Re: [PATCH-next v3 3/5] cgroup/cpuset: Made cpuset_attach_old_cs track task group leaders

[Guopeng Zhang <guopeng.zhang@linux.dev>]

在 2026/5/27 23:37, Waiman Long 写道:
> 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
> or disabling of v2 cpuset controller will move all the tasks in child
> cpusets to the parent cpuset.
> 
> In the former case, t 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().
> 
> Suggested-by: Ridong Chen <ridong.chen@linux.dev>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 4457c4f11fce..b233a71f9b7c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2967,6 +2967,20 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  /*
>   * cpuset_can_attach() and cpuset_attach() specific internal data
>   * Protected by cpuset_mutex
> + *
> + * The cpuset_attach_old_cs is used mainly by cpuset_migrate_mm() tp get the
> + * old_mems_allowed value. There are two ways that many-to-one cpuset migration
> + * can happen:
Hi Waiman,

I applied this series locally and ran some of my test cases. I didn't
observe any issue so far.

While doing a static/checkpatch pass, I noticed a few minor issues in
patches 3, 4 and 5. They are all non-functional nits.

For this patch, I only noticed a couple of small wording/typo nits in
the new comment:

s/tp get/to get/

Best,
Guopeng
> + * 1) A multithread application with threads in different cpusets is wholely
> + *    moved 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 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.
>   */
>  static struct cpuset *cpuset_attach_old_cs;
>  static bool attach_cpus_updated;
> @@ -3069,6 +3083,10 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>  		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)