cgroup: Skip subtree root in cgroup_update_dfl_csses()

The cgroup_update_dfl_csses() function updates css associations when a
cgroup's subtree_control file is modified. Any changes made to a cgroup's
subtree_control file, however, will only affect its descendants but not
the cgroup itself. So there is no point in migrating csses associated
with that cgroup. We can skip them instead.
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -2971,6 +2971,15 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {
 		struct cgrp_cset_link *link;
 
+		/*
+		 * As cgroup_update_dfl_csses() is only called by
+		 * cgroup_apply_control(). The csses associated with the
+		 * given cgrp will not be affected by changes made to
+		 * its subtree_control file. We can skip them.
+		 */
+		if (dsct == cgrp)
+			continue;
+
 		list_for_each_entry(link, &dsct->cset_links, cset_link)
 			cgroup_migrate_add_src(link->cset, dsct, &mgctx);
 	}