Re: [cgroup/for-6.20 PATCH v2 2/4] cgroup/cpuset: Consistently compute effective_xcpus in update_cpumasks_hier()

[Waiman Long <llong@redhat.com>]

On 1/3/26 9:48 PM, Chen Ridong wrote:
>
> On 2026/1/2 3:15, Waiman Long wrote:
>> Since commit f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check()
>> & make update_cpumasks_hier() handle remote partition"), the
>> compute_effective_exclusive_cpumask() helper was extended to
>> strip exclusive CPUs from siblings when computing effective_xcpus
>> (cpuset.cpus.exclusive.effective). This helper was later renamed to
>> compute_excpus() in commit 86bbbd1f33ab ("cpuset: Refactor exclusive
>> CPU mask computation logic").
>>
>> This helper is supposed to be used consistently to compute
>> effective_xcpus. However, there is an exception within the callback
>> critical section in update_cpumasks_hier() when exclusive_cpus of a
>> valid partition root is empty. This can cause effective_xcpus value to
>> differ depending on where exactly it is last computed. Fix this by using
>> compute_excpus() in this case to give a consistent result.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 14 +++++---------
>>   1 file changed, 5 insertions(+), 9 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index da2b3b51630e..37d118a9ad4d 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -2168,17 +2168,13 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>>   		spin_lock_irq(&callback_lock);
>>   		cpumask_copy(cp->effective_cpus, tmp->new_cpus);
>>   		cp->partition_root_state = new_prs;
>> -		if (!cpumask_empty(cp->exclusive_cpus) && (cp != cs))
>> -			compute_excpus(cp, cp->effective_xcpus);
>> -
>>   		/*
>> -		 * Make sure effective_xcpus is properly set for a valid
>> -		 * partition root.
>> +		 * Need to compute effective_xcpus if either exclusive_cpus
>> +		 * is non-empty or it is a valid partition root.
>>   		 */
>> -		if ((new_prs > 0) && cpumask_empty(cp->exclusive_cpus))
>> -			cpumask_and(cp->effective_xcpus,
>> -				    cp->cpus_allowed, parent->effective_xcpus);
>> -		else if (new_prs < 0)
>> +		if ((new_prs > 0) || !cpumask_empty(cp->exclusive_cpus))
>> +			compute_excpus(cp, cp->effective_xcpus);
>> +		if (new_prs < 0)
>>   			reset_partition_data(cp);
>>   		spin_unlock_irq(&callback_lock);
>>   
> The code resets partition data only for new_prs < 0. My understanding is that a partition is invalid
> when new_prs <= 0. Shouldn't reset_partition_data() also be called when new_prs = 0? Is there a
> specific reason to skip the reset in that case?

update_cpumasks_hier() is called when changes in a cpuset or hotplug 
affects other cpusets in the hierarchy. With respect to changes in 
partition state, it is either from valid to invalid or vice versa. It 
will not change from a valid partition to member. The only way new_prs = 
0 is when old_prs = 0. Even if the affected cpuset is processed again in 
update_cpumask_hier(), any state change from valid partition to member 
(update_prstate()), reset_partition_data() should have been called 
there. That is why we only care about when new_prs != 0.

The code isn't wrong here. However I can change the condition to 
(new_prs <= 0) if it makes it easier to understand.

Cheers,
Longman