linux-kernel - Re: [PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

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Date:   Tue, 16 Aug 2022 20:13:25 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
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>,
        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 3/3] cgroup/cpuset: Keep user set cpus affinity


On 8/16/22 18:19, Tejun Heo wrote:
> Hello,
>
> On Tue, Aug 16, 2022 at 06:11:03PM -0400, Waiman Long wrote:
>> It is hard to synchronize different subsystems atomically without running
>> into locking issue. Let me think about what can be done in this case.
> I have a hard time seeing why this would be particularly difficult. cpuset
> just needs to make the latest cpumask available to sched core in an easily
> accessible form and whenever that changes, trigger a set_cpus_allowed call.
> There's no need to entangle operations across the whole subsystems. All
> that's needed to be communicated is the current cpumask.
>
>> Is using a sequence number to check for race with retry good enough?
> It seems unnecessarily fragile and complicated to me. If we're gonna change
> it, let's change it right.

Thanks for the suggestion. I think I get what you want. I am going to 
migrate the cpuset_set_cpus_allowed_ptr() logic into 
set_cpus_allowed_ptr() itself. IOW, if user_cpus_ptr is defined, it will 
be an additional mask to be applied on top. It does affect all callers 
of set_cpus_allowed_ptr() though. I am going to drop this cpuset 
specific patch.

BTW, I will be on PTO starting tomorrow until next Tuesday. So I will be 
slow in responding to emails.

Cheers,
Longman

Cheers,
Longman