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Message-ID: <606ed69e-8ad0-45d5-9de7-48739df7f48d@redhat.com>
Date:   Thu, 28 Jul 2022 15:21:26 -0400
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Subject: Re: [PATCH 1/2] cgroup/cpuset: Keep current cpus list if cpus
 affinity was explicitly set
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To:     Tejun Heo <tj@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220728005815.1715522-1-longman@redhat.com>
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From:   Waiman Long <longman@redhat.com>
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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