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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 20 Aug 2020 09:35:46 +0200
Message-ID: <CAKfTPtATpDB+ihpMf854nTdDfy4Bw_BU=21PRHVczEcE42271Q@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: reduce preemption with IDLE tasks
 runable(Internet mail)
To:     =?UTF-8?B?YmVuYmppYW5nKOiSi+W9qik=?= <benbjiang@tencent.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Jiang Biao <benbjiang@gmail.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Thu, 20 Aug 2020 at 02:13, benbjiang(=E8=92=8B=E5=BD=AA) <benbjiang@tenc=
ent.com> wrote:
>
>
>
> > On Aug 19, 2020, at 10:55 PM, Vincent Guittot <vincent.guittot@linaro.o=
rg> wrote:
> >
> > On Wed, 19 Aug 2020 at 16:27, benbjiang(=E8=92=8B=E5=BD=AA) <benbjiang@=
tencent.com> wrote:
> >>
> >>
> >>
> >>> On Aug 19, 2020, at 7:55 PM, Dietmar Eggemann <dietmar.eggemann@arm.c=
om> wrote:
> >>>
> >>> On 19/08/2020 13:05, Vincent Guittot wrote:
> >>>> On Wed, 19 Aug 2020 at 12:46, Dietmar Eggemann <dietmar.eggemann@arm=
.com> wrote:
> >>>>>
> >>>>> On 17/08/2020 14:05, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>
> >>>>>>
> >>>>>>> On Aug 17, 2020, at 4:57 PM, Dietmar Eggemann <dietmar.eggemann@a=
rm.com> wrote:
> >>>>>>>
> >>>>>>> On 14/08/2020 01:55, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>>> Hi,
> >>>>>>>>
> >>>>>>>>> On Aug 13, 2020, at 2:39 AM, Dietmar Eggemann <dietmar.eggemann=
@arm.com> wrote:
> >>>>>>>>>
> >>>>>>>>> On 12/08/2020 05:19, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>>>>> Hi,
> >>>>>>>>>>
> >>>>>>>>>>> On Aug 11, 2020, at 11:54 PM, Dietmar Eggemann <dietmar.eggem=
ann@arm.com> wrote:
> >>>>>>>>>>>
> >>>>>>>>>>> On 11/08/2020 02:41, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>>>>>>> Hi,
> >>>>>>>>>>>>
> >>>>>>>>>>>>> On Aug 10, 2020, at 9:24 PM, Dietmar Eggemann <dietmar.egge=
mann@arm.com> wrote:
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> On 06/08/2020 17:52, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>>>>>>>>> Hi,
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> On Aug 6, 2020, at 9:29 PM, Dietmar Eggemann <dietmar.egg=
emann@arm.com> wrote:
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> On 03/08/2020 13:26, benbjiang(=E8=92=8B=E5=BD=AA) wrote:
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> On Aug 3, 2020, at 4:16 PM, Dietmar Eggemann <dietmar.e=
ggemann@arm.com> wrote:
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> On 01/08/2020 04:32, Jiang Biao wrote:
> >>>>>>>>>>>>>>>>>> From: Jiang Biao <benbjiang@tencent.com>
> >>>>>
> >>>>> [...]
> >>>>>
> >>>>>>> Are you sure about this?
> >>>>>> Yes. :)
> >>>>>>>
> >>>>>>> The math is telling me for the:
> >>>>>>>
> >>>>>>> idle task:      (3 / (1024 + 1024 + 3))^(-1) * 4ms =3D 2735ms
> >>>>>>>
> >>>>>>> normal task: (1024 / (1024 + 1024 + 3))^(-1) * 4ms =3D    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 t=
o.
> >>>>>
> >>>>> 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 =3D 6ms * (1 + ilog(ncpus)) . On a 8 cores sys=
tem,
> >>>> normal task will run around 12ms in one shoot and the idle task stil=
l
> >>>> 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 doe=
s
> >>> not have other requirements preventing this.
> >> That could work for increasing the period between 2 runs. But could no=
t
> >> reduce the single runtime of idle task I guess, which means normal tas=
k
> >> 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
> That=E2=80=99s true. But in the VM case, when VM are busy(MWAIT passthrou=
gh
> or running cpu eating works), the 1-tick scheduling latency could be
> detected by cyclictest running in the VM.
>
> OTOH, we compensate vruntime in place_entity() to boot waking
> without distinguish SCHED_IDLE task, do you think it=E2=80=99s necessary =
to
> do that? like
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4115,7 +4115,7 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_en=
tity *se, int initial)
>                 vruntime +=3D sched_vslice(cfs_rq, se);
>
>         /* sleeps up to a single latency don't count. */
> -       if (!initial) {
> +       if (!initial && likely(!task_has_idle_policy(task_of(se)))) {
>                 unsigned long thresh =3D sysctl_sched_latency;

Yeah, this is a good improvement.
Does it solve your problem ?

>
> >
> >> OTOH, cgroups(shares) could introduce extra complexity. :)
> >>
> >> I wonder if there=E2=80=99s any possibility to make SCHED_IDLEs=E2=80=
=99 priorities absolutely
> >> lower than SCHED_NORMAL(OTHER), which means no weights/shares
> >> for them, and they run only when no other task=E2=80=99s 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_idl=
e task
>
> Still for VM case, different VMs have been much isolated from each other,
> priority inversion issue could be very rare, we=E2=80=99re trying to make=
 offline tasks
> absoultly harmless to online tasks. :)
>
> Thanks a lot for your time.
> Regards,
> Jiang
>
> >
> >> could avoid it by load-balance more aggressively, or it(priority inver=
sion)
> >> could be ignored in some special case.
> >>
> >>>
>