memcg: simplify mem_cgroup_iter

Current implementation of mem_cgroup_iter has to consider both css and
memcg to find out whether no group has been found (css==NULL - aka the
loop is completed) and that no memcg is associated with the found node
(!memcg - aka css_tryget failed because the group is no longer alive).
This leads to awkward tweaks like tests for css && !memcg to skip the
current node.

It will be much easier if we got rid off css variable altogether and only
rely on memcg.  In order to do that the iteration part has to skip dead
nodes.  This sounds natural to me and as a nice side effect we will get a
simple invariant that memcg is always alive when non-NULL and all nodes
have been visited otherwise.

We could get rid of the surrounding while loop but keep it in for now to
make review easier. It will go away in the following patch.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizefan@huawei.com>
Cc: Ying Han <yinghan@google.com>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 408a5c75d77d3dc309b4dbf37589e8574b0e261a..614d0d9c0debcf90fd810b4424687532726c21bb 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1116,7 +1116,6 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
        rcu_read_lock();
        while (!memcg) {
                struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
-               struct cgroup_subsys_state *css = NULL;
 
                if (reclaim) {
                        int nid = zone_to_nid(reclaim->zone);
@@ -1159,51 +1158,50 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
                 * explicit visit.
                 */
                if (!last_visited) {
-                       css = &root->css;
+                       memcg = root;
                } else {
                        struct cgroup *prev_cgroup, *next_cgroup;
 
                        prev_cgroup = (last_visited == root) ? NULL
                                : last_visited->css.cgroup;
-                       next_cgroup = cgroup_next_descendant_pre(prev_cgroup,
-                                       root->css.cgroup);
-                       if (next_cgroup)
-                               css = cgroup_subsys_state(next_cgroup,
-                                               mem_cgroup_subsys_id);
-               }
+skip_node:
+                       next_cgroup = cgroup_next_descendant_pre(
+                                       prev_cgroup, root->css.cgroup);
 
-               /*
-                * Even if we found a group we have to make sure it is alive.
-                * css && !memcg means that the groups should be skipped and
-                * we should continue the tree walk.
-                * last_visited css is safe to use because it is protected by
-                * css_get and the tree walk is rcu safe.
-                */
-               if (css == &root->css || (css && css_tryget(css)))
-                       memcg = mem_cgroup_from_css(css);
+                       /*
+                        * Even if we found a group we have to make sure it is
+                        * alive. css && !memcg means that the groups should be
+                        * skipped and we should continue the tree walk.
+                        * last_visited css is safe to use because it is
+                        * protected by css_get and the tree walk is rcu safe.
+                        */
+                       if (next_cgroup) {
+                               struct mem_cgroup *mem = mem_cgroup_from_cont(
+                                               next_cgroup);
+                               if (css_tryget(&mem->css))
+                                       memcg = mem;
+                               else {
+                                       prev_cgroup = next_cgroup;
+                                       goto skip_node;
+                               }
+                       }
+               }
 
                if (reclaim) {
-                       struct mem_cgroup *curr = memcg;
-
                        if (last_visited)
                                css_put(&last_visited->css);
 
-                       if (css && !memcg)
-                               curr = mem_cgroup_from_css(css);
-
-                       iter->last_visited = curr;
+                       iter->last_visited = memcg;
                        smp_wmb();
                        iter->last_dead_count = dead_count;
 
-                       if (!css)
+                       if (!memcg)
                                iter->generation++;
                        else if (!prev && memcg)
                                reclaim->generation = iter->generation;
-               } else if (css && !memcg) {
-                       last_visited = mem_cgroup_from_css(css);
                }
 
-               if (prev && !css)
+               if (prev && !memcg)
                        goto out_unlock;
        }
 out_unlock: