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

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

On 6/23/2026 6:45 AM, 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 adding the empty mask checks and
> inheriting the parent's versions if empty.
> 
> [1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com
> 
> Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index aff86acea701..bc0207fd6e57 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3925,6 +3925,14 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   	compute_effective_cpumask(&new_cpus, cs, parent);
>   	nodes_and(new_mems, cs->mems_allowed, parent->effective_mems);
>   
> +	if (is_in_v2_mode()) {
> +		/* Inherit parent's effective_cpus/mems if empty */
> +		if (cpumask_empty(&new_cpus))
> +			cpumask_copy(&new_cpus, parent->effective_cpus);
> +		if (nodes_empty(new_mems))
> +			new_mems = parent->effective_mems;
> +	}
> +
>   	if (!tmp || !cs->partition_root_state)
>   		goto update_tasks;
>   

I noticed that compute_effective_cpumask(...) is called in several 
places, so I think the logic should be consolidated into that function.

```
static void compute_effective_cpumask(struct cpumask *new_cpus,
				      struct cpuset *cs, struct cpuset *parent)
{
	cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
	if (cpumask_empty(&new_cpus) && is_in_v2_mode())
		cpumask_copy(&new_cpus, parent->effective_cpus);
}

```

Similarly, for new_mems, should we introduce a dedicated helper like 
compute_effective_nodemask? The same fallback logic is needed in 
update_nodemasks_hier:


```
static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
{
...
		bool has_mems = nodes_and(*new_mems, cp->mems_allowed, 
parent->effective_mems);

		/*
		 * If it becomes empty, inherit the effective mask of the
		 * parent, which is guaranteed to have some MEMs.
		 */
		if (is_in_v2_mode() && !has_mems)
			*new_mems = parent->effective_mems;
...
```

-- 
Best regards
Ridong