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References: <20230416213406.2966521-1-davidai@google.com> <d83950c4-7458-aeea-f341-327c163704a8@arm.com>
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From:   David Dai <davidai@google.com>
Date:   Wed, 19 Apr 2023 18:11:46 -0700
Message-ID: <CABN1KC+_HDi_i2zzpZVbqiUP5-QB9YrE5wzLqr==_wOemaCXzA@mail.gmail.com>
Subject: Re: [RFC PATCH v1] sched/uclamp: Introduce SCHED_FLAG_RESET_UCLAMP_ON_FORK
 flag
To:     Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Qais Yousef <qyousef@google.com>,
        Quentin Perret <qperret@google.com>,
        Saravana Kannan <saravanak@google.com>,
        kernel-team@android.com, linux-kernel@vger.kernel.org
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On Tue, Apr 18, 2023 at 10:18=E2=80=AFPM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>

Hi Dietmar, thanks for your time,

> On 16/04/2023 23:34, David Dai wrote:
> > A userspace service may manage uclamp dynamically for individual tasks =
and
> > a child task will unintentionally inherit a pesudo-random uclamp settin=
g.
> > This could result in the child task being stuck with a static uclamp va=
lue
>
> Could you explain this with a little bit more detail? Why isn't the
> child task also managed by the userspace service?

See Qais=E2=80=99 reply that contains more detail on how it=E2=80=99s being=
 used in
Android. In general, if a dynamic userspace service will adjust uclamp
on the fly for a given task, but has no knowledge or control over if
or when a task forks. Depending on the timing of the fork, a child
task may inherit a very large or a small uclamp_min or uclamp_max
value. The intent of this patch is to provide more flexibility to the
uclamp APIs such that child tasks do not get stuck with a poor uclamp
value when spawned while retaining other sched attributes. When
RESET_ON_FORK is set on the parent task, it will reset uclamp values
for the child but also reset other sched attributes as well.

>
> The child task will only make a difference if it's on the rq.
>
> Does this issue happen with uclamp mainline or only with Android's
> slightly different version (max- vs. sum aggregation)?

I=E2=80=99m using the version of uclamp that=E2=80=99s in Linus=E2=80=99 tr=
ee. How uclamp is
aggregated is unrelated to the problem I=E2=80=99m trying to solve with thi=
s
patch. Which is to extend the uclamp APIs to have finer control for
the uclamp inheritance of child tasks.

Thanks,
David

>
> > that results in poor performance or poor power.
> >
> > Using SCHED_FLAG_RESET_ON_FORK is too coarse for this usecase and will
> > reset other useful scheduler attributes. Adding a
> > SCHED_FLAG_RESET_UCLAMP_ON_FORK will allow userspace to have finer cont=
rol
> > over scheduler attributes of child processes.
>
> [...]
>