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

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Date:   Mon, 23 Aug 2021 10:40:18 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
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, 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.