sch, dsq, reenq_flags, scx_locked_rq());
}

/**
 * scx_bpf_reenqueue_local - Re-enqueue tasks on a local DSQ
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Iterate over all of the tasks currently enqueued on the local DSQ of the
 * caller's CPU, and re-enqueue them in the BPF scheduler. Can be called from
 * anywhere.
 *
 * This is now a special case of scx_bpf_dsq_reenq() and may be removed in the
 * future.
 */
__bpf_kfunc void scx_bpf_reenqueue_local___v2(const struct bpf_prog_aux *aux)
{
	scx_bpf_dsq_reenq(SCX_DSQ_LOCAL, 0, aux);
}

__bpf_kfunc_end_defs();

static s32 __bstr_format(struct scx_sched *sch, u64 *data_buf, char *line_buf,
			 size_t line_size, char *fmt, unsigned long long *data,
			 u32 data__sz)
{
	struct bpf_bprintf_data bprintf_data = { .get_bin_args = true };
	s32 ret;

	if (data__sz % 8 || data__sz > MAX_BPRINTF_VARARGS * 8 ||
	    (data__sz && !data)) {
		scx_error(sch, "invalid data=%p and data__sz=%u", (void *)data, data__sz);
		return -EINVAL;
	}

	ret = copy_from_kernel_nofault(data_buf, data, data__sz);
	if (ret < 0) {
		scx_error(sch, "failed to read data fields (%d)", ret);
		return ret;
	}

	ret = bpf_bprintf_prepare(fmt, UINT_MAX, data_buf, data__sz / 8,
				  &bprintf_data);
	if (ret < 0) {
		scx_error(sch, "format preparation failed (%d)", ret);
		return ret;
	}

	ret = bstr_printf(line_buf, line_size, fmt,
			  bprintf_data.bin_args);
	bpf_bprintf_cleanup(&bprintf_data);
	if (ret < 0) {
		scx_error(sch, "(\"%s\", %p, %u) failed to format", fmt, data, data__sz);
		return ret;
	}

	return ret;
}

static s32 bstr_format(struct scx_sched *sch, struct scx_bstr_buf *buf,
		       char *fmt, unsigned long long *data, u32 data__sz)
{
	return __bstr_format(sch, buf->data, buf->line, sizeof(buf->line),
			     fmt, data, data__sz);
}

__bpf_kfunc_start_defs();

/**
 * scx_bpf_exit_bstr - Gracefully exit the BPF scheduler.
 * @exit_code: Exit value to pass to user space via struct scx_exit_info.
 * @fmt: error message format string
 * @data: format string parameters packaged using ___bpf_fill() macro
 * @data__sz: @data len, must end in '__sz' for the verifier
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Indicate that the BPF scheduler wants to exit gracefully, and initiate ops
 * disabling.
 */
__bpf_kfunc void scx_bpf_exit_bstr(s64 exit_code, char *fmt,
				   unsigned long long *data, u32 data__sz,
				   const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	unsigned long flags;

	raw_spin_lock_irqsave(&scx_exit_bstr_buf_lock, flags);
	sch = scx_prog_sched(aux);
	if (likely(sch) &&
	    bstr_format(sch, &scx_exit_bstr_buf, fmt, data, data__sz) >= 0)
		scx_exit(sch, SCX_EXIT_UNREG_BPF, exit_code, "%s", scx_exit_bstr_buf.line);
	raw_spin_unlock_irqrestore(&scx_exit_bstr_buf_lock, flags);
}

