[PATCH-next] cgroup/cpuset: Fix incorrect CPU accounting with parent's effective_xcpus expansion

[Waiman Long <longman@redhat.com>]

Commit ec0254b9eb49 ("cgroup/cpuset: Release invalid isolated CPUs with
a shrunk root partition parent") fixes incorrect isolated_cpus accounting
when contraction of a parent's effective_xcpus can cause a child partition
to lose isolated CPUs. Expansion of parent's effective_cpus can also allow
a child partition to gain more effective_xcpus. However, the current code
doesn't handle this case correctly leading to incorrect effective_cpus
and isolated_cpus accounting. Fix that by looking for this possible
expansion of effective_xcpus and perform the proper accounting action.

New test cases are added to test this effective_xcpus expansion scenarios.

Reported-by: Guopeng Zhang <guopeng.zhang@linux.dev>
Link: https://lore.kernel.org/lkml/20261027123837.100909-1-guopeng.zhang@linux.dev
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c                            | 12 ++++++++++++
 tools/testing/selftests/cgroup/test_cpuset_prs.sh | 15 +++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 3219f927c35e..39c81a0a8ab9 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2097,6 +2097,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 							     parent->effective_xcpus);
 			if (!cpumask_empty(tmp->addmask))
 				isolated_cpus_release(cs, tmp->addmask);
+			/* Expansion of parent's effective_xcpus is handled below */
 		}
 		if (nocpu) {
 			part_error = PERR_NOCPUS;
@@ -2106,11 +2107,22 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 						     parent->effective_xcpus);
 		} else if ((parent_prs == PRS_ISOLATED) &&
 			   ((old_prs == PRS_ROOT) || (old_prs == -PRS_ROOT))) {
+			/* Parent partition state changed to PRS_ISOLATED */
 			part_error = PERR_ISOPARENT;
 			if (old_prs == PRS_ROOT) {
 				cpumask_copy(tmp->addmask, cs->effective_xcpus);
 				adding = true;
 			}
+		} else if (is_partition_valid(cs) &&
+			   !cpumask_equal(tmp->delmask, cs->effective_xcpus)) {
+			/*
+			 * If there is an expansion of parent's effective_xcpus,
+			 * we may take more xcpus from parent. The parent's
+			 * effective_cpus should still have active CPUs or nocpu
+			 * should have been set.
+			 */
+			deleting = cpumask_andnot(tmp->delmask, tmp->delmask,
+						  cs->effective_xcpus);
 		} else if (is_partition_invalid(cs)) {
 			struct cgroup_subsys_state *css;
 			struct cpuset *child;
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 83ef6c781af3..844efdf029cd 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -531,6 +531,21 @@ TEST_MATRIX=(
 	" C1-4:P1 C3-4:P2   .      .    T:C4-6   .      .      .     0 A1:5-6|A2:4 A1:P1|A2:P2 4"
 	" C1-4:P1 C1-2:P2   .      .    T:C4-6   .      .      .     0 A1:4-6|A2:4-6 A1:P1|A2:P-2 ."
 
+	# The extra CPUs gained by a child partition when its parent expands the
+	# exclusive CPUs have to be taken away from the parent.
+	" C2-6:X2-5:P1 CX3-6:P1 \
+	                    .      .    X2-6     .      .      .     0 A1:2|A2:3-6|XA2:3-6 A1:P1|A2:P1 ."
+	" C2-6:X2-5:P1 CX3-6:P2 \
+	                    .      .    X2-6     .      .      .     0 A1:2|A2:3-6|XA2:3-6 A1:P1|A2:P2 3-6"
+
+	# Both expansion and contraction of parent's effective_xcpus will be
+	# correctly handled in child partition and the child can be invalidated
+	# if a populated parent could lose all its effective_cpus.
+	" C2-6:X2-5:P1 CX3,5-6:P2 \
+	                    .      .    X4-6     .      .      .     0 A1:4|A2:5-6|XA2:5-6 A1:P1|A2:P2 5-6"
+	" C2-6:X2-5:P1 CX3-6:P2 \
+	                    .      .   T:X4-6    .      .      .     0 A1:4-6|A2:4-6 A1:P1|A2:P-2 ."
+
 	#
 	#  old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
 	#  ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------
-- 
2.56.0