linux-kernel - Re: [PATCH next] cpuset: fix warning when attaching tasks with offline CPUs

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Message-ID: <386c58cb-2330-4627-90c5-9f548f2a9a3e@huaweicloud.com>
Date: Tue, 15 Jul 2025 10:29:17 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: longman@...hat.com, tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org, lujialin4@...wei.com,
 chenridong@...wei.com
Subject: Re: [PATCH next] cpuset: fix warning when attaching tasks with
 offline CPUs



On 2025/7/14 19:59, Peter Zijlstra wrote:
> On Mon, Jul 14, 2025 at 07:30:39PM +0800, Chen Ridong wrote:
> 
>>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>>> index f74d04429a29..5401adbdffa6 100644
>>>> --- a/kernel/cgroup/cpuset.c
>>>> +++ b/kernel/cgroup/cpuset.c
>>>> @@ -3121,7 +3121,7 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>>>>  	if (cs != &top_cpuset)
>>>>  		guarantee_active_cpus(task, cpus_attach);
>>>>  	else
>>>> -		cpumask_andnot(cpus_attach, task_cpu_possible_mask(task),
>>>> +		cpumask_andnot(cpus_attach, cpu_active_mask,
>>>>  			       subpartitions_cpus);
>>>
>>> This breaks things. Any task mask must be a subset of
>>> task_cpu_possible_mask() at all times. It might not be able to run
>>> outside of that mask.
>>
>> Hi Peter,
>>
>> Thanks for your feedback. I'm afraid I don't fully understand what you
>> mean by "breaks things".  Could you please explain in more detail?
>>
>> To clarify my current understanding: this patch simply changes the
>> cpus_attach initialization from task_cpu_possible_mask(task) to
>> cpu_active_mask. The intention is that when CPUs are offlined and
>> tasks get migrated to root cpuset, we shouldn't try to migrate tasks
>> to offline CPUs. And since cpu_active_mask is a subset of
>> cpu_possible_mask, I thought this would be safe. Did I miss anything?
> 
> task_cpu_possible_mask() is the mask a task *MUST* stay inside of.
> 
> Specifically, this was introduced for ARMv9 where some CPUs drop the
> capability to run ARM32 instructions. Trying to schedule an ARM32 task
> on a CPU that does not support that instruction set is an immediate and
> fatal fail.
> 
> Your change results in in something akin to:
> 
>   set_cpus_allowed_task(task, cpu_active_mask & ~subpartition_cpus);
> 
> Which does not honor the task_cpu_possible_mask() constraint.

Thanks for your patience. See now.

Best regards,
Ridong