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mm: memcontrol: convert reclaim iterator to simple css refcounting
The memcg reclaim iterators use a complicated weak reference scheme to prevent pinning cgroups indefinitely in the absence of memory pressure. However, during the ongoing cgroup core rework, css lifetime has been decoupled such that a pinned css no longer interferes with removal of the user-visible cgroup, and all this complexity is now unnecessary. [mhocko@suse.cz: ensure that the cached reference is always released] Signed-off-by: Johannes Weiner <hannes@cmpxchg.org> Cc: Vladimir Davydov <vdavydov@parallels.com> Cc: David Rientjes <rientjes@google.com> Cc: Tejun Heo <tj@kernel.org> Signed-off-by: Michal Hocko <mhocko@suse.cz> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Johannes Weiner
Linus Torvalds
parent
5b1efc027c
commit
5ac8fb31ad
264
mm/memcontrol.c
264
mm/memcontrol.c
@ -143,14 +143,8 @@ struct mem_cgroup_stat_cpu {
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unsigned long targets[MEM_CGROUP_NTARGETS];
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};
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struct mem_cgroup_reclaim_iter {
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/*
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* last scanned hierarchy member. Valid only if last_dead_count
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* matches memcg->dead_count of the hierarchy root group.
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*/
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struct mem_cgroup *last_visited;
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int last_dead_count;
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struct reclaim_iter {
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struct mem_cgroup *position;
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/* scan generation, increased every round-trip */
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unsigned int generation;
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};
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@ -162,7 +156,7 @@ struct mem_cgroup_per_zone {
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struct lruvec lruvec;
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unsigned long lru_size[NR_LRU_LISTS];
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struct mem_cgroup_reclaim_iter reclaim_iter[DEF_PRIORITY + 1];
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struct reclaim_iter iter[DEF_PRIORITY + 1];
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struct rb_node tree_node; /* RB tree node */
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unsigned long usage_in_excess;/* Set to the value by which */
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@ -346,7 +340,6 @@ struct mem_cgroup {
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struct mem_cgroup_stat_cpu nocpu_base;
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spinlock_t pcp_counter_lock;
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atomic_t dead_count;
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#if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_INET)
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struct cg_proto tcp_mem;
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#endif
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@ -1067,122 +1060,6 @@ static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
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return memcg;
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}
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/*
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* Returns a next (in a pre-order walk) alive memcg (with elevated css
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* ref. count) or NULL if the whole root's subtree has been visited.
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*
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* helper function to be used by mem_cgroup_iter
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*/
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static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
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struct mem_cgroup *last_visited)
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{
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struct cgroup_subsys_state *prev_css, *next_css;
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prev_css = last_visited ? &last_visited->css : NULL;
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skip_node:
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next_css = css_next_descendant_pre(prev_css, &root->css);
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/*
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* Even if we found a group we have to make sure it is
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* alive. css && !memcg means that the groups should be
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* skipped and we should continue the tree walk.
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* last_visited css is safe to use because it is
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* protected by css_get and the tree walk is rcu safe.
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*
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* We do not take a reference on the root of the tree walk
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* because we might race with the root removal when it would
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* be the only node in the iterated hierarchy and mem_cgroup_iter
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* would end up in an endless loop because it expects that at
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* least one valid node will be returned. Root cannot disappear
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* because caller of the iterator should hold it already so
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* skipping css reference should be safe.
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*/
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if (next_css) {
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struct mem_cgroup *memcg = mem_cgroup_from_css(next_css);
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if (next_css == &root->css)
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return memcg;
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if (css_tryget_online(next_css)) {
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/*
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* Make sure the memcg is initialized:
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* mem_cgroup_css_online() orders the the
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* initialization against setting the flag.
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*/
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if (smp_load_acquire(&memcg->initialized))
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return memcg;
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css_put(next_css);
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}
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prev_css = next_css;
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goto skip_node;
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}
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return NULL;
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}
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static void mem_cgroup_iter_invalidate(struct mem_cgroup *root)
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{
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/*
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* When a group in the hierarchy below root is destroyed, the
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* hierarchy iterator can no longer be trusted since it might
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* have pointed to the destroyed group. Invalidate it.
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*/
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atomic_inc(&root->dead_count);
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}
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static struct mem_cgroup *
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mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
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struct mem_cgroup *root,
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int *sequence)
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{
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struct mem_cgroup *position = NULL;
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/*
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* A cgroup destruction happens in two stages: offlining and
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* release. They are separated by a RCU grace period.
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*
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* If the iterator is valid, we may still race with an
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* offlining. The RCU lock ensures the object won't be
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* released, tryget will fail if we lost the race.
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*/
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*sequence = atomic_read(&root->dead_count);
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if (iter->last_dead_count == *sequence) {
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smp_rmb();
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position = iter->last_visited;
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/*
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* We cannot take a reference to root because we might race
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* with root removal and returning NULL would end up in
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* an endless loop on the iterator user level when root
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* would be returned all the time.
