sched: Bail out of yield_to when source and target runqueue has one task (7b270f60) · Commits · 戴 / test

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:

Peter Zijlstra <peterz@infradead.org> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi) Reviewed-by:

Srikar Dronamraju <srikar@linux.vnet.ibm.com> Signed-off-by:

Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> Acked-by:

Andrew Jones <drjones@redhat.com> Tested-by:

Chegu Vinod <chegu_vinod@hp.com> Signed-off-by:

Gleb Natapov <gleb@redhat.com>

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;

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