cgroup: suppress spurious de-populated events

During task migration, tasks may transfer between two css_sets which
are associated with the same cgroup.  If those tasks are the only
tasks in the cgroup, this currently triggers a spurious de-populated
event on the cgroup.

Fix it by bumping up populated count before bumping it down during
migration to ensure that it doesn't reach zero spuriously.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e97772b42dfb796f22bbd86dc6d600dae1f38f8e..5d452e7fcb4fecabfe09c667ffd37ce5c59f21ab 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -678,6 +678,9 @@ static void css_set_move_task(struct task_struct *task,
 {
        lockdep_assert_held(&css_set_lock);
 
+       if (to_cset && !css_set_populated(to_cset))
+               css_set_update_populated(to_cset, true);
+
        if (from_cset) {
                struct css_task_iter *it, *pos;
 
@@ -711,8 +714,6 @@ static void css_set_move_task(struct task_struct *task,
                 */
                WARN_ON_ONCE(task->flags & PF_EXITING);
 
-               if (!css_set_populated(to_cset))
-                       css_set_update_populated(to_cset, true);
                rcu_assign_pointer(task->cgroups, to_cset);
                list_add_tail(&task->cg_list, use_mg_tasks ? &to_cset->mg_tasks :
                                                             &to_cset->tasks);