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sched: Bail out of yield_to when source and target runqueue has one task

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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>
This commit is contained in:
Peter Zijlstra 2013-01-22 13:09:13 +05:30 committed by Gleb Natapov
parent c7c9c56ca2
commit 7b270f6099
1 changed files with 19 additions and 6 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

25
kernel/sched/core.c
View file

@ -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 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
}
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 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
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;