/**
 * scx_bpf_error_bstr - Indicate fatal error
 * @fmt: error message format string
 * @data: format string parameters packaged using ___bpf_fill() macro
 * @data__sz: @data len, must end in '__sz' for the verifier
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Indicate that the BPF scheduler encountered a fatal error and initiate ops
 * disabling.
 */
__bpf_kfunc void scx_bpf_error_bstr(char *fmt, unsigned long long *data,
				    u32 data__sz, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	unsigned long flags;

	raw_spin_lock_irqsave(&scx_exit_bstr_buf_lock, flags);
	sch = scx_prog_sched(aux);
	if (likely(sch) &&
	    bstr_format(sch, &scx_exit_bstr_buf, fmt, data, data__sz) >= 0)
		scx_exit(sch, SCX_EXIT_ERROR_BPF, 0, "%s", scx_exit_bstr_buf.line);
	raw_spin_unlock_irqrestore(&scx_exit_bstr_buf_lock, flags);
}

/**
 * scx_bpf_dump_bstr - Generate extra debug dump specific to the BPF scheduler
 * @fmt: format string
 * @data: format string parameters packaged using ___bpf_fill() macro
 * @data__sz: @data len, must end in '__sz' for the verifier
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * To be called through scx_bpf_dump() helper from ops.dump(), dump_cpu() and
 * dump_task() to generate extra debug dump specific to the BPF scheduler.
 *
 * The extra dump may be multiple lines. A single line may be split over
 * multiple calls. The last line is automatically terminated.
 */
__bpf_kfunc void scx_bpf_dump_bstr(char *fmt, unsigned long long *data,
				   u32 data__sz, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct scx_dump_data *dd = &scx_dump_data;
	struct scx_bstr_buf *buf = &dd->buf;
	s32 ret;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return;

	if (raw_smp_processor_id() != dd->cpu) {
		scx_error(sch, "scx_bpf_dump() must only be called from ops.dump() and friends");
		return;
	}

	/* append the formatted string to the line buf */
	ret = __bstr_format(sch, buf->data, buf->line + dd->cursor,
			    sizeof(buf->line) - dd->cursor, fmt, data, data__sz);
	if (ret < 0) {
		dump_line(dd->s, "%s[!] (\"%s\", %p, %u) failed to format (%d)",
			  dd->prefix, fmt, data, data__sz, ret);
		return;
	}

	dd->cursor += ret;
	dd->cursor = min_t(s32, dd->cursor, sizeof(buf->line));

	if (!dd->cursor)
		return;

	/*
	 * If the line buf overflowed or ends in a newline, flush it into the
	 * dump. This is to allow the caller to generate a single line over
	 * multiple calls. As ops_dump_flush() can also handle multiple lines in
	 * the line buf, the only case which can lead to an unexpected
	 * truncation is when the caller keeps generating newlines in the middle
	 * instead of the end consecutively. Don't do that.
	 */
	if (dd->cursor >= sizeof(buf->line) || buf->line[dd->cursor - 1] == '\n')
		ops_dump_flush();
}

/**
 * scx_bpf_cpuperf_cap - Query the maximum relative capacity of a CPU
 * @cpu: CPU of interest
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Return the maximum relative capacity of @cpu in relation to the most
 * performant CPU in the system. The return value is in the range [1,
 * %SCX_CPUPERF_ONE]. See scx_bpf_cpuperf_cur().
 */
__bpf_kfunc u32 scx_bpf_cpuperf_cap(s32 cpu, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (likely(sch) && ops_cpu_valid(sch, cpu, NULL))
		return arch_scale_cpu_capacity(cpu);
	else
		return SCX_CPUPERF_ONE;
}

/**
 * scx_bpf_cpuperf_cur - Query the current relative performance of a CPU
 * @cpu: CPU of interest
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Return the current relative performance of @cpu in relation to its maximum.
 * The return value is in the range [1, %SCX_CPUPERF_ONE].
 *
 * The current performance level of a CPU in relation to the maximum performance
 * available in the system can be calculated as follows:
 *
 *   scx_bpf_cpuperf_cap() * scx_bpf_cpuperf_cur() / %SCX_CPUPERF_ONE
 *
 * The result is in the range [1, %SCX_CPUPERF_ONE].
 */
__bpf_kfunc u32 scx_bpf_cpuperf_cur(s32 cpu, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (likely(sch) && ops_cpu_valid(sch, cpu, NULL))
		return arch_scale_freq_capacity(cpu);
	else
		return SCX_CPUPERF_ONE;
}

