Re: [PATCH] cgroup/cpuset: Fix task checking when enabling a partition

[Waiman Long <longman@redhat.com>]

On 10/20/26 9:36 AM, Guopeng Zhang wrote:
> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
>
> Creating a child partition can fail when tasks are present in its
> descendants, even though the parent has no tasks of its own.
>
> On a cgroup v2 system with CPUs 0-7 online:
>
>      cd /sys/fs/cgroup
>      echo +cpuset > cgroup.subtree_control
>      mkdir A
>      echo 1-2 > A/cpuset.cpus
>      echo root > A/cpuset.cpus.partition
>      echo +cpuset > A/cgroup.subtree_control
>      mkdir A/B
>      echo 1-2 > A/B/cpuset.cpus
>      echo +cpuset > A/B/cgroup.subtree_control
>      mkdir A/B/C
>      echo 1-2 > A/B/C/cpuset.cpus
>      echo $$ > A/B/C/cgroup.procs
>      echo root > A/B/cpuset.cpus.partition
>      cat A/B/cpuset.cpus.partition
>      root invalid (Parent unable to distribute cpu downstream)
>      cat A/cpuset.cpus.effective
>      1-2
>
> B should be a valid root, leaving A with an empty effective CPU list.
> The task in C belongs to B's partition and does not need CPUs in A.
>
> partition_is_populated() is called with B as excluded_child, but
> continue only skips B in the pre-order walk. The walk still visits C
> and finds the task there, so A is considered populated.
>
> Fix this by skipping the whole excluded subtree with
> css_rightmost_descendant(), as is already done for valid child
> partitions. Also add a test case to test_cpuset_prs.sh with a task in
> a grandchild cpuset.
>
> Fixes: b1bcaed1e39a ("cpuset: Treat cpusets in attaching as populated")
> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
> ---
>   kernel/cgroup/cpuset.c                            | 4 ++--
>   tools/testing/selftests/cgroup/test_cpuset_prs.sh | 3 +++
>   2 files changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 585a982249ad..71644482faa7 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -478,10 +478,10 @@ static inline bool partition_is_populated(struct cpuset *cs,
>   
>   	rcu_read_lock();
>   	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
> -		if (cp == cs || cp == excluded_child)
> +		if (cp == cs)
>   			continue;
>   
> -		if (is_partition_valid(cp)) {
> +		if (cp == excluded_child || is_partition_valid(cp)) {
>   			pos_css = css_rightmost_descendant(pos_css);
>   			continue;
>   		}
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index d04aad697b96..9978c560b398 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -452,6 +452,9 @@ TEST_MATRIX=(
>   	# Enabling partition with child cpusets is allowed
>   	"   C0-1    C1      .    C2-3    P1      .      .      .     0 A1:0-1|A2:1 A1:P1"
>   
> +	# Tasks below a new partition belong to it, not to its parent.
> +	" C1-2:P1 C1-2 C1-2:T    .      .      P1     .      .     0 A1:|A2:1-2|A3:1-2 A1:P1|A2:P1"
> +
>   	# A partition root with non-partition root parent is invalid| but it
>   	# can be made valid if its parent becomes a partition root too.
>   	"   C0-1    C1      .    C2-3     .      P2     .      .     0 A1:0-1|A2:1 A1:P0|A2:P-2"
Reviewed-by: Waiman Long <longman@redhat.com>