linux-kernel - Re: [PATCH 2/2] sched: adjust SCHED

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Date:   Thu, 12 Aug 2021 14:09:15 -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 2/2] sched: adjust SCHED_IDLE interactions

> > @@ -697,8 +699,18 @@ 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)w);
> > +       if (sched_feat(BASE_SLICE)) {
> > +               /*
> > +                * SCHED_IDLE entities are not subject to min_granularity if
> > +                * they are competing with non SCHED_IDLE entities. As a result,
> > +                * non SCHED_IDLE entities will have reduced latency to get back
> > +                * on cpu, at the cost of increased context switch frequency of
> > +                * SCHED_IDLE entities.
> > +                */
>
> Ensuring that the entity will have a minimum runtime has been added to
> ensure that we let enough time to move forward.
> If you exclude sched_idle entities from this min runtime, the
> sched_slice of an idle_entity will be really small.
> I don't have details of your example above but I can imagine that it's
> a 16 cpus system which means a sysctl_sched_min_granularity=3.75ms
> which explains the 4ms running time of an idle entity
> For a 16 cpus system, the sched_slice of an idle_entity in your
> example in the cover letter is: 6*(1+log2(16))*3/1027=87us. Of course
> this become even worse with more threads and cgroups or thread with
> ncie prio -19
>
> This value is then used to set the next hrtimer event in SCHED_HRTICK
> and 87us is too small to make any progress
>
> The 1ms of your test comes from the tick which could be a good
> candidate for a min value or the
> normalized_sysctl_sched_min_granularity which has the advantage of not
> increasing with number of CPU

Fair point, this shouldn't completely ignore min granularity. Something like

unsigned int sysctl_sched_idle_min_granularity = NSEC_PER_MSEC;

(and still only using this value instead of the default
min_granularity when the SCHED_IDLE entity is competing with normal
entities)

> > @@ -4216,7 +4228,15 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int initial)
> >                 if (sched_feat(GENTLE_FAIR_SLEEPERS))
> >                         thresh >>= 1;
> >
> > -               vruntime -= thresh;
> > +               /*
> > +                * Don't give sleep credit to a SCHED_IDLE entity if we're
> > +                * placing it onto a cfs_rq with non SCHED_IDLE entities.
> > +                */
> > +               if (!se_is_idle(se) ||
> > +                   cfs_rq->h_nr_running == cfs_rq->idle_h_nr_running)
>
> Can't this condition above create unfairness between idle entities ?
> idle thread 1 wake up while normal thread is running
> normal thread thread sleeps immediately after
> idle thread 2 wakes up just after and gets some credits compared to the 1st one.

Yes, this sacrifices some idle<->idle fairness when there is a normal
thread that comes and goes. One alternative is to simply further
reduce thresh for idle entities. That will interfere with idle<->idle
fairness when there are no normal threads, which is why I opted for
the former. On second thought though, the former fairness issue seems
more problematic. Thoughts on applying a smaller sleep credit
threshold universally to idle entities?