Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source andtarget runqueue has one task

[Chegu Vinod]

On 11/27/2012 2:30 AM, 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..
>
> .
Thanks Raghu. Will try to get this latest patch set evaluated and get 
back to you.

Vinod

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