From b414dc09a31d41d696093a4cce9fb2853a5ecd4e Mon Sep 17 00:00:00 2001
From: Li Zefan <lizefan@huawei.com>
Date: Wed, 31 Jul 2013 09:51:06 +0800
Subject: cgroup: document how cgroup IDs are assigned

As cgroup id has been used in netprio cgroup and will be used in memcg,
it's important to make it clear how a cgroup id is allocated.

For example, in netprio cgroup, the id is used as index of anarray.

Signed-off-by: Li Zefan <lizefan@huwei.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index cca570e..4dfcd0e 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -161,7 +161,13 @@ struct cgroup_name {
 struct cgroup {
 	unsigned long flags;		/* "unsigned long" so bitops work */
 
-	int id;				/* idr allocated in-hierarchy ID */
+	/*
+	 * idr allocated in-hierarchy ID.
+	 *
+	 * The ID of the root cgroup is always 0, and a new cgroup
+	 * will be assigned with a smallest available ID.
+	 */
+	int id;
 
 	/*
 	 * We link our 'sibling' struct into our parent's 'children'.
-- 
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