Re: [PATCH-next 2/6] cgroup/cpuset: Properly compute effective xcpus in update_parent_effective_cpumask()

[Waiman Long <longman@redhat.com>]

On 10/22/26 9:53 PM, Guopeng Zhang wrote:
>
> 在 2026/10/22 23:00, Waiman Long 写道:
>> On 10/22/26 9:45 AM, Guopeng Zhang wrote:
>>> Hi Longman,
>>>
>>> 在 2026/10/22 11:16, Waiman Long 写道:
>>>> Since the introduction of cpuset.cpus.exclusive in commit
>>>> e2ffe502ba45 ("cgroup/cpuset: Add cpuset.cpus.exclusive for v2"), a
>>>> compute_effective_exclusive_cpumask() helper which is later renamed to
>>>> compute_excpus() is added to compute the effective xcpus of a partition
>>>> root. However update_parent_effective_cpumask() is not updated in all
>>>> proper places to use the new helper to check if a given cpuset should
>>>> remain a valid partition root. This can cause a cpuset to incorrectly
>>>> remain a valid partition root even if no effective xcpus is left.
>>>>
>>>> Fix that problem by using compute_excpus() to compute the effective xcpus
>>>> into delmask in all cases except partcmd_disable and partcmd_invalidate
>>>> and use it for tasks_nocpu_error() check as well as checking its
>>>> emptiness. This enables us to catch invalid partition root more
>>>> accurately in update_parent_effective_cpumask().
>>>>
>>> I re-tested the repro from the link below on top of this series
>>> (applied on cgroup/for-7.4-fixes), and it still fails:
>>>
>>>     https://lore.kernel.org/all/20261010075947.333549-3-guopeng.zhang@linux.dev/
>>>
>>>     cd /sys/fs/cgroup
>>>     echo +cpuset > cgroup.subtree_control
>>>     mkdir A
>>>     echo 0-7 > A/cpuset.cpus
>>>     echo 0-7 > A/cpuset.cpus.exclusive
>>>     echo root > A/cpuset.cpus.partition
>>>     cat A/cpuset.cpus.partition
>>>     echo 1 > A/cpuset.cpus.exclusive
>>>     cat A/cpuset.cpus.partition
>>>
>>> The last command still shows "root invalid (Parent unable to distribute
>>> cpu downstream)".
>>>
>>> In this case compute_excpus() returns the stale user mask unchanged.
>>> user_xcpus() is gone from update_parent_effective_cpumask(), but the
>>> dependency just moved one level down: the new code calls
>>> compute_excpus(cs, tmp->delmask), and its first line is
>>>
>>>     cpumask_and(excpus, user_xcpus(cs), parent->effective_xcpus)
>>>
>>> Here user_xcpus(cs) is cs->exclusive_cpus, which still holds the old
>>> 0-7 when compute_excpus() runs: update_exclusive_cpumask() publishes
>>> cs->exclusive_cpus only after partition_cpus_change(). Nothing corrects
>>> it either - the sibling trim is skipped as exclusive_cpus is non-empty,
>>> and for a direct child of the root cpuset the intersection with
>>> parent->effective_xcpus is a no-op. So the stale 0-7 mask still reaches
>>> tasks_nocpu_error() and PERR_NOCPUS is set again. Please point it out
>>> if I got the analysis wrong.
>>>
>>> I have rebased my fix on top of this series and the partition recovers
>>> correctly with it applied. Should the fix be folded into this patch,
>>> or is there a better way to fix it? Happy to hear your thoughts.
>> You are right. My patch series won't fix this particular issue. However, converting an invalid partition back to a valid one is done on a best effort basis. There is no guarantee that it will always happen. Just echo "root" to "cpuset.cpus.partition" again will make it valid again. So it is not a real bug from my point of view. It is just a nice to have feature.
>>
> Thanks for the explanation. This helps me better understand the
> intended behavior of partition recovery.

I did find the reason why your test case isn't working. I have made some 
adjustment in patch 2 which should make your test case work now with the 
v2 series.

Cheers,
Longman