Re: [PATCH-next 3/6] cgroup/cpuset: Fix incorrect effective_cpus bug in update_cpumasks_hier()

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

On 10/22/2026 11:16 AM, Waiman Long wrote:
> With the introduction of cpuset.cpus.exclusive, the computation of
> effective_cpus differs depending on if a cpuset is a valid partition
> root or not.
> 
> In update_cpumasks_hier(), effective_cpus is computed early
> to check if the current subtree can be skipped. Later on
> update_parent_effective_cpumask() may be called which can change the
> partition state from valid to invalid or vice versa. If the partition
> state changes, the previously computed effective_cpus may no longer be
> valid. Fix that by recomputing effective_cpus again when that happens.
> 
> Also add a new test case into test_cpuset_prs.sh to verify this change.
> 
> Fixes: e2ffe502ba45 ("cgroup/cpuset: Add cpuset.cpus.exclusive for v2")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c                            | 10 ++++++++--
>   tools/testing/selftests/cgroup/test_cpuset_prs.sh |  3 +++
>   2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index db6e4663f4be..7b9062326f35 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -2316,10 +2316,16 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>   		if (update_parent) {
>   			update_parent_effective_cpumask(cp, partcmd_update, NULL, tmp);
>   			/*
> -			 * The cpuset partition_root_state may become
> -			 * invalid. Capture it.
> +			 * Capture current partition_root_state as it may have
> +			 * changed. Recompute effective cpumask if changed.
>   			 */
>   			new_prs = cp->partition_root_state;
> +			if (new_prs != old_prs) {
> +				if (new_prs > 0)
> +					compute_partition_effective_cpumask(cp, tmp->new_cpus);
> +				else
> +					compute_effective_cpumask(tmp->new_cpus, cp, parent);
> +			}
>   		}
>   

If cs is a partition (remote or local), we call 
compute_partition_effective_cpumask to compute the effective cpumask; otherwise 
we should call compute_effective_cpumask. Wouldn't it be better to call 
compute_partition_effective_cpumask whenever cs is a valid partition, so we can 
eliminate the branch outside?

```
static void compute_effective_cpumask(struct cpumask *new_cpus,
				      struct cpuset *cs, struct cpuset *parent)
{
	bool has_cpus;

	if (is_remote_partition (cs) ||
	    (is_partition_valid(cs) && is_partition_valid(parent)))
		return compute_partition_effective_cpumask(cs, &new_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);
}
```

>   		spin_lock_irq(&callback_lock);
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 47791591002f..8595b6b4b7bf 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -364,6 +364,9 @@ TEST_MATRIX=(
>   	" C1-4:X2-4 C2-4:X3-4:P1 C3-4:X4:P2 \
>   				   .      .      X5      .      .    0 A1:1-4|A2:2-4|A3:3-4 \
>   								       A1:P0|A2:P-1|A3:P-2 ."
> +	" C1-5:X3-5:P2 C3-5:X4-5:P2 C4-5:X5:P2 \
> +				   .    X1-4     .       .      .    0 A1:1-3|A2:4|A3:4 \
> +								       A1:P2|A2:P2|A3:P-2 1-4"
>   	" C1-4:X2-4 C1-4:X3-4:P1 C3-4:X4:P2 \
>   				   .      .      .      X1      .    0 A1:1-2|A2:3-4|A3:3-4 \
>   								       A1:P0|A2:P1|A3:P-2 ."

-- 
Best regards
Ridong