[PATCH 1/4] cgroup/cpuset: Reset partition data when an invalid partition stays invalid

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

From: Ridong Chen <chenridong@xiaomi.com>

When cpuset.cpus or cpuset.cpus.exclusive of a partition root is
changed, update_cpumask() and update_exclusive_cpumask() copy the new
effective_xcpus computed for the trial cpuset into the cpuset, and call
reset_partition_data() only if the partition was valid before and is
invalid now:

	if ((old_prs > 0) && !is_partition_valid(cs))
		reset_partition_data(cs);

An invalid partition root (old_prs < 0) that stays invalid is not
covered. Its effective_xcpus is then left with the trial value, which
is computed from user_xcpus() and so falls back to cpuset.cpus when
cpuset.cpus.exclusive is cleared. As an invalid partition root does not
own any exclusive CPUs, and rm_siblings_excl_cpus() treats a non-empty
effective_xcpus of a sibling without cpuset.cpus.exclusive as owned,
those CPUs can then no longer be claimed by a sibling:

  # A1: cpuset.cpus=2-3, cpuset.cpus.exclusive=2-3, root, with tasks
  # offline CPUs 2-3 -> A1 becomes invalid
  echo > A1/cpuset.cpus.exclusive
  cat A1/cpuset.cpus.exclusive.effective    # 2-3, expected empty
  echo 3 > B1/cpuset.cpus.exclusive         # -EINVAL

Reset the partition data whenever the cpuset is a partition root, valid
or not, and ends up invalid. Add a test case for it.

Fixes: f62a5d39368e ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Link: https://sashiko.dev/#/patchset/20261101114206.2434398-1-ridong.chen%40linux.dev?part=15
Signed-off-by: Ridong Chen <chenridong@xiaomi.com>
---
 kernel/cgroup/cpuset.c                            | 4 ++--
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 4 ++++
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3052413e41e7..bc16fd1b1c5e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2607,7 +2607,7 @@ static int update_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->cpus_allowed, trialcs->cpus_allowed);
 	cpumask_copy(cs->effective_xcpus, trialcs->effective_xcpus);
-	if ((old_prs > 0) && !is_partition_valid(cs))
+	if (old_prs && !is_partition_valid(cs))
 		reset_partition_data(cs);
 	spin_unlock_irq(&callback_lock);
 
@@ -2672,7 +2672,7 @@ static int update_exclusive_cpumask(struct cpuset *cs, struct cpuset *trialcs,
 	spin_lock_irq(&callback_lock);
 	cpumask_copy(cs->exclusive_cpus, trialcs->exclusive_cpus);
 	cpumask_copy(cs->effective_xcpus, trialcs->effective_xcpus);
-	if ((old_prs > 0) && !is_partition_valid(cs))
+	if (old_prs && !is_partition_valid(cs))
 		reset_partition_data(cs);
 	spin_unlock_irq(&callback_lock);
 
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index bdca6c471f87..9c0c78ca6329 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -434,6 +434,10 @@ TEST_MATRIX=(
 	# cpuset.cpus is allowed, but those extra CPUs are ignored.
 	"  C2-3:P1   C3:P1  .      .      .     C2-4    .      .     0 A1:|A2:2-3 A1:P1|A2:P1"
 
+	# Clearing cpuset.cpus.exclusive of an invalid partition releases its
+	# exclusive CPUs, so a sibling can claim them.
+	" CX2-3:P1    .     .     C3  T:O2=0:O3=0:X  .     .      X3   0 XA1:|XB1:3 A1:P-1"
+
 	# A child cannot recover until its exclusive CPUs are available again.
 	" CX1-4:P1 CX3-4:P1 .     .     X1-2     .      .      .     0 A1:1-2|A2:1-2|XA2: A1:P1|A2:P-1"
 	" CX1-4:P1 CX3-4:P1 .     .  X1-2:X1-4   .      .      .     0 A1:1-2|A2:3-4|XA2:3-4 A1:P1|A2:P1"
-- 
2.34.1