linux-kernel - Re: [PATCH] sched: Fast idling of CPU when system is partially loaded

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Date:	Fri, 13 Jun 2014 12:18:47 -0700
From:	Jason Low <jason.low2@...com>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	Davidlohr Bueson <davidlohr@...com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Fast idling of CPU when system is partially
 loaded

On Fri, 2014-06-13 at 09:28 -0700, Tim Chen wrote:
> On Thu, 2014-06-12 at 23:01 -0700, Jason Low wrote:
> > On Thu, 2014-06-12 at 14:25 -0700, Tim Chen wrote:
> > 
> > > Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> > > ---
> > >  kernel/sched/core.c  | 12 ++++++++----
> > >  kernel/sched/fair.c  | 23 +++++++++++++++++++++--
> > >  kernel/sched/sched.h | 10 ++++++++--
> > >  3 files changed, 37 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index c6b9879..4f57221 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -2630,7 +2630,7 @@ static inline struct task_struct *
> > >  pick_next_task(struct rq *rq, struct task_struct *prev)
> > >  {
> > >  	const struct sched_class *class = &fair_sched_class;
> > > -	struct task_struct *p;
> > > +	struct task_struct *p = NULL;
> > >  
> > >  	/*
> > >  	 * Optimization: we know that if all tasks are in
> > > @@ -2638,9 +2638,13 @@ pick_next_task(struct rq *rq, struct task_struct *prev)
> > >  	 */
> > >  	if (likely(prev->sched_class == class &&
> > >  		   rq->nr_running == rq->cfs.h_nr_running)) {
> > > -		p = fair_sched_class.pick_next_task(rq, prev);
> > > -		if (unlikely(p == RETRY_TASK))
> > > -			goto again;
> > > +
> > > +		/* If no cpu has more than 1 task, skip */
> > > +		if (rq->nr_running > 0 || rq->rd->overload) {
> > 
> > Hi Tim,
> > 
> > If it is skipping if no cpu has more than 1 task, should the
> > above have the additional check for (rq->nr_running > 1) instead
> > of (rq->nr_running > 0)?
> 
> If you have a job on your local cpu, you do want to have the scheduler
> pick the task to run.

I see. In that case, if a CPU is going idle, it still needs to call
idle_balance() to update rq->idle_stamp and rq->next_balance (even if we
skip calling the expensive load_balance).

What do you think about moving the overload check to the top of
idle_balance():

        this_rq->idle_stamp = rq_clock(this_rq);
 
-       if (this_rq->avg_idle < sysctl_sched_migration_cost) {
+       if (this_rq->avg_idle < sysctl_sched_migration_cost ||
+           !this_rq->rd->overload) {
                rcu_read_lock();
                sd = rcu_dereference_check_sched_domain(this_rq->sd);
                if (sd)


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