linux-kernel - Re: [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU hotplug

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Message-ID: <2870d179-a2db-44ee-9183-11efe446ebd9@redhat.com>
Date: Thu, 7 Aug 2025 09:54:54 -0400
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Ingo Molnar <mingo@...nel.org>, Juri Lelli <juri.lelli@...hat.com>,
 "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [PATCH 2/3] cgroup/cpuset.c: Fix a partition error with CPU
 hotplug

On 8/6/25 10:44 PM, Chen Ridong wrote:
>
> On 2025/8/7 1:24, Waiman Long wrote:
>> It was found during testing that an invalid leaf partition with an
>> empty effective exclusive CPU list can become a valid empty partition
>> with no CPU afer an offline/online operation of an unrelated CPU. An
>> empty partition root is allowed in the special case that it has no
>> task in its cgroup and has distributed out all its CPUs to its child
>> partitions. That is certainly not the case here.
>>
>> The problem is in the cpumask_subsets() test in the hotplug case
>> (update with no new mask) of update_parent_effective_cpumask() as it
>> also returns true if the effective exclusive CPU list is empty. Fix that
>> by addding the cpumask_empty() test to root out this exception case.
>> Also add the cpumask_empty() test in cpuset_hotplug_update_tasks()
>> to avoid calling update_parent_effective_cpumask() for this special case.
>>
>> Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 7 ++++---
>>   1 file changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index bf149246e001..d993e058a663 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1843,7 +1843,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>>   			if (is_partition_valid(cs))
>>   				adding = cpumask_and(tmp->addmask,
>>   						xcpus, parent->effective_xcpus);
>> -		} else if (is_partition_invalid(cs) &&
>> +		} else if (is_partition_invalid(cs) && !cpumask_empty(xcpus) &&
>>   			   cpumask_subset(xcpus, parent->effective_xcpus)) {
>>   			struct cgroup_subsys_state *css;
>>   			struct cpuset *child;
> This path looks good to me.
>
> However, I found the update_parent_effective_cpumask function a bit difficult to follow due to its
> complexity.
>
> To improve readability, could we refactor the partcmd_enable, partcmd_disable, partcmd_update and
> partcmd_invalidate logic into separate, well-defined function blocks?  I'd be happy to take
> ownership of this refactoring work if you agree with the approach.

I agree that the code can be a bit hard to read. You are more than 
welcome to improve the readability of the code if you have time.

Cheers,
Longman