/**
 * scx_bpf_cpuperf_set - Set the relative performance target of a CPU
 * @cpu: CPU of interest
 * @perf: target performance level [0, %SCX_CPUPERF_ONE]
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Set the target performance level of @cpu to @perf. @perf is in linear
 * relative scale between 0 and %SCX_CPUPERF_ONE. This determines how the
 * schedutil cpufreq governor chooses the target frequency.
 *
 * The actual performance level chosen, CPU grouping, and the overhead and
 * latency of the operations are dependent on the hardware and cpufreq driver in
 * use. Consult hardware and cpufreq documentation for more information. The
 * current performance level can be monitored using scx_bpf_cpuperf_cur().
 */
__bpf_kfunc void scx_bpf_cpuperf_set(s32 cpu, u32 perf, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return;

	if (unlikely(perf > SCX_CPUPERF_ONE)) {
		scx_error(sch, "Invalid cpuperf target %u for CPU %d", perf, cpu);
		return;
	}

	if (ops_cpu_valid(sch, cpu, NULL)) {
		struct rq *rq = cpu_rq(cpu), *locked_rq = scx_locked_rq();
		struct rq_flags rf;

		/*
		 * When called with an rq lock held, restrict the operation
		 * to the corresponding CPU to prevent ABBA deadlocks.
		 */
		if (locked_rq && rq != locked_rq) {
			scx_error(sch, "Invalid target CPU %d", cpu);
			return;
		}

		/*
		 * If no rq lock is held, allow to operate on any CPU by
		 * acquiring the corresponding rq lock.
		 */
		if (!locked_rq) {
			rq_lock_irqsave(rq, &rf);
			update_rq_clock(rq);
		}

		rq->scx.cpuperf_target = perf;
		cpufreq_update_util(rq, 0);

		if (!locked_rq)
			rq_unlock_irqrestore(rq, &rf);
	}
}

/**
 * scx_bpf_nr_node_ids - Return the number of possible node IDs
 *
 * All valid node IDs in the system are smaller than the returned value.
 */
__bpf_kfunc u32 scx_bpf_nr_node_ids(void)
{
	return nr_node_ids;
}

/**
 * scx_bpf_nr_cpu_ids - Return the number of possible CPU IDs
 *
 * All valid CPU IDs in the system are smaller than the returned value.
 */
__bpf_kfunc u32 scx_bpf_nr_cpu_ids(void)
{
	return nr_cpu_ids;
}

/**
 * scx_bpf_get_possible_cpumask - Get a referenced kptr to cpu_possible_mask
 */
__bpf_kfunc const struct cpumask *scx_bpf_get_possible_cpumask(void)
{
	return cpu_possible_mask;
}

/**
 * scx_bpf_get_online_cpumask - Get a referenced kptr to cpu_online_mask
 */
__bpf_kfunc const struct cpumask *scx_bpf_get_online_cpumask(void)
{
	return cpu_online_mask;
}

/**
 * scx_bpf_put_cpumask - Release a possible/online cpumask
 * @cpumask: cpumask to release
 */
__bpf_kfunc void scx_bpf_put_cpumask(const struct cpumask *cpumask)
{
	/*
	 * Empty function body because we aren't actually acquiring or releasing
	 * a reference to a global cpumask, which is read-only in the caller and
	 * is never released. The acquire / release semantics here are just used
	 * to make the cpumask is a trusted pointer in the caller.
	 */
}

/**
 * scx_bpf_task_running - Is task currently running?
 * @p: task of interest
 */
__bpf_kfunc bool scx_bpf_task_running(const struct task_struct *p)
{
	return task_rq(p)->curr == p;
}

