xref: /linux/kernel/time/tick-sched.c (revision 7fc2cd2e4b398c57c9cf961cfea05eadbf34c05c)

// SPDX-License-Identifier: GPL-2.0
/*
 *  Copyright(C) 2005-2006, Thomas Gleixner <tglx@linutronix.de>
 *  Copyright(C) 2005-2007, Red Hat, Inc., Ingo Molnar
 *  Copyright(C) 2006-2007  Timesys Corp., Thomas Gleixner
 *
 *  NOHZ implementation for low and high resolution timers
 *
 *  Started by: Thomas Gleixner and Ingo Molnar
 */
#include <linux/compiler.h>
#include <linux/cpu.h>
#include <linux/err.h>
#include <linux/hrtimer.h>
#include <linux/interrupt.h>
#include <linux/kernel_stat.h>
#include <linux/percpu.h>
#include <linux/nmi.h>
#include <linux/profile.h>
#include <linux/sched/signal.h>
#include <linux/sched/clock.h>
#include <linux/sched/stat.h>
#include <linux/sched/nohz.h>
#include <linux/sched/loadavg.h>
#include <linux/module.h>
#include <linux/irq_work.h>
#include <linux/posix-timers.h>
#include <linux/context_tracking.h>
#include <linux/mm.h>

#include <asm/irq_regs.h>

#include "tick-internal.h"

#include <trace/events/timer.h>

/*
 * Per-CPU nohz control structure
 */
static DEFINE_PER_CPU(struct tick_sched, tick_cpu_sched);

struct tick_sched *tick_get_tick_sched(int cpu)
{
	return &per_cpu(tick_cpu_sched, cpu);
}

/*
 * The time when the last jiffy update happened. Write access must hold
 * jiffies_lock and jiffies_seq. tick_nohz_next_event() needs to get a
 * consistent view of jiffies and last_jiffies_update.
 */
static ktime_t last_jiffies_update;

/*
 * Must be called with interrupts disabled !
 */
static void tick_do_update_jiffies64(ktime_t now)
{
	unsigned long ticks = 1;
	ktime_t delta, nextp;

	/*
	 * 64-bit can do a quick check without holding the jiffies lock and
	 * without looking at the sequence count. The smp_load_acquire()
	 * pairs with the update done later in this function.
	 *
	 * 32-bit cannot do that because the store of 'tick_next_period'
	 * consists of two 32-bit stores, and the first store could be
	 * moved by the CPU to a random point in the future.
	 */
	if (IS_ENABLED(CONFIG_64BIT)) {
		if (ktime_before(now, smp_load_acquire(&tick_next_period)))
			return;
	} else {
		unsigned int seq;

		/*
		 * Avoid contention on 'jiffies_lock' and protect the quick
		 * check with the sequence count.
		 */
		do {
			seq = read_seqcount_begin(&jiffies_seq);
			nextp = tick_next_period;
		} while (read_seqcount_retry(&jiffies_seq, seq));

		if (ktime_before(now, nextp))
			return;
	}

	/* Quick check failed, i.e. update is required. */
	raw_spin_lock(&jiffies_lock);
	/*
	 * Re-evaluate with the lock held. Another CPU might have done the
	 * update already.
	 */
	if (ktime_before(now, tick_next_period)) {
		raw_spin_unlock(&jiffies_lock);
		return;
	}

	write_seqcount_begin(&jiffies_seq);

	delta = ktime_sub(now, tick_next_period);
	if (unlikely(delta >= TICK_NSEC)) {
		/* Slow path for long idle sleep times */
		s64 incr = TICK_NSEC;

		ticks += ktime_divns(delta, incr);

		last_jiffies_update = ktime_add_ns(last_jiffies_update,
						   incr * ticks);
	} else {
		last_jiffies_update = ktime_add_ns(last_jiffies_update,
						   TICK_NSEC);
	}

	/* Advance jiffies to complete the 'jiffies_seq' protected job */
	jiffies_64 += ticks;

	/* Keep the tick_next_period variable up to date */
	nextp = ktime_add_ns(last_jiffies_update, TICK_NSEC);

	if (IS_ENABLED(CONFIG_64BIT)) {
		/*
		 * Pairs with smp_load_acquire() in the lockless quick
		 * check above, and ensures that the update to 'jiffies_64' is
		 * not reordered vs. the store to 'tick_next_period', neither
		 * by the compiler nor by the CPU.
		 */
		smp_store_release(&tick_next_period, nextp);
	} else {
		/*
		 * A plain store is good enough on 32-bit, as the quick check
		 * above is protected by the sequence count.
		 */
		tick_next_period = nextp;
	}

	/*
	 * Release the sequence count. calc_global_load() below is not
	 * protected by it, but 'jiffies_lock' needs to be held to prevent
	 * concurrent invocations.
	 */
	write_seqcount_end(&jiffies_seq);

	calc_global_load();

	raw_spin_unlock(&jiffies_lock);
	update_wall_time();
}

/*
 * Initialize and return retrieve the jiffies update.
 */
static ktime_t tick_init_jiffy_update(void)
{
	ktime_t period;

	raw_spin_lock(&jiffies_lock);
	write_seqcount_begin(&jiffies_seq);

	/* Have we started the jiffies update yet ? */
	if (last_jiffies_update == 0) {
		u32 rem;

		/*
		 * Ensure that the tick is aligned to a multiple of
		 * TICK_NSEC.
		 */
		div_u64_rem(tick_next_period, TICK_NSEC, &rem);
		if (rem)
			tick_next_period += TICK_NSEC - rem;

		last_jiffies_update = tick_next_period;
	}
	period = last_jiffies_update;

	write_seqcount_end(&jiffies_seq);
	raw_spin_unlock(&jiffies_lock);

	return period;
}

static inline int tick_sched_flag_test(struct tick_sched *ts,
				       unsigned long flag)
{
	return !!(ts->flags & flag);
}

static inline void tick_sched_flag_set(struct tick_sched *ts,
				       unsigned long flag)
{
	lockdep_assert_irqs_disabled();
	ts->flags |= flag;
}

static inline void tick_sched_flag_clear(struct tick_sched *ts,
					 unsigned long flag)
{
	lockdep_assert_irqs_disabled();
	ts->flags &= ~flag;
}

/*
 * Allow only one non-timekeeper CPU at a time update jiffies from
 * the timer tick.
 *
 * Returns true if update was run.
 */
static bool tick_limited_update_jiffies64(struct tick_sched *ts, ktime_t now)
{
	static atomic_t in_progress;
	int inp;

	inp = atomic_read(&in_progress);
	if (inp || !atomic_try_cmpxchg(&in_progress, &inp, 1))
		return false;

	if (ts->last_tick_jiffies == jiffies)
		tick_do_update_jiffies64(now);
	atomic_set(&in_progress, 0);
	return true;
}

