Re: [PATCH-next 1/6] cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition

[Waiman Long <longman@redhat.com>]

On 10/23/26 2:24 AM, Ridong Chen wrote:
>
>
> On 10/22/2026 11:16 AM, Waiman Long wrote:
>> Commit 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus
>> update in update_cpumasks_hier()") searches for the closet partition
>> root ancestor to determine if the isolated CPUs in a to be invalidated
>> isolation partition should be removed from isolated_cpus or not.
>>
>> As reported by sashiko, this method may not work when there is a deeply
>> nested isolated partition hierarchy. For instance, Consider a nested
>> hierarchy A0 -> A1 -> A2 -> A3 where all four are PRS_ISOLATED:
>>
>>    A0: effective_xcpus = 0-4
>>    A1: effective_xcpus = 1-4
>>    A2: effective_xcpus = 2-4
>>    A3: effective_xcpus = 3-4
>>
>> When A1 shrinks its exclusive_cpus from 1-4 to 1-2, A2 will update it
>> effective_xcpus to 2. A3 will find its closet partition root ancestor
>> A2 and decides that its effective_xcpus 3-4 should be removed from
>> isolated_cpus which is incorrect.
>>
>> Address this corner case by searching for the furthest isolated 
>> partition
>> root ancestor instead to decide if its isolated CPUs should be returned
>> to isolated_cpus.
>>
>> Fixes: 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus 
>> update in update_cpumasks_hier()")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 36 +++++++++++++++++-------------------
>>   1 file changed, 17 insertions(+), 19 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 585a982249ad..7ead8c3b7974 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -2156,22 +2156,20 @@ static void 
>> compute_partition_effective_cpumask(struct cpuset *cs,
>>   }
>>     /*
>> - * ancestor_partition - Find and return the closest valid partition 
>> root ancestor
>> + * iancestor_partition - Return furthest isolated partition root 
>> ancestor available
>>    * @cs: the current cpuset
>> - *
>> - * Return: The closet partition root ancestor cpuset
>> + * Return: The furthest isolated partition root ancestor available 
>> or NULL if
>> + *       not found.
>>    */
>> -static struct cpuset *ancestor_partition(struct cpuset *cs)
>> +static struct cpuset *iancestor_partition(struct cpuset *cs)
>
> Nit.
>
> iancestor is confusing.
> Can we renmae it furthest_isolated_ancestor?

Yes, I can make the name change. Thanks for the suggestion.

Cheers,
Longman