/**
 * scx_bpf_task_cpu - CPU a task is currently associated with
 * @p: task of interest
 */
__bpf_kfunc s32 scx_bpf_task_cpu(const struct task_struct *p)
{
	return task_cpu(p);
}

/**
 * scx_bpf_cpu_rq - Fetch the rq of a CPU
 * @cpu: CPU of the rq
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 */
__bpf_kfunc struct rq *scx_bpf_cpu_rq(s32 cpu, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return NULL;

	if (!ops_cpu_valid(sch, cpu, NULL))
		return NULL;

	if (!sch->warned_deprecated_rq) {
		printk_deferred(KERN_WARNING "sched_ext: %s() is deprecated; "
				"use scx_bpf_locked_rq() when holding rq lock "
				"or scx_bpf_cpu_curr() to read remote curr safely.\n", __func__);
		sch->warned_deprecated_rq = true;
	}

	return cpu_rq(cpu);
}

/**
 * scx_bpf_locked_rq - Return the rq currently locked by SCX
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Returns the rq if a rq lock is currently held by SCX.
 * Otherwise emits an error and returns NULL.
 */
__bpf_kfunc struct rq *scx_bpf_locked_rq(const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;
	struct rq *rq;

	guard(preempt)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return NULL;

	rq = scx_locked_rq();
	if (!rq) {
		scx_error(sch, "accessing rq without holding rq lock");
		return NULL;
	}

	return rq;
}

/**
 * scx_bpf_cpu_curr - Return remote CPU's curr task
 * @cpu: CPU of interest
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * Callers must hold RCU read lock (KF_RCU).
 */
__bpf_kfunc struct task_struct *scx_bpf_cpu_curr(s32 cpu, const struct bpf_prog_aux *aux)
{
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		return NULL;

	if (!ops_cpu_valid(sch, cpu, NULL))
		return NULL;

	return rcu_dereference(cpu_rq(cpu)->curr);
}

/**
 * scx_bpf_now - Returns a high-performance monotonically non-decreasing
 * clock for the current CPU. The clock returned is in nanoseconds.
 *
 * It provides the following properties:
 *
 * 1) High performance: Many BPF schedulers call bpf_ktime_get_ns() frequently
 *  to account for execution time and track tasks' runtime properties.
 *  Unfortunately, in some hardware platforms, bpf_ktime_get_ns() -- which
 *  eventually reads a hardware timestamp counter -- is neither performant nor
 *  scalable. scx_bpf_now() aims to provide a high-performance clock by
 *  using the rq clock in the scheduler core whenever possible.
 *
 * 2) High enough resolution for the BPF scheduler use cases: In most BPF
 *  scheduler use cases, the required clock resolution is lower than the most
 *  accurate hardware clock (e.g., rdtsc in x86). scx_bpf_now() basically
 *  uses the rq clock in the scheduler core whenever it is valid. It considers
 *  that the rq clock is valid from the time the rq clock is updated
 *  (update_rq_clock) until the rq is unlocked (rq_unpin_lock).
 *
 * 3) Monotonically non-decreasing clock for the same CPU: scx_bpf_now()
 *  guarantees the clock never goes backward when comparing them in the same
 *  CPU. On the other hand, when comparing clocks in different CPUs, there
 *  is no such guarantee -- the clock can go backward. It provides a
 *  monotonically *non-decreasing* clock so that it would provide the same
 *  clock values in two different scx_bpf_now() calls in the same CPU
 *  during the same period of when the rq clock is valid.
 */
__bpf_kfunc u64 scx_bpf_now(void)
{
	struct rq *rq;
	u64 clock;

	preempt_disable();

