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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 14 Nov 2022 17:57:30 +0100
Message-ID: <CAKfTPtDiKF7x0obCFXmNsmJdsJ9yXzAf4vuOVJmN_eO_bA8KHQ@mail.gmail.com>
Subject: Re: [PATCH v8 6/9] sched/fair: Add sched group latency support
To:     Patrick Bellasi <patrick.bellasi@matbug.net>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org, parth@linux.ibm.com,
        qyousef@layalina.io, chris.hyser@oracle.com,
        David.Laight@aculab.com, pjt@google.com, pavel@ucw.cz,
        tj@kernel.org, qperret@google.com, tim.c.chen@linux.intel.com,
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On Mon, 14 Nov 2022 at 17:20, Patrick Bellasi
<patrick.bellasi@matbug.net> wrote:
>
> Hi Vincent,
>
> On 10-Nov 18:50, Vincent Guittot wrote:
>
> [...]
>
> > diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> > index be4a77baf784..a4866cd4e58c 100644
> > --- a/Documentation/admin-guide/cgroup-v2.rst
> > +++ b/Documentation/admin-guide/cgroup-v2.rst
> > @@ -1095,6 +1095,16 @@ All time durations are in microseconds.
> >          values similar to the sched_setattr(2). This maximum utilization
> >          value is used to clamp the task specific maximum utilization clamp.
> >
> > +  cpu.latency.nice
> > +     A read-write single value file which exists on non-root
> > +     cgroups.  The default is "0".
> > +
> > +     The nice value is in the range [-20, 19].
> > +
> > +     This interface file allows reading and setting latency using the
> > +     same values used by sched_setattr(2). The latency_nice of a group is
> > + used to limit the impact of the latency_nice of a task outside the
> > + group.
>
> This control model is not clear to me.
>
> It does not seem matching what we have for uclamp, where the cgroup values are
> used to restrict how much a task can ask or give (in terms of latency here).
>
> in the uclamp's requested-vs-effective values model:
>
> A) a task can "request" (or give up) latency as much as it likes
>
> B) the cgroup in which the task is in any moment limits wthe maximum
>    latency a task can "request" (or give up)
>
> C) the system wide knob set the "request" limit for the root cgroup an any task
>    not in a cgroup.
>
> This model seems to be what we should use here too.
>
> IOW, for each task compute an "effective" latency_nice value which is defined
> starting for a task "requested" latency value and by restricting this value
> based on the (B) cgroup value and the (C) system wide value.
>
> That's what we do in uclamp_eff_get():
>    https://elixir.bootlin.com/linux/v6.0/source/kernel/sched/core.c#L1484
>
> Why such a model cannot be used for latency_nice too?
> Am I missing something?

Have you read the previous email thread on the subject ?

As I mentioned previously we don't need an effective latency for this
patchset because the current use of cgroup latency_nice is done at
each scheduling level just like the cgroup weight is used at each
level.

Regards,
Vincent

>
>
> Best,
> patrick
>
> --
> #include <best/regards.h>
>
> Patrick Bellasi