#define MAX_STALLED_JIFFIES 5

static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
{
	int tick_cpu, cpu = smp_processor_id();

	/*
	 * Check if the do_timer duty was dropped. We don't care about
	 * concurrency: This happens only when the CPU in charge went
	 * into a long sleep. If two CPUs happen to assign themselves to
	 * this duty, then the jiffies update is still serialized by
	 * 'jiffies_lock'.
	 *
	 * If nohz_full is enabled, this should not happen because the
	 * 'tick_do_timer_cpu' CPU never relinquishes.
	 */
	tick_cpu = READ_ONCE(tick_do_timer_cpu);

	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) && unlikely(tick_cpu == TICK_DO_TIMER_NONE)) {
#ifdef CONFIG_NO_HZ_FULL
		WARN_ON_ONCE(tick_nohz_full_running);
#endif
		WRITE_ONCE(tick_do_timer_cpu, cpu);
		tick_cpu = cpu;
	}

	/* Check if jiffies need an update */
	if (tick_cpu == cpu)
		tick_do_update_jiffies64(now);

	/*
	 * If the jiffies update stalled for too long (timekeeper in stop_machine()
	 * or VMEXIT'ed for several msecs), force an update.
	 */
	if (ts->last_tick_jiffies != jiffies) {
		ts->stalled_jiffies = 0;
		ts->last_tick_jiffies = READ_ONCE(jiffies);
	} else {
		if (++ts->stalled_jiffies >= MAX_STALLED_JIFFIES) {
			if (tick_limited_update_jiffies64(ts, now)) {
				ts->stalled_jiffies = 0;
				ts->last_tick_jiffies = READ_ONCE(jiffies);
			}
		}
	}

	if (tick_sched_flag_test(ts, TS_FLAG_INIDLE))
		ts->got_idle_tick = 1;
}

static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
{
	/*
	 * When we are idle and the tick is stopped, we have to touch
	 * the watchdog as we might not schedule for a really long
	 * time. This happens on completely idle SMP systems while
	 * waiting on the login prompt. We also increment the "start of
	 * idle" jiffy stamp so the idle accounting adjustment we do
	 * when we go busy again does not account too many ticks.
	 */
	if (IS_ENABLED(CONFIG_NO_HZ_COMMON) &&
	    tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
		touch_softlockup_watchdog_sched();
		if (is_idle_task(current))
			ts->idle_jiffies++;
		/*
		 * In case the current tick fired too early past its expected
		 * expiration, make sure we don't bypass the next clock reprogramming
		 * to the same deadline.
		 */
		ts->next_tick = 0;
	}

	update_process_times(user_mode(regs));
	profile_tick(CPU_PROFILING);
}

/*
 * We rearm the timer until we get disabled by the idle code.
 * Called with interrupts disabled.
 */
static enum hrtimer_restart tick_nohz_handler(struct hrtimer *timer)
{
	struct tick_sched *ts =	container_of(timer, struct tick_sched, sched_timer);
	struct pt_regs *regs = get_irq_regs();
	ktime_t now = ktime_get();

	tick_sched_do_timer(ts, now);

	/*
	 * Do not call when we are not in IRQ context and have
	 * no valid 'regs' pointer
	 */
	if (regs)
		tick_sched_handle(ts, regs);
	else
		ts->next_tick = 0;

	/*
	 * In dynticks mode, tick reprogram is deferred:
	 * - to the idle task if in dynticks-idle
	 * - to IRQ exit if in full-dynticks.
	 */
	if (unlikely(tick_sched_flag_test(ts, TS_FLAG_STOPPED)))
		return HRTIMER_NORESTART;

	hrtimer_forward(timer, now, TICK_NSEC);

	return HRTIMER_RESTART;
}

#ifdef CONFIG_NO_HZ_FULL
cpumask_var_t tick_nohz_full_mask;
EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
bool tick_nohz_full_running;
EXPORT_SYMBOL_GPL(tick_nohz_full_running);
static atomic_t tick_dep_mask;

static bool check_tick_dependency(atomic_t *dep)
{
	int val = atomic_read(dep);

	if (val & TICK_DEP_MASK_POSIX_TIMER) {
		trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER);
		return true;
	}

	if (val & TICK_DEP_MASK_PERF_EVENTS) {
		trace_tick_stop(0, TICK_DEP_MASK_PERF_EVENTS);
		return true;
	}

	if (val & TICK_DEP_MASK_SCHED) {
		trace_tick_stop(0, TICK_DEP_MASK_SCHED);
		return true;
	}

	if (val & TICK_DEP_MASK_CLOCK_UNSTABLE) {
		trace_tick_stop(0, TICK_DEP_MASK_CLOCK_UNSTABLE);
		return true;
	}

	if (val & TICK_DEP_MASK_RCU) {
		trace_tick_stop(0, TICK_DEP_MASK_RCU);
		return true;
	}

	if (val & TICK_DEP_MASK_RCU_EXP) {
		trace_tick_stop(0, TICK_DEP_MASK_RCU_EXP);
		return true;
	}

	return false;
}

static bool can_stop_full_tick(int cpu, struct tick_sched *ts)
{
	lockdep_assert_irqs_disabled();

	if (unlikely(!cpu_online(cpu)))
		return false;

	if (check_tick_dependency(&tick_dep_mask))
		return false;

	if (check_tick_dependency(&ts->tick_dep_mask))
		return false;

	if (check_tick_dependency(&current->tick_dep_mask))
		return false;

	if (check_tick_dependency(&current->signal->tick_dep_mask))
		return false;

	return true;
}

static void nohz_full_kick_func(struct irq_work *work)
{
	/* Empty, the tick restart happens on tick_nohz_irq_exit() */
}

static DEFINE_PER_CPU(struct irq_work, nohz_full_kick_work) =
	IRQ_WORK_INIT_HARD(nohz_full_kick_func);

/*
 * Kick this CPU if it's full dynticks in order to force it to
 * re-evaluate its dependency on the tick and restart it if necessary.
 * This kick, unlike tick_nohz_full_kick_cpu() and tick_nohz_full_kick_all(),
 * is NMI safe.
 */
static void tick_nohz_full_kick(void)
{
	if (!tick_nohz_full_cpu(smp_processor_id()))
		return;

	irq_work_queue(this_cpu_ptr(&nohz_full_kick_work));
}

/*
 * Kick the CPU if it's full dynticks in order to force it to
 * re-evaluate its dependency on the tick and restart it if necessary.
 */
void tick_nohz_full_kick_cpu(int cpu)
{
	if (!tick_nohz_full_cpu(cpu))
		return;

	irq_work_queue_on(&per_cpu(nohz_full_kick_work, cpu), cpu);
}

static void tick_nohz_kick_task(struct task_struct *tsk)
{
	int cpu;

