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

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Message-ID: <401bae73-3063-e0ab-c288-2c6e3be75fc5@redhat.com>
Date:   Mon, 15 Aug 2022 09:52:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 8/15/22 04:57, Peter Zijlstra wrote:
> 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.

Since I don't have a full history for this particular patch series that 
add user_cpus_ptr, I am hesitant to change the current behavior for 
arm64 systems. However, given the statement that user_cpus_ptr is for 
tracking "requested affinity" which I assume is when user applications 
call sched_setaffinity(). It does make sense we may not really need this 
if sched_setaffinity() is never called.


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

I will take a closer look to try to simplify the rule here.

Cheers,
Longman