Re: [PATCH -next] cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/11/26 11:26, Waiman Long wrote:
> On 11/25/25 10:17 PM, Chen Ridong wrote:
>>
>> On 2025/11/26 10:33, Waiman Long wrote:
>>> On 11/25/25 8:01 PM, Chen Ridong wrote:
>>>> On 2025/11/26 2:16, Waiman Long wrote:
>>>>>> active CPUs, preventing partition_sched_domains from being invoked with
>>>>>> offline CPUs.
>>>>>>
>>>>>> Signed-off-by: Chen Ridong <chenridong@huawei.com>
>>>>>> ---
>>>>>>     kernel/cgroup/cpuset.c | 29 ++++++-----------------------
>>>>>>     1 file changed, 6 insertions(+), 23 deletions(-)
>>>>>>
>>>>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>>>>> index daf813386260..1ac58e3f26b4 100644
>>>>>> --- a/kernel/cgroup/cpuset.c
>>>>>> +++ b/kernel/cgroup/cpuset.c
>>>>>> @@ -1084,11 +1084,10 @@ void dl_rebuild_rd_accounting(void)
>>>>>>      */
>>>>>>     void rebuild_sched_domains_locked(void)
>>>>>>     {
>>>>>> -    struct cgroup_subsys_state *pos_css;
>>>>>>         struct sched_domain_attr *attr;
>>>>>>         cpumask_var_t *doms;
>>>>>> -    struct cpuset *cs;
>>>>>>         int ndoms;
>>>>>> +    int i;
>>>>>>           lockdep_assert_cpus_held();
>>>>>>         lockdep_assert_held(&cpuset_mutex);
>>>>> In fact, the following code and the comments above in rebuild_sched_domains_locked() are also no
>>>>> longer relevant. So you may remove them as well.
>>>>>
>>>>>           if (!top_cpuset.nr_subparts_cpus &&
>>>>>               !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>>>>>                   return;
>>>>>
>>>> Thank you for reminding me.
>>>>
>>>> I initially retained this code because I believed it was still required for cgroup v1, as I
>>>> recalled
>>>> that synchronous operation is exclusive to cgroup v2.
>>>>
>>>> However, upon re-examining the code, I confirm it can be safely removed. For cgroup v1,
>>>> rebuild_sched_domains_locked is called synchronously, and only the migration task (handled by
>>>> cpuset_migrate_tasks_workfn) operates asynchronously. Consequently, cpuset_hotplug_workfn is
>>>> guaranteed to complete before the hotplug workflow finishes.
>>> Yes, v1 still have a task migration part that is done asynchronously because of the lock ordering
>>> issue. Even if this code has to be left because of v1, you should still update the comment to
>>> reflect that. Please try to keep the comment updated to help others to have a better understanding
>>> of what the code is doing.
>>>
>>> Thanks,
>>> Longman
>>>
>> Hi Longman,
>>
>> Just to confirm (in case I misunderstood): I believe it is safe to remove the check on
>> top_cpuset.effective_cpus (for both cgroup v1 and v2). I will proceed to remove both the
>> corresponding code and its associated comment(not update the comment).
>>
>>            if (!top_cpuset.nr_subparts_cpus &&
>>                !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>>                    return;
>>
>> Additionally, I should add a comment to clarify the rationale for introducing the
>> WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)) warning.
>>
>> Does this approach look good to you? Please let me know if I’ve missed anything or if further
>> adjustments are needed.
>>
> Yes, that is good for me. I was just talking about a hypothetical situation, not that you have to
> update the comment.
> 

See. Thanks.
-- 
Best regards,
Ridong