	/*
	 * If the task is not running, run_posix_cpu_timers()
	 * has nothing to elapse, and an IPI can then be optimized out.
	 *
	 * activate_task()                      STORE p->tick_dep_mask
	 *   STORE p->on_rq
	 * __schedule() (switch to task 'p')    smp_mb() (atomic_fetch_or())
	 *   LOCK rq->lock                      LOAD p->on_rq
	 *   smp_mb__after_spin_lock()
	 *   tick_nohz_task_switch()
	 *     LOAD p->tick_dep_mask
	 *
	 * XXX given a task picks up the dependency on schedule(), should we
	 * only care about tasks that are currently on the CPU instead of all
	 * that are on the runqueue?
	 *
	 * That is, does this want to be: task_on_cpu() / task_curr()?
	 */
	if (!sched_task_on_rq(tsk))
		return;

	/*
	 * If the task concurrently migrates to another CPU,
	 * we guarantee it sees the new tick dependency upon
	 * schedule.
	 *
	 * set_task_cpu(p, cpu);
	 *   STORE p->cpu = @cpu
	 * __schedule() (switch to task 'p')
	 *   LOCK rq->lock
	 *   smp_mb__after_spin_lock()          STORE p->tick_dep_mask
	 *   tick_nohz_task_switch()            smp_mb() (atomic_fetch_or())
	 *      LOAD p->tick_dep_mask           LOAD p->cpu
	 */
	cpu = task_cpu(tsk);

	preempt_disable();
	if (cpu_online(cpu))
		tick_nohz_full_kick_cpu(cpu);
	preempt_enable();
}

/*
 * Kick all full dynticks CPUs in order to force these to re-evaluate
 * their dependency on the tick and restart it if necessary.
 */
static void tick_nohz_full_kick_all(void)
{
	int cpu;

	if (!tick_nohz_full_running)
		return;

	preempt_disable();
	for_each_cpu_and(cpu, tick_nohz_full_mask, cpu_online_mask)
		tick_nohz_full_kick_cpu(cpu);
	preempt_enable();
}

static void tick_nohz_dep_set_all(atomic_t *dep,
				  enum tick_dep_bits bit)
{
	int prev;

	prev = atomic_fetch_or(BIT(bit), dep);
	if (!prev)
		tick_nohz_full_kick_all();
}

/*
 * Set a global tick dependency. Used by perf events that rely on freq and
 * unstable clocks.
 */
void tick_nohz_dep_set(enum tick_dep_bits bit)
{
	tick_nohz_dep_set_all(&tick_dep_mask, bit);
}

void tick_nohz_dep_clear(enum tick_dep_bits bit)
{
	atomic_andnot(BIT(bit), &tick_dep_mask);
}

/*
 * Set per-CPU tick dependency. Used by scheduler and perf events in order to
 * manage event-throttling.
 */
void tick_nohz_dep_set_cpu(int cpu, enum tick_dep_bits bit)
{
	int prev;
	struct tick_sched *ts;

	ts = per_cpu_ptr(&tick_cpu_sched, cpu);

	prev = atomic_fetch_or(BIT(bit), &ts->tick_dep_mask);
	if (!prev) {
		preempt_disable();
		/* Perf needs local kick that is NMI safe */
		if (cpu == smp_processor_id()) {
			tick_nohz_full_kick();
		} else {
			/* Remote IRQ work not NMI-safe */
			if (!WARN_ON_ONCE(in_nmi()))
				tick_nohz_full_kick_cpu(cpu);
		}
		preempt_enable();
	}
}
EXPORT_SYMBOL_GPL(tick_nohz_dep_set_cpu);

void tick_nohz_dep_clear_cpu(int cpu, enum tick_dep_bits bit)
{
	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);

	atomic_andnot(BIT(bit), &ts->tick_dep_mask);
}
EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_cpu);

/*
 * Set a per-task tick dependency. RCU needs this. Also posix CPU timers
 * in order to elapse per task timers.
 */
void tick_nohz_dep_set_task(struct task_struct *tsk, enum tick_dep_bits bit)
{
	if (!atomic_fetch_or(BIT(bit), &tsk->tick_dep_mask))
		tick_nohz_kick_task(tsk);
}
EXPORT_SYMBOL_GPL(tick_nohz_dep_set_task);

void tick_nohz_dep_clear_task(struct task_struct *tsk, enum tick_dep_bits bit)
{
	atomic_andnot(BIT(bit), &tsk->tick_dep_mask);
}
EXPORT_SYMBOL_GPL(tick_nohz_dep_clear_task);

/*
 * Set a per-taskgroup tick dependency. Posix CPU timers need this in order to elapse
 * per process timers.
 */
void tick_nohz_dep_set_signal(struct task_struct *tsk,
			      enum tick_dep_bits bit)
{
	int prev;
	struct signal_struct *sig = tsk->signal;

	prev = atomic_fetch_or(BIT(bit), &sig->tick_dep_mask);
	if (!prev) {
		struct task_struct *t;

		lockdep_assert_held(&tsk->sighand->siglock);
		__for_each_thread(sig, t)
			tick_nohz_kick_task(t);
	}
}

void tick_nohz_dep_clear_signal(struct signal_struct *sig, enum tick_dep_bits bit)
{
	atomic_andnot(BIT(bit), &sig->tick_dep_mask);
}

/*
 * Re-evaluate the need for the tick as we switch the current task.
 * It might need the tick due to per task/process properties:
 * perf events, posix CPU timers, ...
 */
void __tick_nohz_task_switch(void)
{
	struct tick_sched *ts;

	if (!tick_nohz_full_cpu(smp_processor_id()))
		return;

	ts = this_cpu_ptr(&tick_cpu_sched);

	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
		if (atomic_read(&current->tick_dep_mask) ||
		    atomic_read(&current->signal->tick_dep_mask))
			tick_nohz_full_kick();
	}
}

