linux-kernel - Re: [PATCH v7 6/9] sched/fair: Add sched group latency support

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Message-ID: <CAKfTPtCfL7KZSyJmGRaW0KhGtkdZkk0JJDF0w_sgiSmZ-x-DSA@mail.gmail.com>
Date:   Fri, 4 Nov 2022 11:57:39 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Qais Yousef <qyousef@...alina.io>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        dietmar.eggemann@....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,
        patrick.bellasi@...bug.net, David.Laight@...lab.com,
        pjt@...gle.com, pavel@....cz, tj@...nel.org, qperret@...gle.com,
        tim.c.chen@...ux.intel.com, joshdon@...gle.com, timj@....org,
        kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org
Subject: Re: [PATCH v7 6/9] sched/fair: Add sched group latency support

On Fri, 4 Nov 2022 at 11:48, Joel Fernandes <joel@...lfernandes.org> wrote:
>
> On Fri, Nov 4, 2022 at 10:37 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> [...]
> > > during *same CPU* competition between different groups by juggling
> > > around the wakeup-preemption window -- which maybe is good for
> > > Android.
> > >
> > > OTOH, the “prefer idle” flag in android that Qais is referring to,
> > > will need a completely different method as I cannot see how a nice
> > > value can communicate that (that can complement Vincent's changes
> > > here). And it will need to have a per-task interface as well. We have
> >
> > Why a negative latency_nice value condition can't be used ? or latency -20  ?
>
> That's overloading the meaning of a value, the whole nice thing is
> supposed to be "relative to something". So you are being nice to
> something else. Here -20 means you are not being nice. But in fact you
> are, because you are avoiding hurting something else by going to an
> idle CPU. So it becomes really weird.

Looking for an idle CPU 1st is already the default behavior of CFS.
Here we speak about an EAS specific behavior where we want to forgot
the "full" EAS policy for some tasks and favor latency by spreading
and looking for an idle cpu

>
> Also, why would -19 or -18 not be a value instead to cause wakeup to
> prefer an idle CPU? It confuses the user on how to choose value and we
> should refrain from that IMHO.

IIRC, the 1st idea was to say any negative value but then using the
lowest one can be seen as an addon to the wakeup preemption