linux-kernel - Re: [PATCH v5 1/3] sched: Use user_cpus_ptr for saving user provided cpumask in sched

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Message-ID: <YvymwlTkdsVDtmRB@worktop.programming.kicks-ass.net>
Date:   Wed, 17 Aug 2022 10:28:50 +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>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 1/3] sched: Use user_cpus_ptr for saving user provided
 cpumask in sched_setaffinity()

On Tue, Aug 16, 2022 at 03:27:32PM -0400, Waiman Long wrote:

> This will be some changes in behavior for arm64 systems with asymmetric
> CPUs in some corner cases. For instance, if sched_setaffinity()
> has never been called and there is a cpuset change before
> relax_compatible_cpus_allowed_ptr() is called, its subsequent call will
> follow what the cpuset allows but not what the previous cpu affinity
> setting allows.

That's arguably a correctness fix, no? That is, the save/restore should
not have been allowed to revert to an earlier cpuset state.