Re: [PATCH v4 2/6] memcg: rework mem_cgroup_iter to use cgroup iterators

[Kamezawa Hiroyuki]

(2013/02/14 22:26), 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 initialized sufficiently for iteration in mem_cgroup_css_alloc
> already and css reference counting enforces that the group is alive for
> both the last seen cgroup and the found one resp. it signals that the
> group is dead and it should be skipped.
> 
> 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 css keeps it alive even
> though the group have been removed (parked waiting for the last dput so
> that it can be freed).
> 
> Per node-zone-prio iter_lock has been introduced to ensure that
> css_tryget and iter->last_visited is set atomically. Otherwise two
> racing walkers could both take a references and only one release it
> leading to a css leak (which pins cgroup dentry).
> 
> V3
> - introduce iter_lock
> V2
> - use css_{get,put} for iter->last_visited rather than
>    mem_cgroup_{get,put} because it is stronger wrt. cgroup life cycle
> - cgroup_next_descendant_pre expects NULL pos for the first iterartion
>    otherwise it might loop endlessly for intermediate node without any
>    children.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> ---
>   mm/memcontrol.c |   86 +++++++++++++++++++++++++++++++++++++++++++------------
>   1 file changed, 68 insertions(+), 18 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index f3d1bfe..e9f5c47 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -144,10 +144,12 @@ 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 css ref count */
> +	struct mem_cgroup *last_visited;
>   	/* scan generation, increased every round-trip */
>   	unsigned int generation;
> +	/* lock to protect the position and generation */
> +	spinlock_t iter_lock;
>   };
>   
>   /*
> @@ -1130,7 +1132,7 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   				   struct mem_cgroup_reclaim_cookie *reclaim)
>   {
>   	struct mem_cgroup *memcg = NULL;
> -	int id = 0;
> +	struct mem_cgroup *last_visited = NULL;
>   
>   	if (mem_cgroup_disabled())
>   		return NULL;
> @@ -1139,7 +1141,7 @@ 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 (!root->use_hierarchy && root != root_mem_cgroup) {
>   		if (prev)
> @@ -1147,9 +1149,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   		return root;
>   	}
>   
> +	rcu_read_lock();
>   	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);
> @@ -1158,31 +1161,74 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
>   
>   			mz = mem_cgroup_zoneinfo(root, nid, zid);
>   			iter = &mz->reclaim_iter[reclaim->priority];
> -			if (prev && reclaim->generation != iter->generation)
> -				goto out_css_put;
> -			id = iter->position;
> +			spin_lock(&iter->iter_lock);
> +			last_visited = iter->last_visited;
> +			if (prev && reclaim->generation != iter->generation) {
> +				if (last_visited) {
> +					css_put(&last_visited->css);
> +					iter->last_visited = NULL;
> +				}
> +				spin_unlock(&iter->iter_lock);
> +				goto out_unlock;
> +			}
>   		}
>   
> -		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 an
> +		 * explicit visit.
> +		 */
> +		if (!last_visited) {
> +			css = &root->css;
> +		} 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);
> +		}
> +
> +		/*
> +		 * 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);
>   
>   		if (reclaim) {
> -			iter->position = id;
> +			struct mem_cgroup *curr = memcg;
> +
> +			if (last_visited)
> +				css_put(&last_visited->css);
> +
> +			if (css && !memcg)
> +				curr = mem_cgroup_from_css(css);
> +
> +			/* make sure that the cached memcg is not removed */
> +			if (curr)
> +				css_get(&curr->css);
I'm sorry if I miss something...

This curr is  curr == memcg = mem_cgroup_from_css(css) <= already try_get() done.

double refcounted ?

Thanks,
-Kame


> +			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);
>   		}
>   
>   		if (prev && !css)
> -			goto out_css_put;
> +			goto out_unlock;
>   	}
> +out_unlock:
> +	rcu_read_unlock();
>   out_css_put:
>   	if (prev && prev != root)
>   		css_put(&prev->css);
> @@ -6052,8 +6098,12 @@ static int alloc_mem_cgroup_per_zone_info(struct mem_cgroup *memcg, int node)
>   		return 1;
>   
>   	for (zone = 0; zone < MAX_NR_ZONES; zone++) {
> +		int prio;
> +
>   		mz = &pn->zoneinfo[zone];
>   		lruvec_init(&mz->lruvec);
> +		for (prio = 0; prio < DEF_PRIORITY + 1; prio++)
> +			spin_lock_init(&mz->reclaim_iter[prio].iter_lock);
>   		mz->usage_in_excess = 0;
>   		mz->on_tree = false;
>   		mz->memcg = memcg;
> 


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