linux-kernel - Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and target runqueue has one task

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Date:	Tue, 27 Nov 2012 08:04:56 -0600
From:	Andrew Theurer <habanero@...ux.vnet.ibm.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
Cc:	Andrew Jones <drjones@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Chegu Vinod <chegu_vinod@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>
Subject: Re: [PATCH V3 RFC 1/2] sched: Bail out of yield_to when source and
 target runqueue has one task

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@...radead.org>
> >>
> >> 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@...radead.org>
> >> Raghavendra, Checking the rq length of target vcpu condition added.(thanks Avi)
> >> Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> >> Signed-off-by: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
> >> ---
> >>
> >>   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@...hat.com>
> >
> 
> 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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