linux-kernel - Re: [PATCH v4 6/8] sched/fair: Add sched group latency support

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Message-ID: <CAKfTPtBCgJwwvEbKYTQZaczE+z+akoJy5607tFF897CK7w+WUg@mail.gmail.com>
Date:   Tue, 20 Sep 2022 09:02:54 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        parth@...ux.ibm.com, qais.yousef@....com, chris.hyser@...cle.com,
        valentin.schneider@....com, patrick.bellasi@...bug.net,
        David.Laight@...lab.com, pjt@...gle.com, pavel@....cz,
        qperret@...gle.com, tim.c.chen@...ux.intel.com, joshdon@...gle.com
Subject: Re: [PATCH v4 6/8] sched/fair: Add sched group latency support

On Mon, 19 Sept 2022 at 19:34, Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Mon, Sep 19, 2022 at 01:55:15PM +0200, Dietmar Eggemann wrote:
> > s/valentin.schneider@....com//
> >
> > On 16/09/2022 10:03, Vincent Guittot wrote:
> > > Task can set its latency priority, which is then used to decide to preempt
> > > the current running entity of the cfs, but sched group entities still have
> > > the default latency offset.
> > >
> > > Add a latency field in task group to set the latency offset of the
> > > sched_eneities of the group, which will be used against other entities in
> >
> > s/sched_eneities/sched_entity
> >
> > > the parent cfs when deciding which entity to schedule first.
> >
> > So latency for cgroups does not follow any (existing) Resource
> > Distribution Model/Scheme (Documentation/admin-guide/cgroup-v2.rst)?
> > Latency values are only used to compare sched entities at the same level.
>
> I think it'd still result in a hierarchical behavior as scheduling is done
> recursively top-down. Right, vincent?

Correct

>
> It doesn't fit any of the resource distribution model. But it's rather
> difficult to map latencies to existing concepts of resources and we have a
> precedence in the cpu controller (.idle) which behaves similarly, so as long
> as the behavior is hierarchical, I think it's okay.
>
> Thanks.
>
> --
> tejun