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

[Guopeng Zhang <guopeng.zhang@linux.dev>]

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"
-- 
2.43.0