Message-ID: <ca664da8-0f47-06b2-a94c-82b2f9a1c3aa@redhat.com>
Date:   Fri, 17 Mar 2023 10:59:26 -0400
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Subject: Re: [PATCH 3/5] cgroup/cpuset: Find another usable CPU if none found
 in current cpuset
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To:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.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>
References: <20230306200849.376804-1-longman@redhat.com>
 <20230306200849.376804-4-longman@redhat.com>
 <20230314181749.5b4k6selbgdhl3up@blackpad>
 <58a1a878-fa0b-285d-3e43-2b5103d3c770@redhat.com>
 <20230317122708.ax3m2d4zijkfdzjq@blackpad>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230317122708.ax3m2d4zijkfdzjq@blackpad>
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On 3/17/23 08:27, Michal Koutný wrote:
> 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 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)?

Yes, killing it is one possible solution.

(3) doesn't work if the affinity change is due to hot cpu removal. So 
that leaves this patch or (2) as the only alternative. I would like to 
hear what Will and Tejun thinks about it.

I am going to remove this patch from the series for the time being.

Thanks,
Longman