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

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Message-ID: <bf8f21be-7249-fc27-9704-211d0f5a12b1@arm.com>
Date:   Thu, 20 Apr 2023 11:37:01 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     David Dai <davidai@...gle.com>
Cc:     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>,
        Saravana Kannan <saravanak@...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 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.

[...]

>> 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.