/* Get the boot-time nohz CPU list from the kernel parameters. */
void __init tick_nohz_full_setup(cpumask_var_t cpumask)
{
	alloc_bootmem_cpumask_var(&tick_nohz_full_mask);
	cpumask_copy(tick_nohz_full_mask, cpumask);
	tick_nohz_full_running = true;
}

bool tick_nohz_cpu_hotpluggable(unsigned int cpu)
{
	/*
	 * The 'tick_do_timer_cpu' CPU handles housekeeping duty (unbound
	 * timers, workqueues, timekeeping, ...) on behalf of full dynticks
	 * CPUs. It must remain online when nohz full is enabled.
	 */
	if (tick_nohz_full_running && READ_ONCE(tick_do_timer_cpu) == cpu)
		return false;
	return true;
}

static int tick_nohz_cpu_down(unsigned int cpu)
{
	return tick_nohz_cpu_hotpluggable(cpu) ? 0 : -EBUSY;
}

void __init tick_nohz_init(void)
{
	int cpu, ret;

	if (!tick_nohz_full_running)
		return;

	/*
	 * Full dynticks uses IRQ work to drive the tick rescheduling on safe
	 * locking contexts. But then we need IRQ work to raise its own
	 * interrupts to avoid circular dependency on the tick.
	 */
	if (!arch_irq_work_has_interrupt()) {
		pr_warn("NO_HZ: Can't run full dynticks because arch doesn't support IRQ work self-IPIs\n");
		cpumask_clear(tick_nohz_full_mask);
		tick_nohz_full_running = false;
		return;
	}

	if (IS_ENABLED(CONFIG_PM_SLEEP_SMP) &&
			!IS_ENABLED(CONFIG_PM_SLEEP_SMP_NONZERO_CPU)) {
		cpu = smp_processor_id();

		if (cpumask_test_cpu(cpu, tick_nohz_full_mask)) {
			pr_warn("NO_HZ: Clearing %d from nohz_full range "
				"for timekeeping\n", cpu);
			cpumask_clear_cpu(cpu, tick_nohz_full_mask);
		}
	}

	for_each_cpu(cpu, tick_nohz_full_mask)
		ct_cpu_track_user(cpu);

	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
					"kernel/nohz:predown", NULL,
					tick_nohz_cpu_down);
	WARN_ON(ret < 0);
	pr_info("NO_HZ: Full dynticks CPUs: %*pbl.\n",
		cpumask_pr_args(tick_nohz_full_mask));
}
#endif /* #ifdef CONFIG_NO_HZ_FULL */

/*
 * NOHZ - aka dynamic tick functionality
 */
#ifdef CONFIG_NO_HZ_COMMON
/*
 * NO HZ enabled ?
 */
bool tick_nohz_enabled __read_mostly  = true;
unsigned long tick_nohz_active  __read_mostly;
/*
 * Enable / Disable tickless mode
 */
static int __init setup_tick_nohz(char *str)
{
	return (kstrtobool(str, &tick_nohz_enabled) == 0);
}

__setup("nohz=", setup_tick_nohz);

bool tick_nohz_tick_stopped(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	return tick_sched_flag_test(ts, TS_FLAG_STOPPED);
}

bool tick_nohz_tick_stopped_cpu(int cpu)
{
	struct tick_sched *ts = per_cpu_ptr(&tick_cpu_sched, cpu);

	return tick_sched_flag_test(ts, TS_FLAG_STOPPED);
}

/**
 * tick_nohz_update_jiffies - update jiffies when idle was interrupted
 * @now: current ktime_t
 *
 * Called from interrupt entry when the CPU was idle
 *
 * In case the sched_tick was stopped on this CPU, we have to check if jiffies
 * must be updated. Otherwise an interrupt handler could use a stale jiffy
 * value. We do this unconditionally on any CPU, as we don't know whether the
 * CPU, which has the update task assigned, is in a long sleep.
 */
static void tick_nohz_update_jiffies(ktime_t now)
{
	unsigned long flags;

	__this_cpu_write(tick_cpu_sched.idle_waketime, now);

	local_irq_save(flags);
	tick_do_update_jiffies64(now);
	local_irq_restore(flags);

	touch_softlockup_watchdog_sched();
}

static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
{
	ktime_t delta;

	if (WARN_ON_ONCE(!tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE)))
		return;

	delta = ktime_sub(now, ts->idle_entrytime);

	write_seqcount_begin(&ts->idle_sleeptime_seq);
	if (nr_iowait_cpu(smp_processor_id()) > 0)
		ts->iowait_sleeptime = ktime_add(ts->iowait_sleeptime, delta);
	else
		ts->idle_sleeptime = ktime_add(ts->idle_sleeptime, delta);

	ts->idle_entrytime = now;
	tick_sched_flag_clear(ts, TS_FLAG_IDLE_ACTIVE);
	write_seqcount_end(&ts->idle_sleeptime_seq);

	sched_clock_idle_wakeup_event();
}

static void tick_nohz_start_idle(struct tick_sched *ts)
{
	write_seqcount_begin(&ts->idle_sleeptime_seq);
	ts->idle_entrytime = ktime_get();
	tick_sched_flag_set(ts, TS_FLAG_IDLE_ACTIVE);
	write_seqcount_end(&ts->idle_sleeptime_seq);

	sched_clock_idle_sleep_event();
}

static u64 get_cpu_sleep_time_us(struct tick_sched *ts, ktime_t *sleeptime,
				 bool compute_delta, u64 *last_update_time)
{
	ktime_t now, idle;
	unsigned int seq;

	if (!tick_nohz_active)
		return -1;

	now = ktime_get();
	if (last_update_time)
		*last_update_time = ktime_to_us(now);

	do {
		seq = read_seqcount_begin(&ts->idle_sleeptime_seq);

		if (tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE) && compute_delta) {
			ktime_t delta = ktime_sub(now, ts->idle_entrytime);

			idle = ktime_add(*sleeptime, delta);
		} else {
			idle = *sleeptime;
		}
	} while (read_seqcount_retry(&ts->idle_sleeptime_seq, seq));

	return ktime_to_us(idle);

}

/**
 * get_cpu_idle_time_us - get the total idle time of a CPU
 * @cpu: CPU number to query
 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
 *
 * Return the cumulative idle time (since boot) for a given
 * CPU, in microseconds. Note that this is partially broken due to
 * the counter of iowait tasks that can be remotely updated without
 * any synchronization. Therefore it is possible to observe backward
 * values within two consecutive reads.
 *
 * This time is measured via accounting rather than sampling,
 * and is as accurate as ktime_get() is.
 *
 * Return: -1 if NOHZ is not enabled, else total idle time of the @cpu
 */
u64 get_cpu_idle_time_us(int cpu, u64 *last_update_time)
{
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

	return get_cpu_sleep_time_us(ts, &ts->idle_sleeptime,
				     !nr_iowait_cpu(cpu), last_update_time);
}
EXPORT_SYMBOL_GPL(get_cpu_idle_time_us);

