[PATCH v2 2/8] cgroup: document how cgroup IDs are assigned

[Li Zefan]

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 an array.

Signed-off-by: Li Zefan <lizefan@xxxxxxxxx>
---
 include/linux/cgroup.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 2bd052d..8c107e9 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'.
-- 
1.8.0.2

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