Re: [PATCH 2/2] cgroup/cpuset: Rebind/migrate mm only for threadgroup leader in cpuset_update_tasks_nodemask()

[Ridong Chen <ridong.chen@linux.dev>]

On 6/23/2026 6:45 AM, Waiman Long wrote:
> As reported by sashiko [1], cpuset_update_tasks_nodemask() will do
> mpol_rebind_mm() and possibly cpuset_migrate_mm() for all threads of
> a multithreaded process. Since commit 3df9ca0a2b8b ("cpuset: migrate
> memory only for threadgroup leaders"), cpuset_attach() had been updated
> to rebind and migrate memory only for threadgroup leaders to mark the
> group leader as the owner of the mm_struct.
> 
> To be consistent and avoid unnecessary performance overhead for heavily
> multithreaded processes, follow the cpuset_attach() example and perform
> memory rebind and migration only for threadgroup leaders.
> 
> Also add a paragraph in cgroup-v2.rst under cpuset.mems that the
> threadgroup leader is the memory owner of that threadgroup. Therefore
> the non-leading threads shouldn't be in other cgroups whose "cpuset.mems"
> doesn't fully overleap that of the group leader.
> 
> [1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   Documentation/admin-guide/cgroup-v2.rst | 7 +++++++
>   kernel/cgroup/cpuset.c                  | 4 ++++
>   2 files changed, 11 insertions(+)
> 
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index 993446ab66d0..341037c7ec9d 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -2527,6 +2527,13 @@ Cpuset Interface Files
>   	a need to change "cpuset.mems" with active tasks, it shouldn't
>   	be done frequently.
>   
> +	For a multithreaded process, the threadgroup leader is
> +	considered the owner of the group's memory. Memory policy
> +	rebinding and migration will only happen with respect to the
> +	threadgroup leader. To avoid unexpected result, non-leading
> +	threads shouldn't be put into another cgroup whose "cpuset.mems"
> +	doesn't full overleap that of the threadgroup leader.
> +
>     cpuset.mems.effective
>   	A read-only multiple values file which exists on all
>   	cpuset-enabled cgroups.
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index bc0207fd6e57..27bc7a466468 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2659,6 +2659,10 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
>   
>   		cpuset_change_task_nodemask(task, &newmems);
>   
> +		/* Rebind and migrate mm only for task group leader */
> +		if (task != task->group_leader)
> +			continue;
> +

Nit.

if (!thread_group_leader(task))
     continue;

>   		mm = get_task_mm(task);
>   		if (!mm)
>   			continue;

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>

-- 
Best regards
Ridong