Commit 7b270f60 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Gleb Natapov
Browse files

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 <>
Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
Reviewed-by: default avatarSrikar Dronamraju <>
Signed-off-by: default avatarRaghavendra K T <>
Acked-by: default avatarAndrew Jones <>
Tested-by: default avatarChegu Vinod <>
Signed-off-by: default avatarGleb Natapov <>
parent c7c9c56c
......@@ -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)
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:
double_rq_unlock(rq, p_rq);
if (yielded)
if (yielded > 0)
return yielded;
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