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

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Date:   Fri, 17 Mar 2023 13:27:08 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...hat.com>
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,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found
 in current cpuset

On Tue, Mar 14, 2023 at 04:22:06PM -0400, Waiman Long <longman@...hat.com> wrote:
> Some arm64 systems can have asymmetric CPUs where certain tasks are only
> runnable on a selected subset of CPUs.

Ah, I'm catching up.

> This information is not captured in the cpuset. As a result,
> task_cpu_possible_mask() may return a mask that have no overlap with
> effective_cpus causing new_cpus to become empty.

I can see that historically, there was an approach of terminating
unaccomodable tasks:
   94f9c00f6460 ("arm64: Remove logic to kill 32-bit tasks on 64-bit-only cores") 
the removal of killing had been made possible with
   df950811f4a8 ("arm64: Prevent offlining first CPU with 32-bit EL0 on mismatched system").

That gives two other alternatives to affinity modification:
2) kill such tasks (not unlike OOM upon memory.max reduction),
3) reject cpuset reduction (violates cgroup v2 delegation).

What do you think about 2)?

Michal

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