	rq = this_rq();
	if (smp_load_acquire(&rq->scx.flags) & SCX_RQ_CLK_VALID) {
		/*
		 * If the rq clock is valid, use the cached rq clock.
		 *
		 * Note that scx_bpf_now() is re-entrant between a process
		 * context and an interrupt context (e.g., timer interrupt).
		 * However, we don't need to consider the race between them
		 * because such race is not observable from a caller.
		 */
		clock = READ_ONCE(rq->scx.clock);
	} else {
		/*
		 * Otherwise, return a fresh rq clock.
		 *
		 * The rq clock is updated outside of the rq lock.
		 * In this case, keep the updated rq clock invalid so the next
		 * kfunc call outside the rq lock gets a fresh rq clock.
		 */
		clock = sched_clock_cpu(cpu_of(rq));
	}

	preempt_enable();

	return clock;
}

static void scx_read_events(struct scx_sched *sch, struct scx_event_stats *events)
{
	struct scx_event_stats *e_cpu;
	int cpu;

	/* Aggregate per-CPU event counters into @events. */
	memset(events, 0, sizeof(*events));
	for_each_possible_cpu(cpu) {
		e_cpu = &per_cpu_ptr(sch->pcpu, cpu)->event_stats;
		scx_agg_event(events, e_cpu, SCX_EV_SELECT_CPU_FALLBACK);
		scx_agg_event(events, e_cpu, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
		scx_agg_event(events, e_cpu, SCX_EV_DISPATCH_KEEP_LAST);
		scx_agg_event(events, e_cpu, SCX_EV_ENQ_SKIP_EXITING);
		scx_agg_event(events, e_cpu, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED);
		scx_agg_event(events, e_cpu, SCX_EV_REENQ_IMMED);
		scx_agg_event(events, e_cpu, SCX_EV_REENQ_LOCAL_REPEAT);
		scx_agg_event(events, e_cpu, SCX_EV_REFILL_SLICE_DFL);
		scx_agg_event(events, e_cpu, SCX_EV_BYPASS_DURATION);
		scx_agg_event(events, e_cpu, SCX_EV_BYPASS_DISPATCH);
		scx_agg_event(events, e_cpu, SCX_EV_BYPASS_ACTIVATE);
		scx_agg_event(events, e_cpu, SCX_EV_INSERT_NOT_OWNED);
		scx_agg_event(events, e_cpu, SCX_EV_SUB_BYPASS_DISPATCH);
	}
}

/*
 * scx_bpf_events - Get a system-wide event counter to
 * @events: output buffer from a BPF program
 * @events__sz: @events len, must end in '__sz'' for the verifier
 */
__bpf_kfunc void scx_bpf_events(struct scx_event_stats *events,
				size_t events__sz)
{
	struct scx_sched *sch;
	struct scx_event_stats e_sys;

	rcu_read_lock();
	sch = rcu_dereference(scx_root);
	if (sch)
		scx_read_events(sch, &e_sys);
	else
		memset(&e_sys, 0, sizeof(e_sys));
	rcu_read_unlock();

	/*
	 * We cannot entirely trust a BPF-provided size since a BPF program
	 * might be compiled against a different vmlinux.h, of which
	 * scx_event_stats would be larger (a newer vmlinux.h) or smaller
	 * (an older vmlinux.h). Hence, we use the smaller size to avoid
	 * memory corruption.
	 */
	events__sz = min(events__sz, sizeof(*events));
	memcpy(events, &e_sys, events__sz);
}

#ifdef CONFIG_CGROUP_SCHED
/**
 * scx_bpf_task_cgroup - Return the sched cgroup of a task
 * @p: task of interest
 * @aux: implicit BPF argument to access bpf_prog_aux hidden from BPF progs
 *
 * @p->sched_task_group->css.cgroup represents the cgroup @p is associated with
 * from the scheduler's POV. SCX operations should use this function to
 * determine @p's current cgroup as, unlike following @p->cgroups,
 * @p->sched_task_group is stable for the duration of the SCX op. See
 * SCX_CALL_OP_TASK() for details.
 */
__bpf_kfunc struct cgroup *scx_bpf_task_cgroup(struct task_struct *p,
					       const struct bpf_prog_aux *aux)
{
	struct task_group *tg = p->sched_task_group;
	struct cgroup *cgrp = &cgrp_dfl_root.cgrp;
	struct scx_sched *sch;

