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

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Message-ID: <CAKfTPtBCUu80dpGneyhsZneTtcqcm3DrVVoqv6rH60oX_vEz+Q@mail.gmail.com>
Date:   Wed, 21 Sep 2022 19:02:57 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     Qais Yousef <qais.yousef@....com>,
        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, 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 Wed, 21 Sept 2022 at 18:48, Tejun Heo <tj@...nel.org> wrote:
>
> Hello,
>
> On Wed, Sep 21, 2022 at 05:07:38PM +0100, Qais Yousef wrote:
> > Wouldn't cpu.latency.nice be enough? I think the latency_offset is
> > implementation detail that userspace shouldn't be concerned about.
>
> One option could be just using the same mapping as cpu.weight so that 100
> maps to neutral, 10000 maps close to -20, 1 maps close to 19. It isn't great
> that the value can't be interpreted in any intuitive way (e.g. a time
> duration based interface would be a lot easier to grok even if it still is
> best effort) but if that's what the per-task interface is gonna be, it'd be
> best to keep cgroup interface in line.

I would prefer a signed range like the [-1000:1000] as the behavior is
different for sensitive and non sensitive task unlike the cpu.weight
which is reflect that a bigger value get more

>
> As for whether a single value would fit the bill, it's again something which
> should be answered for both task and cgroup based interface at the same
> time. That said, my not-too-throught-through opinion is that a single value
> for per-task / per-cgroup interface + system level knobs to fine tune how
> that actually applies is likely enough and probably better than exposing
> exposing a host of internal details to applications directly.
>
> Thanks.
>
> --
> tejun