linux-kernel - Re: [PATCH V3 2/2] sched/fair: Fallback to sched-idle CPU if idle CPU isn't found

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Date:   Mon, 1 Jul 2019 13:33:49 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Subhra Mazumdar <subhra.mazumdar@...cle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>, tkjos@...gle.com,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        quentin.perret@...aro.org, chris.redpath@....com,
        steven.sistare@...cle.com, songliubraving@...com
Subject: Re: [PATCH V3 2/2] sched/fair: Fallback to sched-idle CPU if idle
 CPU isn't found

On 28-06-19, 18:16, Subhra Mazumdar wrote:
> 
> On 6/25/19 10:06 PM, Viresh Kumar wrote:
> > We try to find an idle CPU to run the next task, but in case we don't
> > find an idle CPU it is better to pick a CPU which will run the task the
> > soonest, for performance reason.
> > 
> > A CPU which isn't idle but has only SCHED_IDLE activity queued on it
> > should be a good target based on this criteria as any normal fair task
> > will most likely preempt the currently running SCHED_IDLE task
> > immediately. In fact, choosing a SCHED_IDLE CPU over a fully idle one
> > shall give better results as it should be able to run the task sooner
> > than an idle CPU (which requires to be woken up from an idle state).
> > 
> > This patch updates both fast and slow paths with this optimization.
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> >   kernel/sched/fair.c | 43 +++++++++++++++++++++++++++++++++----------
> >   1 file changed, 33 insertions(+), 10 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 1277adc3e7ed..2e0527fd468c 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5376,6 +5376,15 @@ static struct {
> >   #endif /* CONFIG_NO_HZ_COMMON */
> > +/* CPU only has SCHED_IDLE tasks enqueued */
> > +static int sched_idle_cpu(int cpu)
> > +{
> > +	struct rq *rq = cpu_rq(cpu);
> > +
> > +	return unlikely(rq->nr_running == rq->cfs.idle_h_nr_running &&
> > +			rq->nr_running);
> > +}
> > +
> Shouldn't this check if rq->curr is also sched idle?

Why wouldn't the current set of checks be enough to guarantee that ?

> And why not drop the rq->nr_running non zero check?

Because CPU isn't sched-idle if nr_running and idle_h_nr_running are both 0,
i.e. it is an IDLE cpu in that case. And so I thought it is important to have
this check as well.

-- 
viresh