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cgroup/cpuset: Fix task checking when enabling a partition

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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: b1bcaed1e3 ("cpuset: Treat cpusets in attaching as populated")
Cc: stable@vger.kernel.org # v6.19+
Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
This commit is contained in:
Guopeng Zhang 2026-10-20 21:36:15 +08:00 committed by Tejun Heo
parent bf6fbf46b5
commit f5b5cf2be5
2 changed files with 5 additions and 2 deletions
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4
kernel/cgroup/cpuset.c
View File

@ -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;
}

3
tools/testing/selftests/cgroup/test_cpuset_prs.sh
View File

@ -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"