Re: [PATCH v2 3/3] cgroup/cpuset: Improve check for v1 task migration out of empty cpuset

[Waiman Long <longman@redhat.com>]

On 3/30/26 2:25 PM, Tejun Heo wrote:
> On Sun, Mar 29, 2026 at 01:39:58PM -0400, Waiman Long wrote:
>> With the "cpuset_v2_mode" mount option, cpuset v1 will emulate v2 in
>> how it handles the management of CPUs. As a result, the effective_cpus
>> can differ from cpus_allowed and an empty cpus_allowed will cause
>> effective_cpus to inherit the value from its parent. Therefore task
>> migration out of a cpuset with empty "cpuset.cpus" should no longer
>> be needed in this case.
>>
>> The current code doesn't handle this particular case. Update the code to
>> correctly detect that the cpuset has really no CPUs left by checking
>> effective_cpus instead of cpus_allowed.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset-v1.c | 10 ++++++----
>>   1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset-v1.c b/kernel/cgroup/cpuset-v1.c
>> index 0c818edd0a1d..9855de37d011 100644
>> --- a/kernel/cgroup/cpuset-v1.c
>> +++ b/kernel/cgroup/cpuset-v1.c
>> @@ -261,7 +261,7 @@ static void remove_tasks_in_empty_cpuset(struct cpuset *cs)
>>   	 * has online cpus, so can't be empty).
>>   	 */
>>   	parent = parent_cs(cs);
>> -	while (cpumask_empty(parent->cpus_allowed) ||
>> +	while (cpumask_empty(parent->effective_cpus) ||
>>   			nodes_empty(parent->mems_allowed))
>>   		parent = parent_cs(parent);
>>   
>> @@ -297,14 +297,16 @@ void cpuset1_hotplug_update_tasks(struct cpuset *cs,
>>   
>>   	/*
>>   	 * Don't call cpuset_update_tasks_cpumask() if the cpuset becomes empty,
>> -	 * as the tasks will be migrated to an ancestor.
>> +	 * as the tasks will be migrated to an ancestor. If cpuset_v2_mode mount
>> +	 * option is used, effective_cpus can differ from cpus_allowed. So
>> +	 * checking effective_cpus is more accurate for determining emptiness.
>>   	 */
>> -	if (cpus_updated && !cpumask_empty(cs->cpus_allowed))
>> +	if (cpus_updated && !cpumask_empty(cs->effective_cpus))
>>   		cpuset_update_tasks_cpumask(cs, new_cpus);
>>   	if (mems_updated && !nodes_empty(cs->mems_allowed))
>>   		cpuset_update_tasks_nodemask(cs);
>>   
>> -	is_empty = cpumask_empty(cs->cpus_allowed) ||
>> +	is_empty = cpumask_empty(cs->effective_cpus) ||
>>   		   nodes_empty(cs->mems_allowed);
> Are these meaningful changes? cpuset1_hotplug_update_tasks() is called by
> cpuset_hotplug_update_tasks() when !is_in_v2_mode(). As is_in_v2_mode() says
> yes on cpuset_v2_mode, the above change doesn't really change anything, no?

You are right. I missed that part. So it is really a no-op then. Will 
drop this patch.

Thanks!
Longman