/**
 * get_cpu_iowait_time_us - get the total iowait time of a CPU
 * @cpu: CPU number to query
 * @last_update_time: variable to store update time in. Do not update
 * counters if NULL.
 *
 * Return the cumulative iowait time (since boot) for a given
 * CPU, in microseconds. Note this is partially broken due to
 * the counter of iowait tasks that can be remotely updated without
 * any synchronization. Therefore it is possible to observe backward
 * values within two consecutive reads.
 *
 * This time is measured via accounting rather than sampling,
 * and is as accurate as ktime_get() is.
 *
 * Return: -1 if NOHZ is not enabled, else total iowait time of @cpu
 */
u64 get_cpu_iowait_time_us(int cpu, u64 *last_update_time)
{
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);

	return get_cpu_sleep_time_us(ts, &ts->iowait_sleeptime,
				     nr_iowait_cpu(cpu), last_update_time);
}
EXPORT_SYMBOL_GPL(get_cpu_iowait_time_us);

static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
{
	hrtimer_cancel(&ts->sched_timer);
	hrtimer_set_expires(&ts->sched_timer, ts->last_tick);

	/* Forward the time to expire in the future */
	hrtimer_forward(&ts->sched_timer, now, TICK_NSEC);

	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES)) {
		hrtimer_start_expires(&ts->sched_timer,
				      HRTIMER_MODE_ABS_PINNED_HARD);
	} else {
		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
	}

	/*
	 * Reset to make sure the next tick stop doesn't get fooled by past
	 * cached clock deadline.
	 */
	ts->next_tick = 0;
}

static inline bool local_timer_softirq_pending(void)
{
	return local_timers_pending() & BIT(TIMER_SOFTIRQ);
}

/*
 * Read jiffies and the time when jiffies were updated last
 */
u64 get_jiffies_update(unsigned long *basej)
{
	unsigned long basejiff;
	unsigned int seq;
	u64 basemono;

	do {
		seq = read_seqcount_begin(&jiffies_seq);
		basemono = last_jiffies_update;
		basejiff = jiffies;
	} while (read_seqcount_retry(&jiffies_seq, seq));
	*basej = basejiff;
	return basemono;
}

/**
 * tick_nohz_next_event() - return the clock monotonic based next event
 * @ts:		pointer to tick_sched struct
 * @cpu:	CPU number
 *
 * Return:
 * *%0		- When the next event is a maximum of TICK_NSEC in the future
 *		  and the tick is not stopped yet
 * *%next_event	- Next event based on clock monotonic
 */
static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
{
	u64 basemono, next_tick, delta, expires;
	unsigned long basejiff;
	int tick_cpu;

	basemono = get_jiffies_update(&basejiff);
	ts->last_jiffies = basejiff;
	ts->timer_expires_base = basemono;

	/*
	 * Keep the periodic tick, when RCU, architecture or irq_work
	 * requests it.
	 * Aside of that, check whether the local timer softirq is
	 * pending. If so, its a bad idea to call get_next_timer_interrupt(),
	 * because there is an already expired timer, so it will request
	 * immediate expiry, which rearms the hardware timer with a
	 * minimal delta, which brings us back to this place
	 * immediately. Lather, rinse and repeat...
	 */
	if (rcu_needs_cpu() || arch_needs_cpu() ||
	    irq_work_needs_cpu() || local_timer_softirq_pending()) {
		next_tick = basemono + TICK_NSEC;
	} else {
		/*
		 * Get the next pending timer. If high resolution
		 * timers are enabled this only takes the timer wheel
		 * timers into account. If high resolution timers are
		 * disabled this also looks at the next expiring
		 * hrtimer.
		 */
		next_tick = get_next_timer_interrupt(basejiff, basemono);
		ts->next_timer = next_tick;
	}

	/* Make sure next_tick is never before basemono! */
	if (WARN_ON_ONCE(basemono > next_tick))
		next_tick = basemono;

	/*
	 * If the tick is due in the next period, keep it ticking or
	 * force prod the timer.
	 */
	delta = next_tick - basemono;
	if (delta <= (u64)TICK_NSEC) {
		/*
		 * We've not stopped the tick yet, and there's a timer in the
		 * next period, so no point in stopping it either, bail.
		 */
		if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
			ts->timer_expires = 0;
			goto out;
		}
	}

	/*
	 * If this CPU is the one which had the do_timer() duty last, we limit
	 * the sleep time to the timekeeping 'max_deferment' value.
	 * Otherwise we can sleep as long as we want.
	 */
	delta = timekeeping_max_deferment();
	tick_cpu = READ_ONCE(tick_do_timer_cpu);
	if (tick_cpu != cpu &&
	    (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST)))
		delta = KTIME_MAX;

	/* Calculate the next expiry time */
	if (delta < (KTIME_MAX - basemono))
		expires = basemono + delta;
	else
		expires = KTIME_MAX;

	ts->timer_expires = min_t(u64, expires, next_tick);

out:
	return ts->timer_expires;
}

static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
{
	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
	unsigned long basejiff = ts->last_jiffies;
	u64 basemono = ts->timer_expires_base;
	bool timer_idle = tick_sched_flag_test(ts, TS_FLAG_STOPPED);
	int tick_cpu;
	u64 expires;

	/* Make sure we won't be trying to stop it twice in a row. */
	ts->timer_expires_base = 0;

	/*
	 * Now the tick should be stopped definitely - so the timer base needs
	 * to be marked idle as well to not miss a newly queued timer.
	 */
	expires = timer_base_try_to_set_idle(basejiff, basemono, &timer_idle);
	if (expires > ts->timer_expires) {
		/*
		 * This path could only happen when the first timer was removed
		 * between calculating the possible sleep length and now (when
		 * high resolution mode is not active, timer could also be a
		 * hrtimer).
		 *
		 * We have to stick to the original calculated expiry value to
		 * not stop the tick for too long with a shallow C-state (which
		 * was programmed by cpuidle because of an early next expiration
		 * value).
		 */
		expires = ts->timer_expires;
	}

	/* If the timer base is not idle, retain the not yet stopped tick. */
	if (!timer_idle)
		return;

	/*
	 * If this CPU is the one which updates jiffies, then give up
	 * the assignment and let it be taken by the CPU which runs
	 * the tick timer next, which might be this CPU as well. If we
	 * don't drop this here, the jiffies might be stale and
	 * do_timer() never gets invoked. Keep track of the fact that it
	 * was the one which had the do_timer() duty last.
	 */
	tick_cpu = READ_ONCE(tick_do_timer_cpu);
	if (tick_cpu == cpu) {
		WRITE_ONCE(tick_do_timer_cpu, TICK_DO_TIMER_NONE);
		tick_sched_flag_set(ts, TS_FLAG_DO_TIMER_LAST);
	} else if (tick_cpu != TICK_DO_TIMER_NONE) {
		tick_sched_flag_clear(ts, TS_FLAG_DO_TIMER_LAST);
	}