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*/
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if (position && position != root &&
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!css_tryget_online(&position->css))
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position = NULL;
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}
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return position;
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}
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static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
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struct mem_cgroup *last_visited,
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struct mem_cgroup *new_position,
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struct mem_cgroup *root,
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int sequence)
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{
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/* root reference counting symmetric to mem_cgroup_iter_load */
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if (last_visited && last_visited != root)
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css_put(&last_visited->css);
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/*
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* We store the sequence count from the time @last_visited was
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* loaded successfully instead of rereading it here so that we
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* don't lose destruction events in between. We could have
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* raced with the destruction of @new_position after all.
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*/
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iter->last_visited = new_position;
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smp_wmb();
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iter->last_dead_count = sequence;
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}
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/**
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* mem_cgroup_iter - iterate over memory cgroup hierarchy
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* @root: hierarchy root
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@ -1204,8 +1081,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
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struct mem_cgroup *prev,
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struct mem_cgroup_reclaim_cookie *reclaim)
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{
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struct reclaim_iter *uninitialized_var(iter);
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struct cgroup_subsys_state *css = NULL;
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struct mem_cgroup *memcg = NULL;
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struct mem_cgroup *last_visited = NULL;
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struct mem_cgroup *pos = NULL;
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if (mem_cgroup_disabled())
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return NULL;
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@ -1214,50 +1093,101 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
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root = root_mem_cgroup;
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if (prev && !reclaim)
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last_visited = prev;
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pos = prev;
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if (!root->use_hierarchy && root != root_mem_cgroup) {
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if (prev)
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goto out_css_put;
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goto out;
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return root;
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}
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rcu_read_lock();
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while (!memcg) {
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struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
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int uninitialized_var(seq);
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if (reclaim) {
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struct mem_cgroup_per_zone *mz;
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if (reclaim) {
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struct mem_cgroup_per_zone *mz;
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mz = mem_cgroup_zone_zoneinfo(root, reclaim->zone);
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iter = &mz->reclaim_iter[reclaim->priority];
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if (prev && reclaim->generation != iter->generation) {
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iter->last_visited = NULL;
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goto out_unlock;
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}
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mz = mem_cgroup_zone_zoneinfo(root, reclaim->zone);
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iter = &mz->iter[reclaim->priority];
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last_visited = mem_cgroup_iter_load(iter, root, &seq);
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}
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memcg = __mem_cgroup_iter_next(root, last_visited);
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if (reclaim) {
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mem_cgroup_iter_update(iter, last_visited, memcg, root,
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seq);
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if (!memcg)
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iter->generation++;
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else if (!prev && memcg)
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reclaim->generation = iter->generation;
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}
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if (prev && !memcg)
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if (prev && reclaim->generation != iter->generation)
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goto out_unlock;
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do {
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pos = ACCESS_ONCE(iter->position);
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/*
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* A racing update may change the position and
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* put the last reference, hence css_tryget(),
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* or retry to see the updated position.
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*/
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} while (pos && !css_tryget(&pos->css));
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}
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if (pos)
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css = &pos->css;
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for (;;) {
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css = css_next_descendant_pre(css, &root->css);
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if (!css) {
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/*
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* Reclaimers share the hierarchy walk, and a
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* new one might jump in right at the end of
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* the hierarchy - make sure they see at least
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* one group and restart from the beginning.
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*/
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if (!prev)
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continue;
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break;
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}
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/*
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* Verify the css and acquire a reference. The root
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* is provided by the caller, so we know it's alive
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* and kicking, and don't take an extra reference.
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*/
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memcg = mem_cgroup_from_css(css);
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if (css == &root->css)
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break;
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if (css_tryget_online(css)) {
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/*
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* Make sure the memcg is initialized:
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* mem_cgroup_css_online() orders the the
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* initialization against setting the flag.
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*/
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if (smp_load_acquire(&memcg->initialized))
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break;
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css_put(css);
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}
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memcg = NULL;
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}
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if (reclaim) {
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if (cmpxchg(&iter->position, pos, memcg) == pos) {
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if (memcg)
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css_get(&memcg->css);
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if (pos)
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css_put(&pos->css);
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}
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/*
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* pairs with css_tryget when dereferencing iter->position
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* above.
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*/
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if (pos)
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css_put(&pos->css);
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if (!memcg)
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iter->generation++;
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else if (!prev)
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reclaim->generation = iter->generation;
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}
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out_unlock:
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rcu_read_unlock();
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out_css_put:
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out:
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if (prev && prev != root)
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css_put(&prev->css);
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@ -5447,24 +5377,6 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
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return 0;
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}
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/*
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* Announce all parents that a group from their hierarchy is gone.
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*/
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static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
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{
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struct mem_cgroup *parent = memcg;
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while ((parent = parent_mem_cgroup(parent)))
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mem_cgroup_iter_invalidate(parent);
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/*
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* if the root memcg is not hierarchical we have to check it
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* explicitely.
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*/
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if (!root_mem_cgroup->use_hierarchy)
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mem_cgroup_iter_invalidate(root_mem_cgroup);
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}
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static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
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{
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struct mem_cgroup *memcg = mem_cgroup_from_css(css);
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@ -5485,8 +5397,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
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kmem_cgroup_css_offline(memcg);
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mem_cgroup_invalidate_reclaim_iterators(memcg);
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/*
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* This requires that offlining is serialized. Right now that is
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* guaranteed because css_killed_work_fn() holds the cgroup_mutex.
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