	guard(rcu)();

	sch = scx_prog_sched(aux);
	if (unlikely(!sch))
		goto out;

	if (!scx_kf_arg_task_ok(sch, p))
		goto out;

	cgrp = tg_cgrp(tg);

out:
	cgroup_get(cgrp);
	return cgrp;
}
#endif	/* CONFIG_CGROUP_SCHED */

__bpf_kfunc_end_defs();

BTF_KFUNCS_START(scx_kfunc_ids_any)
BTF_ID_FLAGS(func, scx_bpf_task_set_slice, KF_IMPLICIT_ARGS | KF_RCU);
BTF_ID_FLAGS(func, scx_bpf_task_set_dsq_vtime, KF_IMPLICIT_ARGS | KF_RCU);
BTF_ID_FLAGS(func, scx_bpf_kick_cpu, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dsq_nr_queued, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_destroy_dsq, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dsq_peek, KF_IMPLICIT_ARGS | KF_RCU_PROTECTED | KF_RET_NULL)
BTF_ID_FLAGS(func, scx_bpf_dsq_reenq, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_reenqueue_local___v2, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, bpf_iter_scx_dsq_new, KF_IMPLICIT_ARGS | KF_ITER_NEW | KF_RCU_PROTECTED)
BTF_ID_FLAGS(func, bpf_iter_scx_dsq_next, KF_ITER_NEXT | KF_RET_NULL)
BTF_ID_FLAGS(func, bpf_iter_scx_dsq_destroy, KF_ITER_DESTROY)
BTF_ID_FLAGS(func, scx_bpf_exit_bstr, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_error_bstr, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_dump_bstr, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_cap, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_cur, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_cpuperf_set, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_nr_node_ids)
BTF_ID_FLAGS(func, scx_bpf_nr_cpu_ids)
BTF_ID_FLAGS(func, scx_bpf_get_possible_cpumask, KF_ACQUIRE)
BTF_ID_FLAGS(func, scx_bpf_get_online_cpumask, KF_ACQUIRE)
BTF_ID_FLAGS(func, scx_bpf_put_cpumask, KF_RELEASE)
BTF_ID_FLAGS(func, scx_bpf_task_running, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_task_cpu, KF_RCU)
BTF_ID_FLAGS(func, scx_bpf_cpu_rq, KF_IMPLICIT_ARGS)
BTF_ID_FLAGS(func, scx_bpf_locked_rq, KF_IMPLICIT_ARGS | KF_RET_NULL)
BTF_ID_FLAGS(func, scx_bpf_cpu_curr, KF_IMPLICIT_ARGS | KF_RET_NULL | KF_RCU_PROTECTED)
BTF_ID_FLAGS(func, scx_bpf_now)
BTF_ID_FLAGS(func, scx_bpf_events)
#ifdef CONFIG_CGROUP_SCHED
BTF_ID_FLAGS(func, scx_bpf_task_cgroup, KF_IMPLICIT_ARGS | KF_RCU | KF_ACQUIRE)
#endif
BTF_KFUNCS_END(scx_kfunc_ids_any)

static const struct btf_kfunc_id_set scx_kfunc_set_any = {
	.owner			= THIS_MODULE,
	.set			= &scx_kfunc_ids_any,
	.filter			= scx_kfunc_context_filter,
};

/*
 * Per-op kfunc allow flags. Each bit corresponds to a context-sensitive kfunc
 * group; an op may permit zero or more groups, with the union expressed in
 * scx_kf_allow_flags[]. The verifier-time filter (scx_kfunc_context_filter())
 * consults this table to decide whether a context-sensitive kfunc is callable
 * from a given SCX op.
 */
enum scx_kf_allow_flags {
	SCX_KF_ALLOW_UNLOCKED		= 1 << 0,
	SCX_KF_ALLOW_CPU_RELEASE	= 1 << 1,
	SCX_KF_ALLOW_DISPATCH		= 1 << 2,
	SCX_KF_ALLOW_ENQUEUE		= 1 << 3,
	SCX_KF_ALLOW_SELECT_CPU		= 1 << 4,
};

