linux-kernel - Re: [PATCH 1/8] cgroup: implement cgroup_is

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Date:	Mon, 8 Apr 2013 16:47:50 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Li Zefan <lizefan@...wei.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>,
	Glauber Costa <glommer@...allels.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Cgroups <cgroups@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 1/8] cgroup: implement cgroup_is_ancestor()

On Mon 08-04-13 16:20:11, Li Zefan wrote:
[...]
> @@ -5299,6 +5300,26 @@ struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
>  	return css ? css : ERR_PTR(-ENOENT);
>  }
>  
> +/**
> + * cgroup_is_ancestor - test "root" cgroup is an ancestor of "child"
> + * @child: the cgroup to be tested.
> + * @root: the cgroup supposed to be an ancestor of the child.
> + *
> + * Returns true if "root" is an ancestor of "child" in its hierarchy.
> + */
> +bool cgroup_is_ancestor(struct cgroup *child, struct cgroup *root)
> +{
> +	int depth = child->depth;

Is this functionality helpful for other controllers but memcg?
css_is_ancestor is currently used only by memcg code AFAICS and we can
get the same functionality easily by using something like:
	
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d195f40..37bbbff 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1472,11 +1472,13 @@ void mem_cgroup_update_lru_size(struct lruvec *lruvec, enum lru_list lru,
 bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
 				  struct mem_cgroup *memcg)
 {
-	if (root_memcg == memcg)
-		return true;
-	if (!root_memcg->use_hierarchy || !memcg)
-		return false;
-	return css_is_ancestor(&memcg->css, &root_memcg->css);
+	struct mem_cgroup *parent = memcg;
+	do {
+		if (parent == root_memcg)
+			return true;
+	} while ((parent = parent_mem_cgroup(parent)));
+
+	return false;
 }
 
 static bool mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,

In both cases we have to go up the hierarchy.

> +
> +	if (depth < root->depth)
> +		return false;
> +
> +	while (depth-- != root->depth)
> +		child = child->parent;
> +
> +	return (child == root);
> +}
> +
>  #ifdef CONFIG_CGROUP_DEBUG
>  static struct cgroup_subsys_state *debug_css_alloc(struct cgroup *cont)
>  {
> -- 
> 1.8.0.2
> --
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-- 
Michal Hocko
SUSE Labs
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