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From:   Jiang Biao <benbjiang@gmail.com>
Date:   Wed, 25 Aug 2021 10:43:56 +0800
Message-ID: <CAPJCdBk-8vjg6GtGx3ft3KoyeeXvovxF53kwE7vJGY2eTYOwyg@mail.gmail.com>
Subject: Re: [PATCH v3 3/4] sched: reduce sched slice for SCHED_IDLE entities
To:     Josh Don <joshdon@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Paul Turner <pjt@google.com>,
        Oleg Rombakh <olegrom@google.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Steve Sistare <steven.sistare@oracle.com>,
        Tejun Heo <tj@kernel.org>, Rik van Riel <riel@surriel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
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On Wed, 25 Aug 2021 at 01:04, Josh Don <joshdon@google.com> wrote:
>
> Hi Jiang,
>
> On Tue, Aug 24, 2021 at 3:25 AM Jiang Biao <benbjiang@gmail.com> wrote:
> >
> > Why not just ignore min granularity when normal entities compete with
> > a SCHED_IDLE entity? something like this,
> >
> > @@ -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)
> >  +          && (!se_is_idle(init_se) || sched_idle_cfs_rq(cfs_rq)))
> >  +               slice = max(slice, (u64)sysctl_sched_min_granularity);
> >
> >          return slice;
> >   }
> > If so, there seems no need to introduce sysctl_sched_idle_min_granularity? :)
>
> Ignoring min_gran entirely could lead to some really tiny slices; see
> discussion at https://lkml.org/lkml/2021/8/12/651.
Got it, tiny slices could be a problem in SCHED_HRTICK case.
But the sysctl_sched_idle_min_granularity used in sched_slice() and
sysctl_sched_min_granularity used in check_preempt_tick would have
different semantics for SCHED_IDLE task, which could be functional ok
but a little confusing.

Regards,
Jiang