Date:   Fri, 17 Mar 2023 13:27:08 +0100
From:   Michal =?utf-8?Q?Koutn=C3=BD?= <mkoutny@suse.com>
To:     Waiman Long <longman@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found
 in current cpuset
Message-ID: <20230317122708.ax3m2d4zijkfdzjq@blackpad>
References: <20230306200849.376804-1-longman@redhat.com>
 <20230306200849.376804-4-longman@redhat.com>
 <20230314181749.5b4k6selbgdhl3up@blackpad>
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On Tue, Mar 14, 2023 at 04:22:06PM -0400, Waiman Long <longman@redhat.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 c=
ores")=20
the removal of killing had been made possible with
   df950811f4a8 ("arm64: Prevent offlining first CPU with 32-bit EL0 on mis=
matched 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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