	/* Skip reprogram of event if it's not changed */
	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED) && (expires == ts->next_tick)) {
		/* Sanity check: make sure clockevent is actually programmed */
		if (expires == KTIME_MAX || ts->next_tick == hrtimer_get_expires(&ts->sched_timer))
			return;

		WARN_ONCE(1, "basemono: %llu ts->next_tick: %llu dev->next_event: %llu "
			  "timer->active: %d timer->expires: %llu\n", basemono, ts->next_tick,
			  dev->next_event, hrtimer_active(&ts->sched_timer),
			  hrtimer_get_expires(&ts->sched_timer));
	}

	/*
	 * tick_nohz_stop_tick() can be called several times before
	 * tick_nohz_restart_sched_tick() is called. This happens when
	 * interrupts arrive which do not cause a reschedule. In the first
	 * call we save the current tick time, so we can restart the
	 * scheduler tick in tick_nohz_restart_sched_tick().
	 */
	if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
		calc_load_nohz_start();
		quiet_vmstat();

		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
		tick_sched_flag_set(ts, TS_FLAG_STOPPED);
		trace_tick_stop(1, TICK_DEP_MASK_NONE);
	}

	ts->next_tick = expires;

	/*
	 * If the expiration time == KTIME_MAX, then we simply stop
	 * the tick timer.
	 */
	if (unlikely(expires == KTIME_MAX)) {
		if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
			hrtimer_cancel(&ts->sched_timer);
		else
			tick_program_event(KTIME_MAX, 1);
		return;
	}

	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES)) {
		hrtimer_start(&ts->sched_timer, expires,
			      HRTIMER_MODE_ABS_PINNED_HARD);
	} else {
		hrtimer_set_expires(&ts->sched_timer, expires);
		tick_program_event(expires, 1);
	}
}

static void tick_nohz_retain_tick(struct tick_sched *ts)
{
	ts->timer_expires_base = 0;
}

#ifdef CONFIG_NO_HZ_FULL
static void tick_nohz_full_stop_tick(struct tick_sched *ts, int cpu)
{
	if (tick_nohz_next_event(ts, cpu))
		tick_nohz_stop_tick(ts, cpu);
	else
		tick_nohz_retain_tick(ts);
}
#endif /* CONFIG_NO_HZ_FULL */

static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
{
	/* Update jiffies first */
	tick_do_update_jiffies64(now);

	/*
	 * Clear the timer idle flag, so we avoid IPIs on remote queueing and
	 * the clock forward checks in the enqueue path:
	 */
	timer_clear_idle();

	calc_load_nohz_stop();
	touch_softlockup_watchdog_sched();

	/* Cancel the scheduled timer and restore the tick: */
	tick_sched_flag_clear(ts, TS_FLAG_STOPPED);
	tick_nohz_restart(ts, now);
}

static void __tick_nohz_full_update_tick(struct tick_sched *ts,
					 ktime_t now)
{
#ifdef CONFIG_NO_HZ_FULL
	int cpu = smp_processor_id();

	if (can_stop_full_tick(cpu, ts))
		tick_nohz_full_stop_tick(ts, cpu);
	else if (tick_sched_flag_test(ts, TS_FLAG_STOPPED))
		tick_nohz_restart_sched_tick(ts, now);
#endif
}

static void tick_nohz_full_update_tick(struct tick_sched *ts)
{
	if (!tick_nohz_full_cpu(smp_processor_id()))
		return;

	if (!tick_sched_flag_test(ts, TS_FLAG_NOHZ))
		return;

	__tick_nohz_full_update_tick(ts, ktime_get());
}

/*
 * A pending softirq outside an IRQ (or softirq disabled section) context
 * should be waiting for ksoftirqd to handle it. Therefore we shouldn't
 * reach this code due to the need_resched() early check in can_stop_idle_tick().
 *
 * However if we are between CPUHP_AP_SMPBOOT_THREADS and CPU_TEARDOWN_CPU on the
 * cpu_down() process, softirqs can still be raised while ksoftirqd is parked,
 * triggering the code below, since wakep_softirqd() is ignored.
 *
 */
static bool report_idle_softirq(void)
{
	static int ratelimit;
	unsigned int pending = local_softirq_pending();

	if (likely(!pending))
		return false;

	/* Some softirqs claim to be safe against hotplug and ksoftirqd parking */
	if (!cpu_active(smp_processor_id())) {
		pending &= ~SOFTIRQ_HOTPLUG_SAFE_MASK;
		if (!pending)
			return false;
	}

	/* On RT, softirq handling may be waiting on some lock */
	if (local_bh_blocked())
		return false;

	if (ratelimit < 10) {
		pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
			pending);
		ratelimit++;
	}

	return true;
}

static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
{
	WARN_ON_ONCE(cpu_is_offline(cpu));

	if (unlikely(!tick_sched_flag_test(ts, TS_FLAG_NOHZ)))
		return false;

	if (need_resched())
		return false;

	if (unlikely(report_idle_softirq()))
		return false;

	if (tick_nohz_full_enabled()) {
		int tick_cpu = READ_ONCE(tick_do_timer_cpu);

		/*
		 * Keep the tick alive to guarantee timekeeping progression
		 * if there are full dynticks CPUs around
		 */
		if (tick_cpu == cpu)
			return false;

		/* Should not happen for nohz-full */
		if (WARN_ON_ONCE(tick_cpu == TICK_DO_TIMER_NONE))
			return false;
	}

	return true;
}

/**
 * tick_nohz_idle_stop_tick - stop the idle tick from the idle task
 *
 * When the next event is more than a tick into the future, stop the idle tick
 */
void tick_nohz_idle_stop_tick(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
	int cpu = smp_processor_id();
	ktime_t expires;

	/*
	 * If tick_nohz_get_sleep_length() ran tick_nohz_next_event(), the
	 * tick timer expiration time is known already.
	 */
	if (ts->timer_expires_base)
		expires = ts->timer_expires;
	else if (can_stop_idle_tick(cpu, ts))
		expires = tick_nohz_next_event(ts, cpu);
	else
		return;

	ts->idle_calls++;

	if (expires > 0LL) {
		int was_stopped = tick_sched_flag_test(ts, TS_FLAG_STOPPED);

		tick_nohz_stop_tick(ts, cpu);

		ts->idle_sleeps++;
		ts->idle_expires = expires;

		if (!was_stopped && tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
			ts->idle_jiffies = ts->last_jiffies;
			nohz_balance_enter_idle(cpu);
		}
	} else {
		tick_nohz_retain_tick(ts);
	}
}

void tick_nohz_idle_retain_tick(void)
{
	tick_nohz_retain_tick(this_cpu_ptr(&tick_cpu_sched));
}

