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

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Date:   Wed, 19 Apr 2023 07:18:51 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     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>
Cc:     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 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?

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)?

> 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 control
> over scheduler attributes of child processes.

[...]