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

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Date:   Fri, 21 Apr 2023 17:10:51 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Saravana Kannan <saravanak@...gle.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        David Dai <davidai@...gle.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        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, 20 Apr 2023 at 18:26, Saravana Kannan <saravanak@...gle.com> wrote:
>
> On Thu, Apr 20, 2023 at 6:44 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > On Thu, 20 Apr 2023 at 11:37, 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?
> > >
> > > Do we anticipate a use-case in which uclamp inheritance would be required?
> > >
> > > Let's not over-complicate the sched_[sg]etattr() unnecessarily.
> >
> > I was about to ask the same question and I'm aligned with Dietmar.
> > Use RESET_ON_FORK and set all attributes
>
> That's racy though. If we have an external service (that's only
> responsible for setting uclamp) setting all the attributes, the forked
> thread could also be trying to set some of the attributes. Also, how
> is this external service going to keep track of all the threads being
> forked and set the right attributes for all of them?

My assumption was that you didn't use RESET_ON_FORK because there were
other attributes that you wanted to keep from parent but it doesn't
seem to be the case so use RESET_ON_FORK and if needed the forked
thread will set its other attributes

>
> If it's not considered a UAPI breakage, I'd rather we never inherit uclamp.
>
> -Saravana
>
> >
> > >
> > > [...]
> > >
> > > >> 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.