/*
 * Map each SCX op to the union of kfunc groups it permits, indexed by
 * SCX_OP_IDX(op). Ops not listed only permit kfuncs that are not
 * context-sensitive.
 */
static const u32 scx_kf_allow_flags[] = {
	[SCX_OP_IDX(select_cpu)]	= SCX_KF_ALLOW_SELECT_CPU | SCX_KF_ALLOW_ENQUEUE,
	[SCX_OP_IDX(enqueue)]		= SCX_KF_ALLOW_SELECT_CPU | SCX_KF_ALLOW_ENQUEUE,
	[SCX_OP_IDX(dispatch)]		= SCX_KF_ALLOW_ENQUEUE | SCX_KF_ALLOW_DISPATCH,
	[SCX_OP_IDX(cpu_release)]	= SCX_KF_ALLOW_CPU_RELEASE,
	[SCX_OP_IDX(init_task)]		= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(dump)]		= SCX_KF_ALLOW_UNLOCKED,
#ifdef CONFIG_EXT_GROUP_SCHED
	[SCX_OP_IDX(cgroup_init)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_exit)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_prep_move)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_cancel_move)] = SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_set_weight)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_set_bandwidth)] = SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cgroup_set_idle)]	= SCX_KF_ALLOW_UNLOCKED,
#endif	/* CONFIG_EXT_GROUP_SCHED */
	[SCX_OP_IDX(sub_attach)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(sub_detach)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cpu_online)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(cpu_offline)]	= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(init)]		= SCX_KF_ALLOW_UNLOCKED,
	[SCX_OP_IDX(exit)]		= SCX_KF_ALLOW_UNLOCKED,
};

/*
 * Verifier-time filter for SCX kfuncs. Registered via the .filter field on
 * each per-group btf_kfunc_id_set. The BPF core invokes this for every kfunc
 * call in the registered hook (BPF_PROG_TYPE_STRUCT_OPS or
 * BPF_PROG_TYPE_SYSCALL), regardless of which set originally introduced the
 * kfunc - so the filter must short-circuit on kfuncs it doesn't govern by
 * falling through to "allow" when none of the SCX sets contain the kfunc.
 */
int scx_kfunc_context_filter(const struct bpf_prog *prog, u32 kfunc_id)
{
	bool in_unlocked = btf_id_set8_contains(&scx_kfunc_ids_unlocked, kfunc_id);
	bool in_select_cpu = btf_id_set8_contains(&scx_kfunc_ids_select_cpu, kfunc_id);
	bool in_enqueue = btf_id_set8_contains(&scx_kfunc_ids_enqueue_dispatch, kfunc_id);
	bool in_dispatch = btf_id_set8_contains(&scx_kfunc_ids_dispatch, kfunc_id);
	bool in_cpu_release = btf_id_set8_contains(&scx_kfunc_ids_cpu_release, kfunc_id);
	bool in_idle = btf_id_set8_contains(&scx_kfunc_ids_idle, kfunc_id);
	bool in_any = btf_id_set8_contains(&scx_kfunc_ids_any, kfunc_id);
	u32 moff, flags;

	/* Not an SCX kfunc - allow. */
	if (!(in_unlocked || in_select_cpu || in_enqueue || in_dispatch ||
	      in_cpu_release || in_idle || in_any))
		return 0;

	/* SYSCALL progs (e.g. BPF test_run()) may call unlocked and select_cpu kfuncs. */
	if (prog->type == BPF_PROG_TYPE_SYSCALL)
		return (in_unlocked || in_select_cpu || in_idle || in_any) ? 0 : -EACCES;

	if (prog->type != BPF_PROG_TYPE_STRUCT_OPS)
		return (in_any || in_idle) ? 0 : -EACCES;

