[PATCH 1/2] cgroup/cpuset: Avoid unnecessary cpus & mems update in cpuset_hotplug_update_tasks()

[Waiman Long]

As reported by sashiko [1], cpuset_hotplug_update_tasks() may perform
unnecessary task iteration and updating of tasks' CPU and node masks
when mems_allowed and/or cpus_allowed are not set in cpuset v2. It is
due to the fact that the temporary new_cpus and new_mems masks do not
inherit parent's effective_cpus/mems when they are empty which is the
expected behavior for cpuset v2 since commit 4ec22e9c5a90 ("cpuset:
Enable cpuset controller in default hierarchy").

Fix that and avoid unnecessay work by adding the empty mask checks and
inheriting the parent's versions if empty.

[1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com

Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index aff86acea701..bc0207fd6e57 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3925,6 +3925,14 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	compute_effective_cpumask(&new_cpus, cs, parent);
 	nodes_and(new_mems, cs->mems_allowed, parent->effective_mems);
 
+	if (is_in_v2_mode()) {
+		/* Inherit parent's effective_cpus/mems if empty */
+		if (cpumask_empty(&new_cpus))
+			cpumask_copy(&new_cpus, parent->effective_cpus);
+		if (nodes_empty(new_mems))
+			new_mems = parent->effective_mems;
+	}
+
 	if (!tmp || !cs->partition_root_state)
 		goto update_tasks;
 
-- 
2.54.0