Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source andtarget runqueue has one task
From: Andrew Theurer
Date: Tue Nov 27 2012 - 09:06:29 EST
On Tue, 2012-11-27 at 16:00 +0530, Raghavendra K T wrote:
> On 11/26/2012 07:05 PM, Andrew Jones wrote:
> > On Mon, Nov 26, 2012 at 05:37:54PM +0530, Raghavendra K T wrote:
> >> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxx>
> >> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
> >> Reviewed-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> >> ---
> >>
> >> kernel/sched/core.c | 25 +++++++++++++++++++------
> >> 1 file changed, 19 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >> index 2d8927f..fc219a5 100644
> >> --- a/kernel/sched/core.c
> >> +++ b/kernel/sched/core.c
> >> @@ -4289,7 +4289,10 @@ 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)
> >> {
> >> @@ -4303,6 +4306,15 @@ 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);
> >> @@ -4310,13 +4322,13 @@ 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) {
> >> @@ -4329,11 +4341,12 @@ 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;
> >>
> >
> > Acked-by: Andrew Jones <drjones@xxxxxxxxxx>
> >
>
> Thank you Drew.
>
> Marcelo Gleb.. Please let me know if you have comments / concerns on the
> patches..
>
> Andrew, Vinod, IMO, the patch set looks good for undercommit scenarios
> especially for large guests where we do have overhead of vcpu iteration
> of ple handler..
I agree, looks fine for undercommit scenarios. I do wonder what happens
with 1.5x overcommit, where we might see 1/2 the host cpus with runqueue
of 2 and 1/2 of the host cpus with a runqueue of 1. Even with this
change that scenario still might be fine, but it would be nice to see a
comparison.
-Andrew
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