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

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

On 6/24/2026 7:04 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 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.
> 
> Add new test_cpuset_prs.sh test cases to confirm that effective_cpus
> will inherit the parent's version if cpuset.cpus is empty.
> 
> [1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com
> 
> Suggested-by: Ridong Chen <ridong.chen@linux.dev>
> Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c                        | 45 +++++++++++--------
>   .../selftests/cgroup/test_cpuset_prs.sh       | 11 ++++-
>   2 files changed, 35 insertions(+), 21 deletions(-)
> 
> 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.
>    * The result is valid only if the given cpuset isn't a partition root.
>    */
>   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);
> +	bool has_cpus;
> +
> +	has_cpus = cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
> +	if (!has_cpus && is_in_v2_mode())
> +		cpumask_copy(new_cpus, parent->effective_cpus);
> +}
> +
> +/**
> + * compute_effective_nodemask - Compute the effective nodemask of the cpuset
> + * @new_cpus: the temp variable for the new effective_mems mask
> + * @cs: the cpuset the need to recompute the new effective_mems mask
> + * @parent: the parent cpuset
> + *
> + * For v2, the parent's effective_mems is inherited if nodemask empty.
> + */
> +static void compute_effective_nodemask(nodemask_t *new_mems,
> +				       struct cpuset *cs, struct cpuset *parent)
> +{
> +	bool has_mems;
> +
> +	has_mems = nodes_and(*new_mems, cs->mems_allowed, parent->effective_mems);
> +	if (!has_mems && is_in_v2_mode())
> +		nodes_copy(*new_mems, parent->effective_mems);
>   }
>   
>   /*
> @@ -2148,15 +2171,6 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>   			goto update_parent_effective;
>   		}
>   
> -		/*
> -		 * If it becomes empty, inherit the effective mask of the
> -		 * parent, which is guaranteed to have some CPUs unless
> -		 * it is a partition root that has explicitly distributed
> -		 * out all its CPUs.
> -		 */
> -		if (is_in_v2_mode() && !remote && cpumask_empty(tmp->new_cpus))
> -			cpumask_copy(tmp->new_cpus, parent->effective_cpus);
> -
>   		/*
>   		 * Skip the whole subtree if
>   		 * 1) the cpumask remains the same,
> @@ -2704,14 +2718,7 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
>   	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
>   		struct cpuset *parent = parent_cs(cp);
>   
> -		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;
> +		compute_effective_nodemask(new_mems, cp, parent);
>   
>   		/* Skip the whole subtree if the nodemask remains the same. */
>   		if (nodes_equal(*new_mems, cp->effective_mems)) {
> @@ -3923,7 +3930,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   
>   	parent = parent_cs(cs);
>   	compute_effective_cpumask(&new_cpus, cs, parent);
> -	nodes_and(new_mems, cs->mems_allowed, parent->effective_mems);
> +	compute_effective_nodemask(&new_mems, cs, parent);
>   
>   	if (!tmp || !cs->partition_root_state)
>   		goto update_tasks;
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 0d41aa0d343d..ca9bc38fdb95 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -495,13 +495,20 @@ REMOTE_TEST_MATRIX=(
>   	# Narrowing cpuset.cpus to previously sibling-excluded CPUs should
>   	# not return CPUs that were never actually owned.
>   	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
> -	      .      .     .         C3    .       .     p1:4|c11:1-2|c12:3 \
> +	      .      .     .       C3      .       .     p1:4|c11:1-2|c12:3 \
>   							 p1:P1|c11:P1|c12:P2 3"
>   	# Expanding cpuset.cpus to include a previously sibling-excluded CPU
>   	# after the sibling has become a member should correctly request it.
>   	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
> -	      .      .      P0     C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
> +	      .      .     P0      C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
>   							 p1:P1|c11:P0|c12:P2 2-3"
> +	# Cpusets with empty cpuset.cpus should inherit parent's effective_cpus
> +	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
> +	      .      P1    P1      .       .       .     p1:3-4|p2:5-6|c11:1-2|c12:3-4|c21:5|c22:5-6 \
> +							 p1:P1|p2:P1|c11:P1"
> +	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
> +	      .      P1    P1      .      O5=0     .     p1:3-4|p2:6|c11:1-2|c12:3-4|c21:6|c22:6 \
> +							 p1:P1|p2:P1|c11:P1"
>   )
>   
>   #

LGTM.

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

-- 
Best regards
Ridong