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Commit 7b270f60 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Gleb Natapov
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sched: Bail out of yield_to when source and target runqueue has one task 



In case of undercomitted scenarios, especially in large guests
yield_to overhead is significantly high. when run queue length of
source and target is one, take an opportunity to bail out and return
-ESRCH. This return condition can be further exploited to quickly come
out of PLE handler.

(History: Raghavendra initially worked on break out of kvm ple handler upon
 seeing source runqueue length = 1, but it had to export rq length).
 Peter came up with the elegant idea of return -ESRCH in scheduler core.

Signed-off-by: default avatarPeter Zijlstra <peterz@infradead.org>
Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
Reviewed-by: default avatarSrikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: default avatarRaghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
Acked-by: default avatarAndrew Jones <drjones@redhat.com>
Tested-by: default avatarChegu Vinod <chegu_vinod@hp.com>
Signed-off-by: default avatarGleb Natapov <gleb@redhat.com>
parent c7c9c56c

kernel/sched/core.c

+19 −6
Original line number Diff line number Diff line
@@ -4316,7 +4316,10 @@ EXPORT_SYMBOL(yield); 
 * It's the caller's job to ensure that the target task struct

 * can't go away on us before we can do any checks.

 *

 * Returns true if we indeed boosted the target task.

 * Returns:

 *	true (>0) if we indeed boosted the target task.

 *	false (0) if we failed to boost the target.

 *	-ESRCH if there's no task to yield to.

 */

bool __sched yield_to(struct task_struct *p, bool preempt)

{
@@ -4330,6 +4333,15 @@ bool __sched yield_to(struct task_struct *p, bool preempt) 


again:

	p_rq = task_rq(p);

	/*

	 * If we're the only runnable task on the rq and target rq also

	 * has only one task, there's absolutely no point in yielding.

	 */

	if (rq->nr_running == 1 && p_rq->nr_running == 1) {

		yielded = -ESRCH;

		goto out_irq;

	}



	double_rq_lock(rq, p_rq);

	while (task_rq(p) != p_rq) {

		double_rq_unlock(rq, p_rq);
@@ -4337,13 +4349,13 @@ again: 
	}



	if (!curr->sched_class->yield_to_task)

		goto out;

		goto out_unlock;



	if (curr->sched_class != p->sched_class)

		goto out;

		goto out_unlock;



	if (task_running(p_rq, p) || p->state)

		goto out;

		goto out_unlock;



	yielded = curr->sched_class->yield_to_task(rq, p, preempt);

	if (yielded) {
@@ -4356,11 +4368,12 @@ again: 
			resched_task(p_rq->curr);

	}



out:

out_unlock:

	double_rq_unlock(rq, p_rq);

out_irq:

	local_irq_restore(flags);



	if (yielded)

	if (yielded > 0)

		schedule();



	return yielded;