(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; > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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