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From:   Saravana Kannan <saravanak@google.com>
Date:   Thu, 20 Apr 2023 09:22:27 -0700
Message-ID: <CAGETcx9RGnW_Bq49yVogFo3Eys4UczmXP-6bkZh1q=u+M-_bDg@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:     David Dai <davidai@google.com>, 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>, kernel-team@android.com,
        linux-kernel@vger.kernel.org
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On Thu, Apr 20, 2023 at 2:37=E2=80=AFAM Dietmar Eggemann
<dietmar.eggemann@arm.com> wrote:
>
> On 20/04/2023 03:11, David Dai wrote:
> > 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 task=
s and
> >>> a child task will unintentionally inherit a pesudo-random uclamp sett=
ing.
> >>> This could result in the child task being stuck with a static uclamp =
value
> >>
> >> 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 b=
eing 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.
>
> OK, in this case, why not just change behavior and always reset the
> uclamp values at fork?

Personally, I'd have preferred uclamp was never inherited in the first
place, but wouldn't that be considered as breaking UAPI if we change
it now?

-Saravana

> Do we anticipate a use-case in which uclamp inheritance would be required=
?
>
> Let's not over-complicate the sched_[sg]etattr() unnecessarily.
>
> [...]
>
> >> 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 tree. How uclamp is
> > aggregated is unrelated to the problem I=E2=80=99m trying to solve with=
 this
> > patch. Which is to extend the uclamp APIs to have finer control for
> > the uclamp inheritance of child tasks.
>
> OK, I see.