1 /* SPDX-License-Identifier: GPL-2.0 */ 2 /* 3 * BPF extensible scheduler class: Documentation/scheduler/sched-ext.rst 4 * 5 * Sub-scheduler hierarchy support. 6 * 7 * Copyright (c) 2026 Meta Platforms, Inc. and affiliates. 8 * Copyright (c) 2026 Tejun Heo <tj@kernel.org> 9 */ 10 #ifndef _KERNEL_SCHED_EXT_SUB_H 11 #define _KERNEL_SCHED_EXT_SUB_H 12 13 #include "internal.h" 14 #include "cid.h" 15 16 #ifdef CONFIG_EXT_SUB_SCHED 17 18 struct scx_sched *scx_skip_subtree_pre(struct scx_sched *pos, struct scx_sched *root); 19 struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos, struct scx_sched *root); 20 void scx_set_task_sched(struct task_struct *p, struct scx_sched *sch); 21 struct cgroup *sch_cgroup(struct scx_sched *sch); 22 void set_cgroup_sched(struct cgroup *cgrp, struct scx_sched *sch); 23 void scx_pstack_recursion_on_dispatch(struct bpf_prog *prog); 24 void drain_descendants(struct scx_sched *sch); 25 void scx_sub_disable(struct scx_sched *sch); 26 void scx_sub_enable_workfn(struct kthread_work *work); 27 bool scx_bpf_sub_dispatch(u64 cgroup_id, const struct bpf_prog_aux *aux); 28 void scx_free_pshards(struct scx_sched *sch); 29 s32 scx_alloc_pshards(struct scx_sched *sch); 30 31 static inline const char *sch_cgrp_path(struct scx_sched *sch) 32 { 33 return sch->cgrp_path; 34 } 35 36 #else /* CONFIG_EXT_SUB_SCHED */ 37 38 static inline struct scx_sched *scx_next_descendant_pre(struct scx_sched *pos, struct scx_sched *root) { return pos ? NULL : root; } 39 static inline struct scx_sched *scx_skip_subtree_pre(struct scx_sched *pos, struct scx_sched *root) { return NULL; } 40 static inline void scx_set_task_sched(struct task_struct *p, struct scx_sched *sch) {} 41 static inline struct cgroup *sch_cgroup(struct scx_sched *sch) { return NULL; } 42 static inline const char *sch_cgrp_path(struct scx_sched *sch) { return "/"; } 43 static inline void set_cgroup_sched(struct cgroup *cgrp, struct scx_sched *sch) {} 44 static inline void drain_descendants(struct scx_sched *sch) { } 45 static inline void scx_sub_disable(struct scx_sched *sch) { } 46 static inline void scx_free_pshards(struct scx_sched *sch) {} 47 static inline s32 scx_alloc_pshards(struct scx_sched *sch) { return 0; } 48 49 #endif /* CONFIG_EXT_SUB_SCHED */ 50 51 /** 52 * scx_for_each_descendant_pre - pre-order walk of a sched's descendants 53 * @pos: iteration cursor 54 * @root: sched to walk the descendants of 55 * 56 * Walk @root's descendants. @root is included in the iteration and the first 57 * node to be visited. Must be called with scx_enable_mutex, scx_sched_lock, or 58 * RCU read lock. 59 */ 60 #define scx_for_each_descendant_pre(pos, root) \ 61 for ((pos) = scx_next_descendant_pre(NULL, (root)); (pos); \ 62 (pos) = scx_next_descendant_pre((pos), (root))) 63 64 /* 65 * One user of this function is scx_bpf_dispatch() which can be called 66 * recursively as sub-sched dispatches nest. Always inline to reduce stack usage 67 * from the call frame. 