cgroup/cpuset: Add warnings to catch inconsistency in exclusive CPUs

JIRA: https://issues.redhat.com/browse/RHEL-83455
Upstream Status: https://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git

commit 86888c7
Author: Waiman Long <longman@redhat.com>
Date:   Mon, 7 Apr 2025 17:21:05 -0400

    cgroup/cpuset: Add warnings to catch inconsistency in exclusive CPUs

    Add WARN_ON_ONCE() statements whenever new exclusive CPUs are being
    added to a partition root to catch inconsistency in the way exclusive
    CPUs are being handled in the cpuset code.

    Signed-off-by: Waiman Long <longman@redhat.com>
    Signed-off-by: Tejun Heo <tj@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

kernel/cgroup/cpuset.c

@@ -1711,13 +1711,15 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
 	 * The requested exclusive_cpus must not be allocated to other
 	 * partitions and it can't use up all the root's effective_cpus.
 	 *
-	 * Note that if there is any local partition root above it or
-	 * remote partition root underneath it, its exclusive_cpus must
-	 * have overlapped with subpartitions_cpus.
+	 * The effective_xcpus mask can contain offline CPUs, but there must
+	 * be at least one or more online CPUs present before it can be enabled.
+	 *
+	 * Note that creating a remote partition with any local partition root
+	 * above it or remote partition root underneath it is not allowed.
 	 */
 	compute_effective_exclusive_cpumask(cs, tmp->new_cpus, NULL);
-	if (cpumask_empty(tmp->new_cpus) ||
-	    cpumask_intersects(tmp->new_cpus, subpartitions_cpus) ||
+	WARN_ON_ONCE(cpumask_intersects(tmp->new_cpus, subpartitions_cpus));
+	if (!cpumask_intersects(tmp->new_cpus, cpu_active_mask) ||
 	    cpumask_subset(top_cpuset.effective_cpus, tmp->new_cpus))
 		return PERR_INVCPUS;
 
@@ -1813,6 +1815,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
 	 * left in the top cpuset.
 	 */
 	if (adding) {
+		WARN_ON_ONCE(cpumask_intersects(tmp->addmask, subpartitions_cpus));
 		if (!capable(CAP_SYS_ADMIN))
 			cs->prs_err = PERR_ACCESS;
 		else if (cpumask_intersects(tmp->addmask, subpartitions_cpus) ||
@@ -1922,7 +1925,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 	bool nocpu;
 
 	lockdep_assert_held(&cpuset_mutex);
-	WARN_ON_ONCE(is_remote_partition(cs));
+	WARN_ON_ONCE(is_remote_partition(cs));	/* For local partition only */
 
 	/*
 	 * new_prs will only be changed for the partcmd_update and
@@ -1968,7 +1971,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 		 * exclusive_cpus not set. Sibling conflict should only happen
 		 * if exclusive_cpus isn't set.
 		 */
-		xcpus = tmp->new_cpus;
+		xcpus = tmp->delmask;
 		if (compute_effective_exclusive_cpumask(cs, xcpus, NULL))
 			WARN_ON_ONCE(!cpumask_empty(cs->exclusive_cpus));
 
@@ -1989,9 +1992,20 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 		if (nocpu)
 			return PERR_NOCPUS;
 
-		deleting = cpumask_and(tmp->delmask, xcpus, parent->effective_xcpus);
-		if (deleting)
-			subparts_delta++;
+		/*
+		 * This function will only be called when all the preliminary
+		 * checks have passed. At this point, the following condition
+		 * should hold.
+		 *
+		 * (cs->effective_xcpus & cpu_active_mask) ⊆ parent->effective_cpus
+		 *
+		 * Warn if it is not the case.
+		 */
+		cpumask_and(tmp->new_cpus, xcpus, cpu_active_mask);
+		WARN_ON_ONCE(!cpumask_subset(tmp->new_cpus, parent->effective_cpus));
+
+		deleting = true;
+		subparts_delta++;
 		new_prs = (cmd == partcmd_enable) ? PRS_ROOT : PRS_ISOLATED;
 	} else if (cmd == partcmd_disable) {
 		/*
@@ -2045,6 +2059,15 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
 			deleting = cpumask_and(tmp->delmask, tmp->delmask,
 					       parent->effective_xcpus);
 		}
+		/*
+		 * The new CPUs to be removed from parent's effective CPUs
+		 * must be present.
+		 */
+		if (deleting) {
+			cpumask_and(tmp->new_cpus, tmp->delmask, cpu_active_mask);
+			WARN_ON_ONCE(!cpumask_subset(tmp->new_cpus, parent->effective_cpus));
+		}
+
 		/*
 		 * Make partition invalid if parent's effective_cpus could
 		 * become empty and there are tasks in the parent.