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..894131f47f78 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);
 
-- 
2.52.0

On 2026/1/10 9:32, 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..894131f47f78 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);
>  

Looks good to me.

Reviewed-by: chenridong<huawei@.com>

-- 
Best regards,
Ridong

On 2026/1/12 9:13, Chen Ridong wrote:
> 
> 
> On 2026/1/10 9:32, 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..894131f47f78 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);
>>  
> 
> Looks good to me.
> 

Reviewed-by: Chen Ridong <chenridong@huawei.com>

-- 
Best regards,
Ridong