linux-kernel - Re: [PATCH v3] sched/core: Use empty mask to reset cpumasks in sched

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Message-ID: <ZRvcRgv+HBY5+e9J@gmail.com>
Date:   Tue, 3 Oct 2023 11:17:58 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....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>,
        linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>
Subject: Re: [PATCH v3] sched/core: Use empty mask to reset cpumasks in
 sched_setaffinity()


* Waiman Long <longman@...hat.com> wrote:

> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset. However,
> that affinity is also being inherited by any subsequently created child
> processes which may not want or be aware of that affinity.
> 
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity.  This patch implements such a scheme
> by using an empty cpumask to signal a reset of the cpumasks to the
> default as allowed by the current cpuset.
> 
> Before this patch, passing in an empty cpumask to sched_setaffinity(2)
> will always return an -EINVAL error. With this patch, an alternative
> error of -ENODEV will be returned returned if sched_setaffinity(2)
> has been called before to set up user_cpus_ptr. In this case, the
> user_cpus_ptr that stores the user provided affinity will be cleared and
> the task's CPU affinity will be reset to that of the current cpuset. This
> alternative error code of -ENODEV signals that the no CPU is specified
> and, at the same time, a side effect of resetting cpu affinity to the
> cpuset default.

I agree that this problem needs a solution, but I don't really agree
with the -ENODEV ABI hack.

Why not just return success in that case? The 'reset' of the mask was
successful after all.

Thanks,

	Ingo