Re: [patch v2 3/6] memcg: rework mem_cgroup_iter to use cgroup iterators

[Ying Han <yinghan@google.com>]

On Mon, Nov 26, 2012 at 10:47 AM, Michal Hocko <mhocko@suse.cz> 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 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).
>
> 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@suse.cz>
> ---
>  mm/memcontrol.c |   74 ++++++++++++++++++++++++++++++++++++++++++-------------
>  1 file changed, 57 insertions(+), 17 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 1f5528d..6bcc97b 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -144,8 +144,8 @@ 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 css ref count */
> +       struct mem_cgroup *last_visited;
>         /* scan generation, increased every round-trip */
>         unsigned int generation;
>         /* lock to protect the position and generation */
> @@ -1066,7 +1066,7 @@ 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;
> @@ -1075,7 +1075,7 @@ 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)
> @@ -1083,9 +1083,10 @@ 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);
> @@ -1095,34 +1096,73 @@ 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) {
> +                                       css_put(&last_visited->css);
> +                                       iter->last_visited = NULL;
> +                               }
>                                 spin_unlock(&iter->iter_lock);
> -                               goto out_css_put;
> +                               goto out_unlock;
>                         }
> -                       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 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);
> +                       iter->last_visited = curr;

Here we take extra refcnt for last_visited, and assume it is under
target reclaim which then calls mem_cgroup_iter_break() and we leaked
a refcnt of the
target memcg css. Sorry if i missed the change in iter_break(),
otherwise we might need something like this:

 void mem_cgroup_iter_break(struct mem_cgroup *root,
-                          struct mem_cgroup *prev)
+                          struct mem_cgroup *prev,
+                          struct mem_cgroup_reclaim_cookie *reclaim)
 {
        if (!root)
                root = root_mem_cgroup;
        if (prev && prev != root)
                css_put(&prev->css);
+
+       if (reclaim) {
+               int nid = zone_to_nid(reclaim->zone);
+               int zid = zone_idx(reclaim->zone);
+               struct mem_cgroup *last_visited = NULL;
+               struct mem_cgroup_per_zone *mz;
+               struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
+
+               mz = mem_cgroup_zoneinfo(root, nid, zid);
+               iter = &mz->reclaim_iter[reclaim->priority];
+               spin_lock(&iter->iter_lock);
+               last_visited = iter->last_visited;
+               if (last_visited) {
+                       css_put(&last_visited->css);
+                       iter->last_visited = NULL;
+               }
+               spin_unlock(&iter->iter_lock);
+       }
 }

--Ying



>                         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);
> --
> 1.7.10.4
>

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