From f5b5cf2be574df6f25cbf1b000960340bff9a0bb Mon Sep 17 00:00:00 2001 From: Guopeng Zhang Date: Tue, 20 Oct 2026 21:36:15 +0800 Subject: [PATCH] cgroup/cpuset: Fix task checking when enabling a partition 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") Cc: stable@vger.kernel.org # v6.19+ Signed-off-by: Guopeng Zhang Reviewed-by: Waiman Long Signed-off-by: Tejun Heo --- 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"