Re: [PATCH v2 1/2] cgroup/cpuset: Avoid unnecessary cpus & mems update in cpuset_hotplug_update_tasks()

[Manuel Ebner <manuelebner@mailbox.org>]

On Tue, 2026-06-23 at 19:04 -0400, Waiman Long wrote:
> As reported by sashiko [1], cpuset_hotplug_update_tasks() may perform
> unnecessary task iteration and updating of tasks' CPU and node masks
> when mems_allowed and/or cpus_allowed are not set in cpuset v2. It is
> due to the fact that the temporary new_cpus and new_mems masks do not
> inherit parent's effective_cpus/mems when they are empty which is the
> expected behavior for cpuset v2 since commit 4ec22e9c5a90 ("cpuset:
> Enable cpuset controller in default hierarchy").
> 
> Fix that and avoid unnecessay work by enhancing
> compute_effective_cpumask() to add the empty cpumask check
> and inheriting the parent's versions if empty when in v2. A new
> compute_effective_nodemask() helper is also added to perform similar
> function for new effective_mems.

perform a similar function
or
perform similar functions

> [...]
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index aff86acea701..044ddbf66f8e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1094,12 +1094,35 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct
> cpumask *new_cpus)
>   * @cs: the cpuset the need to recompute the new effective_cpus mask
>   * @parent: the parent cpuset
>   *
> + * For v2, the parent's effective_cpus is inherited if cpumask empty.

+ * For v2, the parent's effective_cpus is inherited if cpumask is empty.

Thanks
 Manuel