cpuset: Remove unnecessary checks in rebuild_sched_domains_locked

Commit 406100f ("cpuset: fix race between hotplug work and later CPU
offline") added a check for empty effective_cpus in partitions for cgroup
v2. However, this check did not account for remote partitions, which were
introduced later.

After commit 2125c00 ("cgroup/cpuset: Make cpuset hotplug processing
synchronous"), cpuset hotplug handling is now synchronous. This eliminates
the race condition with subsequent CPU offline operations that the original
check aimed to fix.

Instead of extending the check to support remote partitions, this patch
removes all the redundant effective_cpus check. Additionally, it adds a
check and warning to verify that all generated sched domains consist of
active CPUs, preventing partition_sched_domains from being invoked with
offline CPUs.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: Chen Ridong

kernel/cgroup/cpuset.c

@@ -1103,53 +1103,33 @@ void dl_rebuild_rd_accounting(void)
  */
 void rebuild_sched_domains_locked(void)
 {
-	struct cgroup_subsys_state *pos_css;
 	struct sched_domain_attr *attr;
 	cpumask_var_t *doms;
-	struct cpuset *cs;
 	int ndoms;
+	int i;
 
 	lockdep_assert_cpus_held();
 	lockdep_assert_held(&cpuset_mutex);
 	force_sd_rebuild = false;
 
-	/*
-	 * If we have raced with CPU hotplug, return early to avoid
-	 * passing doms with offlined cpu to partition_sched_domains().
-	 * Anyways, cpuset_handle_hotplug() will rebuild sched domains.
-	 *
-	 * With no CPUs in any subpartitions, top_cpuset's effective CPUs
-	 * should be the same as the active CPUs, so checking only top_cpuset
-	 * is enough to detect racing CPU offlines.
-	 */
-	if (cpumask_empty(subpartitions_cpus) &&
-	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
-		return;
+	/* Generate domain masks and attrs */
+	ndoms = generate_sched_domains(&doms, &attr);
 
 	/*
-	 * With subpartition CPUs, however, the effective CPUs of a partition
-	 * root should be only a subset of the active CPUs.  Since a CPU in any
-	 * partition root could be offlined, all must be checked.
-	 */
-	if (!cpumask_empty(subpartitions_cpus)) {
-		rcu_read_lock();
-		cpuset_for_each_descendant_pre(cs, pos_css, &top_cpuset) {
-			if (!is_partition_valid(cs)) {
-				pos_css = css_rightmost_descendant(pos_css);
-				continue;
-			}
-			if (!cpumask_subset(cs->effective_cpus,
-					    cpu_active_mask)) {
-				rcu_read_unlock();
-				return;
-			}
-		}
-		rcu_read_unlock();
+	* cpuset_hotplug_workfn is invoked synchronously now, thus this
+	* function should not race with CPU hotplug. And the effective CPUs
+	* must not include any offline CPUs. Passing an offline CPU in the
+	* doms to partition_sched_domains() will trigger a kernel panic.
+	*
+	* We perform a final check here: if the doms contains any
+	* offline CPUs, a warning is emitted and we return directly to
+	* prevent the panic.
+	*/
+	for (i = 0; i < ndoms; ++i) {
+		if (WARN_ON_ONCE(!cpumask_subset(doms[i], cpu_active_mask)))
+			return;
 	}
 
-	/* Generate domain masks and attrs */
-	ndoms = generate_sched_domains(&doms, &attr);
-
 	/* Have scheduler rebuild the domains */
 	partition_sched_domains(ndoms, doms, attr);
 }