Re: [PATCH-next v5 4/6] cgroup/cpuset: Make cpuset_attach_old_cs track task group leaders

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

On 2026/6/2 10:32, Waiman Long wrote:
> There are two possible ways that migration of tasks from multiple source
> cpusets to a target cpuset can happen. Either a multithread application
> with threads in different cpusets is wholely moved to a new cpuset
					^
					wholly
> or disabling of v2 cpuset controller will move all the tasks in child
> cpusets to the parent cpuset.
> 
> In the former case, it is the mm setting of the group leader that really
> matters. So cpuset_attach_old_cs should track the oldcs of the thread
> leader. In the latter case, effective_mems of child cpusets must always
> be a subset of the parent. So no real page migration will be necessary
> no matter which child cpuset is selected as cpuset_attach_old_cs.
> 
> IOW, cpuset_attach_old_cs should be updated to match the latest task
> group leader in cpuset_can_attach(), but fall back to that of the first
> task if there is no group leader in the taskset.
> 
> Suggested-by: Ridong Chen <ridong.chen@linux.dev>
> Signed-off-by: Waiman Long <longman@redhat.com>

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

> ---
>  kernel/cgroup/cpuset.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 5c777b1237a8..60e8149cc907 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2975,6 +2975,10 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  	return 0;
>  }
>  
> +/*
> + * cpuset_can_attach() and cpuset_attach() specific internal data
> + * Protected by cpuset_mutex
> + */
>  static struct cpuset *cpuset_attach_old_cs;
>  
>  /*
> @@ -3065,11 +3069,32 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>  	if (ret)
>  		goto out_unlock;
>  
> +	/*
> +	 * The cpuset_attach_old_cs is used mainly by cpuset_migrate_mm() to get
> +	 * the old_mems_allowed value. There are two ways that many-to-one
> +	 * cpuset migration can happen:
> +	 * 1) A multithread application with threads in different cpusets is
> +	 *    wholely migrated to a new cpuset.
> +	 * 2) Disabling v2 cpuset controller will move all the tasks in child
> +	 *    cpusets to the parent cpuset.
> +	 *
> +	 * In the former case, it is the mm setting of the group leader that
> +	 * really matters. So cpuset_attach_old_cs should track the oldcs of the
> +	 * group leader. It falls back to the oldcs of the first task if there
> +	 * is no group leader in the taskset. In the latter case, effective_mems
> +	 * of child cpusets must always be a subset of the parent. So no real
> +	 * page migration will be necessary no matter which child cpuset is
> +	 * selected as cpuset_attach_old_cs.
> +	 */
>  	cgroup_taskset_for_each(task, css, tset) {
>  		ret = task_can_attach(task);
>  		if (ret)
>  			goto out_unlock;
>  
> +		/* Update cpuset_attach_old_cs to the latest group leader */
> +		if (task == task->group_leader)
> +			cpuset_attach_old_cs = task_cs(task);
> +
>  		if (setsched_check) {
>  			ret = security_task_setscheduler(task);
>  			if (ret)

-- 
Best regards,
Ridong