/**
 * tick_nohz_idle_enter - prepare for entering idle on the current CPU
 *
 * Called when we start the idle loop.
 */
void tick_nohz_idle_enter(void)
{
	struct tick_sched *ts;

	lockdep_assert_irqs_enabled();

	local_irq_disable();

	ts = this_cpu_ptr(&tick_cpu_sched);

	WARN_ON_ONCE(ts->timer_expires_base);

	tick_sched_flag_set(ts, TS_FLAG_INIDLE);
	tick_nohz_start_idle(ts);

	local_irq_enable();
}

/**
 * tick_nohz_irq_exit - Notify the tick about IRQ exit
 *
 * A timer may have been added/modified/deleted either by the current IRQ,
 * or by another place using this IRQ as a notification. This IRQ may have
 * also updated the RCU callback list. These events may require a
 * re-evaluation of the next tick. Depending on the context:
 *
 * 1) If the CPU is idle and no resched is pending, just proceed with idle
 *    time accounting. The next tick will be re-evaluated on the next idle
 *    loop iteration.
 *
 * 2) If the CPU is nohz_full:
 *
 *    2.1) If there is any tick dependency, restart the tick if stopped.
 *
 *    2.2) If there is no tick dependency, (re-)evaluate the next tick and
 *         stop/update it accordingly.
 */
void tick_nohz_irq_exit(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	if (tick_sched_flag_test(ts, TS_FLAG_INIDLE))
		tick_nohz_start_idle(ts);
	else
		tick_nohz_full_update_tick(ts);
}

/**
 * tick_nohz_idle_got_tick - Check whether or not the tick handler has run
 *
 * Return: %true if the tick handler has run, otherwise %false
 */
bool tick_nohz_idle_got_tick(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	if (ts->got_idle_tick) {
		ts->got_idle_tick = 0;
		return true;
	}
	return false;
}

/**
 * tick_nohz_get_next_hrtimer - return the next expiration time for the hrtimer
 * or the tick, whichever expires first. Note that, if the tick has been
 * stopped, it returns the next hrtimer.
 *
 * Called from power state control code with interrupts disabled
 *
 * Return: the next expiration time
 */
ktime_t tick_nohz_get_next_hrtimer(void)
{
	return __this_cpu_read(tick_cpu_device.evtdev)->next_event;
}

/**
 * tick_nohz_get_sleep_length - return the expected length of the current sleep
 * @delta_next: duration until the next event if the tick cannot be stopped
 *
 * Called from power state control code with interrupts disabled.
 *
 * The return value of this function and/or the value returned by it through the
 * @delta_next pointer can be negative which must be taken into account by its
 * callers.
 *
 * Return: the expected length of the current sleep
 */
ktime_t tick_nohz_get_sleep_length(ktime_t *delta_next)
{
	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
	int cpu = smp_processor_id();
	/*
	 * The idle entry time is expected to be a sufficient approximation of
	 * the current time at this point.
	 */
	ktime_t now = ts->idle_entrytime;
	ktime_t next_event;

	WARN_ON_ONCE(!tick_sched_flag_test(ts, TS_FLAG_INIDLE));

	*delta_next = ktime_sub(dev->next_event, now);

	if (!can_stop_idle_tick(cpu, ts))
		return *delta_next;

	next_event = tick_nohz_next_event(ts, cpu);
	if (!next_event)
		return *delta_next;

	/*
	 * If the next highres timer to expire is earlier than 'next_event', the
	 * idle governor needs to know that.
	 */
	next_event = min_t(u64, next_event,
			   hrtimer_next_event_without(&ts->sched_timer));

	return ktime_sub(next_event, now);
}

/**
 * tick_nohz_get_idle_calls_cpu - return the current idle calls counter value
 * for a particular CPU.
 * @cpu: target CPU number
 *
 * Called from the schedutil frequency scaling governor in scheduler context.
 *
 * Return: the current idle calls counter value for @cpu
 */
unsigned long tick_nohz_get_idle_calls_cpu(int cpu)
{
	struct tick_sched *ts = tick_get_tick_sched(cpu);

	return ts->idle_calls;
}

static void tick_nohz_account_idle_time(struct tick_sched *ts,
					ktime_t now)
{
	unsigned long ticks;

	ts->idle_exittime = now;

	if (vtime_accounting_enabled_this_cpu())
		return;
	/*
	 * We stopped the tick in idle. update_process_times() would miss the
	 * time we slept, as it does only a 1 tick accounting.
	 * Enforce that this is accounted to idle !
	 */
	ticks = jiffies - ts->idle_jiffies;
	/*
	 * We might be one off. Do not randomly account a huge number of ticks!
	 */
	if (ticks && ticks < LONG_MAX)
		account_idle_ticks(ticks);
}

void tick_nohz_idle_restart_tick(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED)) {
		ktime_t now = ktime_get();
		tick_nohz_restart_sched_tick(ts, now);
		tick_nohz_account_idle_time(ts, now);
	}
}

static void tick_nohz_idle_update_tick(struct tick_sched *ts, ktime_t now)
{
	if (tick_nohz_full_cpu(smp_processor_id()))
		__tick_nohz_full_update_tick(ts, now);
	else
		tick_nohz_restart_sched_tick(ts, now);

	tick_nohz_account_idle_time(ts, now);
}

/**
 * tick_nohz_idle_exit - Update the tick upon idle task exit
 *
 * When the idle task exits, update the tick depending on the
 * following situations:
 *
 * 1) If the CPU is not in nohz_full mode (most cases), then
 *    restart the tick.
 *
 * 2) If the CPU is in nohz_full mode (corner case):
 *   2.1) If the tick can be kept stopped (no tick dependencies)
 *        then re-evaluate the next tick and try to keep it stopped
 *        as long as possible.
 *   2.2) If the tick has dependencies, restart the tick.
 *
 */
void tick_nohz_idle_exit(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
	bool idle_active, tick_stopped;
	ktime_t now;

	local_irq_disable();

