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Date:   Fri, 24 Mar 2023 10:42:09 -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:     Will Deacon <will@kernel.org>
Cc:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>,
        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,
        Peter Zijlstra <peterz@infradead.org>
References: <20230306200849.376804-1-longman@redhat.com>
 <20230306200849.376804-4-longman@redhat.com>
 <20230314181749.5b4k6selbgdhl3up@blackpad>
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 <ca664da8-0f47-06b2-a94c-82b2f9a1c3aa@redhat.com>
 <20230324143247.GA27199@willie-the-truck>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20230324143247.GA27199@willie-the-truck>
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On 3/24/23 10:32, Will Deacon wrote:
> On Fri, Mar 17, 2023 at 10:59:26AM -0400, Waiman Long wrote:
>> 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.
> The main constraint from the Android side (the lucky ecosystem where these
> SoCs tend to show up) is that existing userspace (including 32-bit binaries)
> continues to function without modification. 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.
>
> 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'm mainly just concerned about introducing any regressions, which is why
> I ran my tests on this series

I agree that killing it may be too draconian. I am withholding this 
patch for now.

Thanks,
Longman