linux-kernel - Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus affinity was explicitly set

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Date:   Thu, 28 Jul 2022 15:21:26 -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>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus
 affinity was explicitly set

On 7/28/22 15:02, Tejun Heo wrote:
> Hello,
>
> On Thu, Jul 28, 2022 at 02:57:28PM -0400, Waiman Long wrote:
>> There can be a counter argument that if a user found out that there is not
>> enough cpus in a cpuset to meet its performance target, one can always
>> increase the number of cpus in the cpuset. Generalizing this behavior to all
>> the tasks irrespective if they have explicitly set cpus affinity before will
>> disallow this use case.
> This is nasty.

That is a nasty example, I know. There may be users depending on the 
existing behavior even if they don't know it. So I am a bit hesitant to 
change the default behavior like that. On the other hand, tasks that 
have explicitly set its cpu affinity certainly don't want to have 
unexpected change to that.

> The real solution here is separating out what user requested
> and the mask that cpuset (or cpu hotplug) needs to apply on top. ie.
> remember what the user requested in a separate cpumask and compute the
> intersection into p->cpus_maks whenever something changes and apply
> fallbacks on that final mask. Multiple parties updating the same variable is
> never gonna lead to anything consistent and we're patching up for whatever
> the immediate use case seems to need at the moment. That said, I'm not
> necessarily against patching it up but if you're interested in delving into
> it deeper, that'd be great.

I believe the current code is already restricting what cpu affinity that 
a user can request by limiting to those allowed by the current cpuset. 
Hotplug is another issue that may need to be addressed. I will update my 
patch to make it handle hotplug in a more graceful way.

Thanks,
Longman