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

[Raghavendra K T]

Hi Ingo,

Could you please take this into x86 tree?

Thanks,
On 12/14/2012 05:59 AM, Marcelo Tosatti wrote:
> Raghavendra,
>
> Please get this integrate through x86 tree (Ingo CC'ed).
>
> 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;
> >
> > --
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