linux-kernel - Re: [PATCH v3 3/4] sched: reduce sched slice for SCHED

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Message-ID: <CAKfTPtDDNgbneGrQ+p8FOKLZ=SqvY59GO+itYwje6MU7xEN9LQ@mail.gmail.com>
Date:   Tue, 24 Aug 2021 09:56:09 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Paul Turner <pjt@...gle.com>,
        Oleg Rombakh <olegrom@...gle.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Steve Sistare <steven.sistare@...cle.com>,
        Tejun Heo <tj@...nel.org>, Rik van Riel <riel@...riel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] sched: reduce sched slice for SCHED_IDLE entities

On Mon, 23 Aug 2021 at 19:40, Josh Don <joshdon@...gle.com> wrote:
>
> On Mon, Aug 23, 2021 at 3:08 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Fri, 20 Aug 2021 at 03:04, Josh Don <joshdon@...gle.com> wrote:
> > >
> > > @@ -684,12 +696,13 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >         for_each_sched_entity(se) {
> > >                 struct load_weight *load;
> > >                 struct load_weight lw;
> > > +               struct cfs_rq *qcfs_rq;
> > >
> > > -               cfs_rq = cfs_rq_of(se);
> > > -               load = &cfs_rq->load;
> > > +               qcfs_rq = cfs_rq_of(se);
> > > +               load = &qcfs_rq->load;
> > >
> > >                 if (unlikely(!se->on_rq)) {
> > > -                       lw = cfs_rq->load;
> > > +                       lw = qcfs_rq->load;
> > >
> > >                         update_load_add(&lw, se->load.weight);
> > >                         load = &lw;
> > > @@ -697,8 +710,14 @@ static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >                 slice = __calc_delta(slice, se->load.weight, load);
> > >         }
> > >
> > > -       if (sched_feat(BASE_SLICE))
> > > -               slice = max(slice, (u64)sysctl_sched_min_granularity);
> > > +       if (sched_feat(BASE_SLICE)) {
> > > +               if (se_is_idle(init_se) && !sched_idle_cfs_rq(cfs_rq))
> >
> > Like for place_entity, we should probably not dynamically switch
> > between the 2 values below depending on the presence or not of non
> > sched idle tasks and always use sysctl_sched_idle_min_granularity
>
> My reasoning here is that sched_slice is something we reasonably
> expect to change as tasks enqueue/dequeue, and unlike place_entity()
> it does not create fairness issues by messing with vruntime.
> Additionally, it would be preferable to use the larger min granularity
> on a cpu running only idle tasks.

Fair enough

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>