linux-kernel - Re: [RFC PATCH v1] sched/uclamp: Introduce SCHED_FLAG_RESET_UCLAMP_ON

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Date:   Thu, 20 Apr 2023 09:22:27 -0700
From:   Saravana Kannan <saravanak@...gle.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     David Dai <davidai@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Qais Yousef <qyousef@...gle.com>,
        Quentin Perret <qperret@...gle.com>, kernel-team@...roid.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1] sched/uclamp: Introduce SCHED_FLAG_RESET_UCLAMP_ON_FORK
 flag

On Thu, Apr 20, 2023 at 2:37 AM Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> On 20/04/2023 03:11, David Dai wrote:
> > On Tue, Apr 18, 2023 at 10:18 PM Dietmar Eggemann
> > <dietmar.eggemann@....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 setting.
> >>> 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’ reply that contains more detail on how it’s 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.
>
> 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’m using the version of uclamp that’s in Linus’ tree. How uclamp is
> > aggregated is unrelated to the problem I’m 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.