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

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Message-ID: <CAKfTPtATf94n4t6EnB5_76ygox26xd9EwvEHiv5cGN=E9BdLgg@mail.gmail.com>
Date:   Wed, 19 Aug 2020 13:05:22 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     benbjiang(蒋彪) <benbjiang@...cent.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 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

Also, you can increase even more the period between 2 runs of idle
task by using cgroups and min shares value : 2

>
> So the idle task runs every ~680ms but only for 1 tick (1ms) (4 times
> less than the normal tasks). The condition 'if (delta_exec >
> ideal_runtime)' in check_preempt_tick() is only true at the 4th tick
> when a normal task runs even though the slice is 3ms.
>
> In the 250 Hz example the sched slice diffs are hidden behind the 4ms tick.
>
> [...]