linux-kernel - Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found in current cpuset

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Message-ID: <17e804ea-64b5-97e1-d5fa-571157e15746@redhat.com>
Date:   Sat, 25 Mar 2023 18:08:18 -0400
From:   Waiman Long <longman@...hat.com>
To:     Michal Koutný <mkoutny@...e.com>,
        Will Deacon <will@...nel.org>
Cc:     Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found
 in current cpuset


On 3/24/23 14:19, Michal Koutný wrote:
> On Fri, Mar 24, 2023 at 02:32:50PM +0000, Will Deacon <will@...nel.org> wrote:
>> So approaches such as killing tasks or rejecting system calls tend not
>> to work as well, since you inevitably get divergent behaviour leading
>> to functional breakage rather than e.g. performance anomalies.
> What about temporary performance drop from 100% to 0% aka freezing the
> tasks for the duration of the mismatching affinity config?

That can be a lot of extra work to freeze it. I will prefer something 
simpler.

Without this patch, I believe it will lead to a cpumask of 0 which will 
cause the scheduler to pick a fallback cpu. It looks like the fallback 
code may be able to pick up the right cpu or it may panic the system 
(less likely).

Cheers,
Longman
>
>
>> Having said that, the behaviour we currently have in mainline seems to
>> be alright, so please don't go out of your way to accomodate these SoCs.
> I see. (Just wondering what you think about the fourth option above.)
>
> Thanks,
> Michal