	WARN_ON_ONCE(!tick_sched_flag_test(ts, TS_FLAG_INIDLE));
	WARN_ON_ONCE(ts->timer_expires_base);

	tick_sched_flag_clear(ts, TS_FLAG_INIDLE);
	idle_active = tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE);
	tick_stopped = tick_sched_flag_test(ts, TS_FLAG_STOPPED);

	if (idle_active || tick_stopped)
		now = ktime_get();

	if (idle_active)
		tick_nohz_stop_idle(ts, now);

	if (tick_stopped)
		tick_nohz_idle_update_tick(ts, now);

	local_irq_enable();
}

/*
 * In low-resolution mode, the tick handler must be implemented directly
 * at the clockevent level. hrtimer can't be used instead, because its
 * infrastructure actually relies on the tick itself as a backend in
 * low-resolution mode (see hrtimer_run_queues()).
 */
static void tick_nohz_lowres_handler(struct clock_event_device *dev)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	dev->next_event = KTIME_MAX;

	if (likely(tick_nohz_handler(&ts->sched_timer) == HRTIMER_RESTART))
		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
}

static inline void tick_nohz_activate(struct tick_sched *ts)
{
	if (!tick_nohz_enabled)
		return;
	tick_sched_flag_set(ts, TS_FLAG_NOHZ);
	/* One update is enough */
	if (!test_and_set_bit(0, &tick_nohz_active))
		timers_update_nohz();
}

/**
 * tick_nohz_switch_to_nohz - switch to NOHZ mode
 */
static void tick_nohz_switch_to_nohz(void)
{
	if (!tick_nohz_enabled)
		return;

	if (tick_switch_to_oneshot(tick_nohz_lowres_handler))
		return;

	/*
	 * Recycle the hrtimer in 'ts', so we can share the
	 * highres code.
	 */
	tick_setup_sched_timer(false);
}

static inline void tick_nohz_irq_enter(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
	ktime_t now;

	if (!tick_sched_flag_test(ts, TS_FLAG_STOPPED | TS_FLAG_IDLE_ACTIVE))
		return;
	now = ktime_get();
	if (tick_sched_flag_test(ts, TS_FLAG_IDLE_ACTIVE))
		tick_nohz_stop_idle(ts, now);
	/*
	 * If all CPUs are idle we may need to update a stale jiffies value.
	 * Note nohz_full is a special case: a timekeeper is guaranteed to stay
	 * alive but it might be busy looping with interrupts disabled in some
	 * rare case (typically stop machine). So we must make sure we have a
	 * last resort.
	 */
	if (tick_sched_flag_test(ts, TS_FLAG_STOPPED))
		tick_nohz_update_jiffies(now);
}

#else

static inline void tick_nohz_switch_to_nohz(void) { }
static inline void tick_nohz_irq_enter(void) { }
static inline void tick_nohz_activate(struct tick_sched *ts) { }

#endif /* CONFIG_NO_HZ_COMMON */

/*
 * Called from irq_enter() to notify about the possible interruption of idle()
 */
void tick_irq_enter(void)
{
	tick_check_oneshot_broadcast_this_cpu();
	tick_nohz_irq_enter();
}

static int sched_skew_tick;

static int __init skew_tick(char *str)
{
	get_option(&str, &sched_skew_tick);

	return 0;
}
early_param("skew_tick", skew_tick);

/**
 * tick_setup_sched_timer - setup the tick emulation timer
 * @hrtimer: whether to use the hrtimer or not
 */
void tick_setup_sched_timer(bool hrtimer)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	/* Emulate tick processing via per-CPU hrtimers: */
	hrtimer_setup(&ts->sched_timer, tick_nohz_handler, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_HARD);

	if (IS_ENABLED(CONFIG_HIGH_RES_TIMERS) && hrtimer)
		tick_sched_flag_set(ts, TS_FLAG_HIGHRES);

	/* Get the next period (per-CPU) */
	hrtimer_set_expires(&ts->sched_timer, tick_init_jiffy_update());

	/* Offset the tick to avert 'jiffies_lock' contention. */
	if (sched_skew_tick) {
		u64 offset = TICK_NSEC >> 1;
		do_div(offset, num_possible_cpus());
		offset *= smp_processor_id();
		hrtimer_add_expires_ns(&ts->sched_timer, offset);
	}

	hrtimer_forward_now(&ts->sched_timer, TICK_NSEC);
	if (IS_ENABLED(CONFIG_HIGH_RES_TIMERS) && hrtimer)
		hrtimer_start_expires(&ts->sched_timer, HRTIMER_MODE_ABS_PINNED_HARD);
	else
		tick_program_event(hrtimer_get_expires(&ts->sched_timer), 1);
	tick_nohz_activate(ts);
}

/*
 * Shut down the tick and make sure the CPU won't try to retake the timekeeping
 * duty before disabling IRQs in idle for the last time.
 */
void tick_sched_timer_dying(int cpu)
{
	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
	ktime_t idle_sleeptime, iowait_sleeptime;
	unsigned long idle_calls, idle_sleeps;

	/* This must happen before hrtimers are migrated! */
	if (tick_sched_flag_test(ts, TS_FLAG_HIGHRES))
		hrtimer_cancel(&ts->sched_timer);

	idle_sleeptime = ts->idle_sleeptime;
	iowait_sleeptime = ts->iowait_sleeptime;
	idle_calls = ts->idle_calls;
	idle_sleeps = ts->idle_sleeps;
	memset(ts, 0, sizeof(*ts));
	ts->idle_sleeptime = idle_sleeptime;
	ts->iowait_sleeptime = iowait_sleeptime;
	ts->idle_calls = idle_calls;
	ts->idle_sleeps = idle_sleeps;
}

/*
 * Async notification about clocksource changes
 */
void tick_clock_notify(void)
{
	int cpu;

	for_each_possible_cpu(cpu)
		set_bit(0, &per_cpu(tick_cpu_sched, cpu).check_clocks);
}

/*
 * Async notification about clock event changes
 */
void tick_oneshot_notify(void)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	set_bit(0, &ts->check_clocks);
}

/*
 * Check if a change happened, which makes oneshot possible.
 *
 * Called cyclically from the hrtimer softirq (driven by the timer
 * softirq). 'allow_nohz' signals that we can switch into low-res NOHZ
 * mode, because high resolution timers are disabled (either compile
 * or runtime). Called with interrupts disabled.
 */
int tick_check_oneshot_change(int allow_nohz)
{
	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);

	if (!test_and_clear_bit(0, &ts->check_clocks))
		return 0;

	if (tick_sched_flag_test(ts, TS_FLAG_NOHZ))
		return 0;

	if (!timekeeping_valid_for_hres() || !tick_is_oneshot_available())
		return 0;

	if (!allow_nohz)
		return 1;

	tick_nohz_switch_to_nohz();
	return 0;
}