68 */ 69 static __always_inline bool 70 scx_dispatch_sched(struct scx_sched *sch, struct rq *rq, 71 struct task_struct *prev, bool nested) 72 { 73 struct scx_dsp_ctx *dspc = &this_cpu_ptr(sch->pcpu)->dsp_ctx; 74 int nr_loops = SCX_DSP_MAX_LOOPS; 75 s32 cpu = cpu_of(rq); 76 bool prev_on_sch = (prev->sched_class == &ext_sched_class) && 77 scx_task_on_sched(sch, prev); 78 79 if (scx_consume_global_dsq(sch, rq)) 80 return true; 81 82 if (scx_bypass_dsp_enabled(sch)) { 83 /* if @sch is bypassing, only the bypass DSQs are active */ 84 if (scx_bypassing(sch, cpu)) 85 return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0); 86 87 #ifdef CONFIG_EXT_SUB_SCHED 88 /* 89 * If @sch isn't bypassing but its children are, @sch is 90 * responsible for making forward progress for both its own 91 * tasks that aren't bypassing and the bypassing descendants' 92 * tasks. The following implements a simple built-in behavior - 93 * let each CPU try to run the bypass DSQ every Nth time. 94 * 95 * Later, if necessary, we can add an ops flag to suppress the 96 * auto-consumption and a kfunc to consume the bypass DSQ and, 97 * so that the BPF scheduler can fully control scheduling of 98 * bypassed tasks. 99 */ 100 struct scx_sched_pcpu *pcpu = per_cpu_ptr(sch->pcpu, cpu); 101 102 if (!(pcpu->bypass_host_seq++ % SCX_BYPASS_HOST_NTH) && 103 scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0)) { 104 __scx_add_event(sch, SCX_EV_SUB_BYPASS_DISPATCH, 1); 105 return true; 106 } 107 #endif /* CONFIG_EXT_SUB_SCHED */ 108 } 109 110 if (unlikely(!SCX_HAS_OP(sch, dispatch)) || !scx_rq_online(rq)) 111 return false; 112 113 dspc->rq = rq; 114 115 /* 116 * The dispatch loop. Because scx_flush_dispatch_buf() may drop the rq 117 * lock, the local DSQ might still end up empty after a successful 118 * ops.dispatch(). If the local DSQ is empty even after ops.dispatch() 119 * produced some tasks, retry. The BPF scheduler may depend on this 120 * looping behavior to simplify its implementation. 121 */ 122 do { 123 dspc->nr_tasks = 0; 124 125 if (nested) { 126 SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu), 127 prev_on_sch ? prev : NULL); 128 } else { 129 /* stash @prev so that nested invocations can access it */ 130 rq->scx.sub_dispatch_prev = prev; 131 SCX_CALL_OP(sch, dispatch, rq, scx_cpu_arg(cpu), 132 prev_on_sch ? prev : NULL); 133 rq->scx.sub_dispatch_prev = NULL; 134 } 135 136 scx_flush_dispatch_buf(sch, rq); 137 138 if ((prev->scx.flags & SCX_TASK_QUEUED) && prev->scx.slice) { 139 rq->scx.flags |= SCX_RQ_BAL_KEEP; 140 return true; 141 } 142 if (rq->scx.local_dsq.nr) 143 return true; 144 if (scx_consume_global_dsq(sch, rq)) 145 return true; 146 147 /* 148 * ops.dispatch() can trap us in this loop by repeatedly 149 * dispatching ineligible tasks. Break out once in a while to 150 * allow the watchdog to run. As IRQ can't be enabled in 151 * balance(), we want to complete this scheduling cycle and then 152 * start a new one. IOW, we want to call resched_curr() on the 153 * next, most likely idle, task, not the current one. Use 154 * __scx_bpf_kick_cpu() for deferred kicking. 155 */ 156 if (unlikely(!--nr_loops)) { 157 scx_kick_cpu(sch, cpu, 0); 158 break; 159 } 160 } while (dspc->nr_tasks); 161 162 /* 163 * Prevent the CPU from going idle while bypassed descendants have tasks 164 * queued. Without this fallback, bypassed tasks could stall if the host 165 * scheduler's ops.dispatch() doesn't yield any tasks. 166 */ 167 if (scx_bypass_dsp_enabled(sch)) 168 return scx_consume_dispatch_q(sch, rq, scx_bypass_dsq(sch, cpu), 0); 169 170 return false; 171 } 172 173 #endif /* _KERNEL_SCHED_EXT_SUB_H */ 174