linux-kernel - Re: [patch -v4 4/8] memcg: enhance memcg iterator to support predicates

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Date:	Mon, 10 Jun 2013 09:48:54 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org, Ying Han <yinghan@...gle.com>,
	Hugh Dickins <hughd@...gle.com>,
	Glauber Costa <glommer@...allels.com>,
	Michel Lespinasse <walken@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, Tejun Heo <tj@...nel.org>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [patch -v4 4/8] memcg: enhance memcg iterator to support
 predicates

Just for the record. I squash the following doc update to the patch in
the next version.
---
diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 811967a..9ca85ff 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -42,11 +42,19 @@ struct mem_cgroup_reclaim_cookie {
 };
 
 enum mem_cgroup_filter_t {
-	VISIT,
-	SKIP,
-	SKIP_TREE,
+	VISIT,		/* visit current node */
+	SKIP,		/* skip the current node and continue traversal */
+	SKIP_TREE,	/* skip the whole subtree and continue traversal */
 };
 
+/*
+ * mem_cgroup_filter_t predicate might instruct mem_cgroup_iter_cond how to
+ * iterate through the hierarchy tree. Each tree element is checked by the
+ * predicate before it is returned by the iterator. If a filter returns
+ * SKIP or SKIP_TREE then the iterator code continues traversal (with the
+ * next node down the hierarchy or the next node that doesn't belong under the
+ * memcg's subtree).
+ */
 typedef enum mem_cgroup_filter_t
 (*mem_cgroup_iter_filter)(struct mem_cgroup *memcg, struct mem_cgroup *root);
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 91740f7..43e955a 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1073,6 +1073,14 @@ skip_node:
 			prev_cgroup = next_cgroup;
 			goto skip_node;
 		case SKIP_TREE:
+			/*
+			 * cgroup_rightmost_descendant is not an optimal way to
+			 * skip through a subtree (especially for imbalanced
+			 * trees leaning to right) but that's what we have right
+			 * now. More effective solution would be traversing
+			 * right-up for first non-NULL without calling
+			 * cgroup_next_descendant_pre afterwards.
+			 */
 			prev_cgroup = cgroup_rightmost_descendant(next_cgroup);
 			goto skip_node;
 		case VISIT:
-- 
Michal Hocko
SUSE Labs
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