Re: [RFC 2/5] memcg: rework mem_cgroup_iter to use cgroup iterators

[Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

(2012/11/14 0:30), Michal Hocko wrote:
> mem_cgroup_iter curently relies on css->id when walking down a group
> hierarchy tree. This is really awkward because the tree walk depends on
> the groups creation ordering. The only guarantee is that a parent node
> is visited before its children.
> Example
>   1) mkdir -p a a/d a/b/c
>   2) mkdir -a a/b/c a/d
> Will create the same trees but the tree walks will be different:
>   1) a, d, b, c
>   2) a, b, c, d
> 
> 574bd9f7 (cgroup: implement generic child / descendant walk macros) has
> introduced generic cgroup tree walkers which provide either pre-order
> or post-order tree walk. This patch converts css->id based iteration
> to pre-order tree walk to keep the semantic with the original iterator
> where parent is always visited before its subtree.
> 
> cgroup_for_each_descendant_pre suggests using post_create and
> pre_destroy for proper synchronization with groups addidition resp.
> removal. This implementation doesn't use those because a new memory
> cgroup is fully initialized in mem_cgroup_create and css_tryget makes
> sure that the group is alive when we encounter it by iterator.
> 
> If the reclaim cookie is used we need to store the last visited group
> into the iterator so we have to be careful that it doesn't disappear in
> the mean time. Elevated reference count on the memcg guarantees that
> the group will not vanish even though it has been already removed from
> the tree. In such a case css_tryget will fail and the iteration is
> retried (groups are linked with RCU safe lists so the forward progress
> is still possible). iter_lock will make sure that only one reclaimer
> will see the last_visited group and the reference count game around it.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.cz>
> ---
>   mm/memcontrol.c |   64 ++++++++++++++++++++++++++++++++++++++++++-------------
>   1 file changed, 49 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 0fe5177..5da1e58 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -142,8 +142,8 @@ struct mem_cgroup_stat_cpu {
>   };
>   
>   struct mem_cgroup_reclaim_iter {
> -	/* css_id of the last scanned hierarchy member */
> -	int position;
> +	/* last scanned hierarchy member with elevated ref count */
> +	struct mem_cgroup *last_visited;
>   	/* scan generation, increased every round-trip */
>   	unsigned int generation;
>   	/* lock to protect the position and generation */
> @@ -1063,8 +1063,8 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   				   struct mem_cgroup *prev,
>   				   struct mem_cgroup_reclaim_cookie *reclaim)
>   {
> -	struct mem_cgroup *memcg = NULL;
> -	int id = 0;
> +	struct mem_cgroup *memcg = NULL,
> +			  *last_visited = NULL;
>   
>   	if (mem_cgroup_disabled())
>   		return NULL;
> @@ -1073,7 +1073,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   		root = root_mem_cgroup;
>   
>   	if (prev && !reclaim)
> -		id = css_id(&prev->css);
> +		last_visited = prev;
>   
>   	if (prev && prev != root)
>   		css_put(&prev->css);
> @@ -1086,7 +1086,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   
>   	while (!memcg) {
>   		struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
> -		struct cgroup_subsys_state *css;
> +		struct cgroup_subsys_state *css = NULL;
>   
>   		if (reclaim) {
>   			int nid = zone_to_nid(reclaim->zone);
> @@ -1096,30 +1096,64 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   			mz = mem_cgroup_zoneinfo(root, nid, zid);
>   			iter = &mz->reclaim_iter[reclaim->priority];
>   			spin_lock(&iter->iter_lock);
> +			last_visited = iter->last_visited;
>   			if (prev && reclaim->generation != iter->generation) {
> +				if (last_visited) {
> +					mem_cgroup_put(last_visited);
> +					iter->last_visited = NULL;
> +				}
>   				spin_unlock(&iter->iter_lock);
>   				return NULL;
>   			}
> -			id = iter->position;
>   		}
>   
>   		rcu_read_lock();
> -		css = css_get_next(&mem_cgroup_subsys, id + 1, &root->css, &id);
> -		if (css) {
> -			if (css == &root->css || css_tryget(css))
> -				memcg = mem_cgroup_from_css(css);
> -		} else
> -			id = 0;
> -		rcu_read_unlock();
> +		/*
> +		 * Root is not visited by cgroup iterators so it needs a special
> +		 * treatment.
> +		 */
> +		if (!last_visited) {
> +			css = &root->css;
> +		} else {
> +			struct cgroup *next_cgroup;
> +
> +			next_cgroup = cgroup_next_descendant_pre(
> +					last_visited->css.cgroup,
> +					root->css.cgroup);

Maybe I miss something but.... last_visited is holded by memcg's refcnt.
The cgroup pointed by css.cgroup is by cgroup's refcnt which can be freed
before memcg is freed and last_visited->css.cgroup is out of RCU cycle.
Is this safe ?

Thanks,
-Kame

> +			if (next_cgroup)
> +				css = cgroup_subsys_state(next_cgroup,
> +						mem_cgroup_subsys_id);
> +		}
> +
> +		/*
> +		 * Even if we find 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.
> +		 */
> +		if (css == &root->css || (css && css_tryget(css)))
> +			memcg = mem_cgroup_from_css(css);
>   
>   		if (reclaim) {
> -			iter->position = id;
> +			struct mem_cgroup *curr = memcg;
> +
> +			if (last_visited)
> +				mem_cgroup_put(last_visited);
> +
> +			if (css && !memcg)
> +				curr = mem_cgroup_from_css(css);
> +			if (curr)
> +				mem_cgroup_get(curr);
> +			iter->last_visited = curr;
> +
>   			if (!css)
>   				iter->generation++;
>   			else if (!prev && memcg)
>   				reclaim->generation = iter->generation;
>   			spin_unlock(&iter->iter_lock);
> +		} else if (css && !memcg) {
> +			last_visited = mem_cgroup_from_css(css);
>   		}
> +		rcu_read_unlock();
>   
>   		if (prev && !css)
>   			return NULL;
> 


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