	/*
	 * add_subprog_and_kfunc() collects all kfunc calls, including dead code
	 * guarded by bpf_ksym_exists(), before check_attach_btf_id() sets
	 * prog->aux->st_ops. Allow all kfuncs when st_ops is not yet set;
	 * do_check_main() re-runs the filter with st_ops set and enforces the
	 * actual restrictions.
	 */
	if (!prog->aux->st_ops)
		return 0;

	/*
	 * Non-SCX struct_ops: SCX kfuncs are not permitted.
	 */
	if (prog->aux->st_ops != &bpf_sched_ext_ops)
		return -EACCES;

	/* SCX struct_ops: check the per-op allow list. */
	if (in_any || in_idle)
		return 0;

	moff = prog->aux->attach_st_ops_member_off;
	flags = scx_kf_allow_flags[SCX_MOFF_IDX(moff)];

	if ((flags & SCX_KF_ALLOW_UNLOCKED) && in_unlocked)
		return 0;
	if ((flags & SCX_KF_ALLOW_CPU_RELEASE) && in_cpu_release)
		return 0;
	if ((flags & SCX_KF_ALLOW_DISPATCH) && in_dispatch)
		return 0;
	if ((flags & SCX_KF_ALLOW_ENQUEUE) && in_enqueue)
		return 0;
	if ((flags & SCX_KF_ALLOW_SELECT_CPU) && in_select_cpu)
		return 0;

	return -EACCES;
}

static int __init scx_init(void)
{
	int ret;

	/*
	 * kfunc registration can't be done from init_sched_ext_class() as
	 * register_btf_kfunc_id_set() needs most of the system to be up.
	 *
	 * Some kfuncs are context-sensitive and can only be called from
	 * specific SCX ops. They are grouped into per-context BTF sets, each
	 * registered with scx_kfunc_context_filter as its .filter callback. The
	 * BPF core dedups identical filter pointers per hook
	 * (btf_populate_kfunc_set()), so the filter is invoked exactly once per
	 * kfunc lookup; it consults scx_kf_allow_flags[] to enforce per-op
	 * restrictions at verify time.
	 */
	if ((ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
					     &scx_kfunc_set_enqueue_dispatch)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
					     &scx_kfunc_set_dispatch)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
					     &scx_kfunc_set_cpu_release)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
					     &scx_kfunc_set_unlocked)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL,
					     &scx_kfunc_set_unlocked)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS,
					     &scx_kfunc_set_any)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING,
					     &scx_kfunc_set_any)) ||
	    (ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL,
					     &scx_kfunc_set_any))) {
		pr_err("sched_ext: Failed to register kfunc sets (%d)\n", ret);
		return ret;
	}

	ret = scx_idle_init();
	if (ret) {
		pr_err("sched_ext: Failed to initialize idle tracking (%d)\n", ret);
		return ret;
	}

	ret = register_bpf_struct_ops(&bpf_sched_ext_ops, sched_ext_ops);
	if (ret) {
		pr_err("sched_ext: Failed to register struct_ops (%d)\n", ret);
		return ret;
	}

	ret = register_pm_notifier(&scx_pm_notifier);
	if (ret) {
		pr_err("sched_ext: Failed to register PM notifier (%d)\n", ret);
		return ret;
	}

	scx_kset = kset_create_and_add("sched_ext", &scx_uevent_ops, kernel_kobj);
	if (!scx_kset) {
		pr_err("sched_ext: Failed to create /sys/kernel/sched_ext\n");
		return -ENOMEM;
	}

	ret = sysfs_create_group(&scx_kset->kobj, &scx_global_attr_group);
	if (ret < 0) {
		pr_err("sched_ext: Failed to add global attributes\n");
		return ret;
	}

	return 0;
}
__initcall(scx_init);