Commit c6e7bd7a authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Ingo Molnar
Browse files

sched/core: Optimize ttwu() spinning on p->on_cpu 



Both Rik and Mel reported seeing ttwu() spend significant time on:

  smp_cond_load_acquire(&p->on_cpu, !VAL);

Attempt to avoid this by queueing the wakeup on the CPU that owns the
p->on_cpu value. This will then allow the ttwu() to complete without
further waiting.

Since we run schedule() with interrupts disabled, the IPI is
guaranteed to happen after p->on_cpu is cleared, this is what makes it
safe to queue early.

Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarMel Gorman <mgorman@techsingularity.net>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Jirka Hladky <jhladky@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: valentin.schneider@arm.com
Cc: Hillf Danton <hdanton@sina.com>
Cc: Rik van Riel <riel@surriel.com>
Link: https://lore.kernel.org/r/20200524202956.27665-2-mgorman@techsingularity.net
parent d505b8af

kernel/sched/core.c

+31 −21
Original line number Diff line number Diff line
@@ -2312,7 +2312,7 @@ static void wake_csd_func(void *info) 
	sched_ttwu_pending();

}



static void ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)

static void __ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)

{

	struct rq *rq = cpu_rq(cpu);
@@ -2354,6 +2354,17 @@ bool cpus_share_cache(int this_cpu, int that_cpu) 
{

	return per_cpu(sd_llc_id, this_cpu) == per_cpu(sd_llc_id, that_cpu);

}



static bool ttwu_queue_remote(struct task_struct *p, int cpu, int wake_flags)

{

	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {

		sched_clock_cpu(cpu); /* Sync clocks across CPUs */

		__ttwu_queue_remote(p, cpu, wake_flags);

		return true;

	}



	return false;

}

#endif /* CONFIG_SMP */



static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags)
@@ -2362,11 +2373,8 @@ static void ttwu_queue(struct task_struct *p, int cpu, int wake_flags) 
	struct rq_flags rf;



#if defined(CONFIG_SMP)

	if (sched_feat(TTWU_QUEUE) && !cpus_share_cache(smp_processor_id(), cpu)) {

		sched_clock_cpu(cpu); /* Sync clocks across CPUs */

		ttwu_queue_remote(p, cpu, wake_flags);

	if (ttwu_queue_remote(p, cpu, wake_flags))

		return;

	}

#endif



	rq_lock(rq, &rf);
@@ -2548,7 +2556,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) 
	if (p->on_rq && ttwu_remote(p, wake_flags))

		goto unlock;



	if (p->in_iowait) {

		delayacct_blkio_end(p);

		atomic_dec(&task_rq(p)->nr_iowait);

	}



#ifdef CONFIG_SMP

	p->sched_contributes_to_load = !!task_contributes_to_load(p);

	p->state = TASK_WAKING;



	/*

	 * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be

	 * possible to, falsely, observe p->on_cpu == 0.
@@ -2570,6 +2586,16 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) 
	 */

	smp_rmb();



	/*

	 * If the owning (remote) CPU is still in the middle of schedule() with

	 * this task as prev, considering queueing p on the remote CPUs wake_list

	 * which potentially sends an IPI instead of spinning on p->on_cpu to

	 * let the waker make forward progress. This is safe because IRQs are

	 * disabled and the IPI will deliver after on_cpu is cleared.

	 */

	if (READ_ONCE(p->on_cpu) && ttwu_queue_remote(p, cpu, wake_flags))

		goto unlock;



	/*

	 * If the owning (remote) CPU is still in the middle of schedule() with

	 * this task as prev, wait until its done referencing the task.
@@ -2581,28 +2607,12 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) 
	 */

	smp_cond_load_acquire(&p->on_cpu, !VAL);



	p->sched_contributes_to_load = !!task_contributes_to_load(p);

	p->state = TASK_WAKING;



	if (p->in_iowait) {

		delayacct_blkio_end(p);

		atomic_dec(&task_rq(p)->nr_iowait);

	}



	cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags);

	if (task_cpu(p) != cpu) {

		wake_flags |= WF_MIGRATED;

		psi_ttwu_dequeue(p);

		set_task_cpu(p, cpu);

	}



#else /* CONFIG_SMP */



	if (p->in_iowait) {

		delayacct_blkio_end(p);

		atomic_dec(&task_rq(p)->nr_iowait);

	}



#endif /* CONFIG_SMP */



	ttwu_queue(p, cpu, wake_flags);

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