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

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Date:   Tue, 3 Oct 2023 12:06:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, 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()

On Thu, Aug 03, 2023 at 10:32:18PM -0400, Waiman Long 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.

So I still don't like this much, the normal state is all bits set:

  $ grep allowed /proc/self/status
  Cpus_allowed:   ff,ffffffff

The all clear bitmask just feels weird for this.