linux-kernel - Re: [PATCH v3 1/3] sched: Use user_cpus_ptr for saving user provided cpumask in sched

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Date:   Mon, 15 Aug 2022 10:57:15 +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 v3 1/3] sched: Use user_cpus_ptr for saving user provided
 cpumask in sched_setaffinity()

On Fri, Aug 12, 2022 at 04:39:27PM -0400, Waiman Long wrote:
> The user_cpus_ptr field is added by commit b90ca8badbd1 ("sched:
> Introduce task_struct::user_cpus_ptr to track requested affinity"). It
> is currently used only by arm64 arch due to possible asymmetric cpu
> setup. This patch extends its usage to save user provided cpumask when
> sched_setaffinity() is called for all arches.
> 
> To preserve the existing arm64 use case, a new cpus_affinity_set flag is
> added to differentiate if user_cpus_ptr is set up by sched_setaffinity()
> or by force_compatible_cpus_allowed_ptr(). user_cpus_ptr
> set by sched_setaffinity() has priority and won't be
> overwritten by force_compatible_cpus_allowed_ptr() or
> relax_compatible_cpus_allowed_ptr().

What why ?! The only possible case where
restrict_cpus_allowed_ptr() will now need that weird new state is when
the affinity has never been set before, in that case cpus_ptr should be
possible_mask.

Please just make a single consistent rule and don't make weird corner
cases like this.