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>

1 files changed, 7 insertions, 1 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index cca570e188fb..4dfcd0e1b73e 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'.