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

[Tejun Heo <tj@kernel.org>]

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?

Thanks.

-- 
tejun