linux-kernel - Re: [PATCH] sched/fair: reduce preemption with IDLE tasks runable(Internet mail)

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Date:   Wed, 19 Aug 2020 16:55:49 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     benbjiang(蒋彪) <benbjiang@...cent.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Jiang Biao <benbjiang@...il.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "bsegall@...gle.com" <bsegall@...gle.com>,
        "mgorman@...e.de" <mgorman@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks
 runable(Internet mail)

On Wed, 19 Aug 2020 at 16:27, benbjiang(蒋彪) <benbjiang@...cent.com> wrote:
>
>
>
> > On Aug 19, 2020, at 7:55 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >
> > On 19/08/2020 13:05, Vincent Guittot wrote:
> >> On Wed, 19 Aug 2020 at 12:46, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>
> >>> On 17/08/2020 14:05, benbjiang(蒋彪) wrote:
> >>>>
> >>>>
> >>>>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>
> >>>>> On 14/08/2020 01:55, benbjiang(蒋彪) wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>>> On Aug 13, 2020, at 2:39 AM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>>>
> >>>>>>> On 12/08/2020 05:19, benbjiang(蒋彪) wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>>> On Aug 11, 2020, at 11:54 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>>>>>
> >>>>>>>>> On 11/08/2020 02:41, benbjiang(蒋彪) wrote:
> >>>>>>>>>> Hi,
> >>>>>>>>>>
> >>>>>>>>>>> On Aug 10, 2020, at 9:24 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> On 06/08/2020 17:52, benbjiang(蒋彪) wrote:
> >>>>>>>>>>>> Hi,
> >>>>>>>>>>>>
> >>>>>>>>>>>>> On Aug 6, 2020, at 9:29 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On 03/08/2020 13:26, benbjiang(蒋彪) wrote:
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote:
> >>>>>>>>>>>>>>>> From: Jiang Biao <benbjiang@...cent.com>
> >>>
> >>> [...]
> >>>
> >>>>> Are you sure about this?
> >>>> Yes. :)
> >>>>>
> >>>>> The math is telling me for the:
> >>>>>
> >>>>> idle task:      (3 / (1024 + 1024 + 3))^(-1) * 4ms = 2735ms
> >>>>>
> >>>>> normal task: (1024 / (1024 + 1024 + 3))^(-1) * 4ms =    8ms
> >>>>>
> >>>>> (4ms - 250 Hz)
> >>>> My tick is 1ms - 1000HZ, which seems reasonable for 600ms? :)
> >>>
> >>> OK, I see.
> >>>
> >>> But here the different sched slices (check_preempt_tick()->
> >>> sched_slice()) between normal tasks and the idle task play a role to.
> >>>
> >>> Normal tasks get ~3ms whereas the idle task gets <0.01ms.
> >>
> >> In fact that depends on the number of CPUs on the system
> >> :sysctl_sched_latency = 6ms * (1 + ilog(ncpus)) . On a 8 cores system,
> >> normal task will run around 12ms in one shoot and the idle task still
> >> one tick period
> >
> > True. This is on a single CPU.
> Agree. :)
>
> >
> >> Also, you can increase even more the period between 2 runs of idle
> >> task by using cgroups and min shares value : 2
> >
> > Ah yes, maybe this is what Jiang wants to do then? If his runtime does
> > not have other requirements preventing this.
> That could work for increasing the period between 2 runs. But could not
> reduce the single runtime of idle task I guess, which means normal task
> could have 1-tick schedule latency because of idle task.

Yes.  An idle task will preempt an always running task during 1 tick
every 680ms. But also you should keep in mind that a waking normal
task will preempt the idle task immediately which means that it will
not add scheduling latency to a normal task but "steal" 0.14% of
normal task throughput (1/680) at most

> OTOH, cgroups(shares) could introduce extra complexity. :)
>
> I wonder if there’s any possibility to make SCHED_IDLEs’ priorities absolutely
> lower than SCHED_NORMAL(OTHER), which means no weights/shares
> for them, and they run only when no other task’s runnable.
> I guess there may be priority inversion issue if we do that. But maybe we

Exactly, that's why we must ensure a minimum running time for sched_idle task

> could avoid it by load-balance more aggressively, or it(priority inversion)
> could be ignored in some special case.
>
> Thx.
> Regard,
> Jiang
>
> >
> > [...]
>