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

[Waiman Long <longman@redhat.com>]

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