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.
Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
---
mm/memcontrol.c | 250 +++++++++++++++++---------------------------------------
1 file changed, 76 insertions(+), 174 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b62972c80055..67dabe8b0aa6 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -143,14 +143,8 @@ struct mem_cgroup_stat_cpu {
unsigned long targets[MEM_CGROUP_NTARGETS];
};
-struct mem_cgroup_reclaim_iter {
- /*
- * last scanned hierarchy member. Valid only if last_dead_count
- * matches memcg->dead_count of the hierarchy root group.
- */
- struct mem_cgroup *last_visited;
- int last_dead_count;
-
+struct reclaim_iter {
+ struct mem_cgroup *position;
/* scan generation, increased every round-trip */
unsigned int generation;
};
@@ -162,7 +156,7 @@ struct mem_cgroup_per_zone {
struct lruvec lruvec;
unsigned long lru_size[NR_LRU_LISTS];
- struct mem_cgroup_reclaim_iter reclaim_iter[DEF_PRIORITY + 1];
+ struct reclaim_iter iter[DEF_PRIORITY + 1];
struct rb_node tree_node; /* RB tree node */
unsigned long usage_in_excess;/* Set to the value by which */
@@ -346,7 +340,6 @@ struct mem_cgroup {
struct mem_cgroup_stat_cpu nocpu_base;
spinlock_t pcp_counter_lock;
- atomic_t dead_count;
#if defined(CONFIG_MEMCG_KMEM) && defined(CONFIG_INET)
struct cg_proto tcp_mem;
#endif
@@ -1067,122 +1060,6 @@ static struct mem_cgroup *get_mem_cgroup_from_mm(struct mm_struct *mm)
return memcg;
}
-/*
- * Returns a next (in a pre-order walk) alive memcg (with elevated css
- * ref. count) or NULL if the whole root's subtree has been visited.
- *
- * helper function to be used by mem_cgroup_iter
- */
-static struct mem_cgroup *__mem_cgroup_iter_next(struct mem_cgroup *root,
- struct mem_cgroup *last_visited)
-{
- struct cgroup_subsys_state *prev_css, *next_css;
-
- prev_css = last_visited ? &last_visited->css : NULL;
-skip_node:
- next_css = css_next_descendant_pre(prev_css, &root->css);
-
- /*
- * 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.
- *
- * We do not take a reference on the root of the tree walk
- * because we might race with the root removal when it would
- * be the only node in the iterated hierarchy and mem_cgroup_iter
- * would end up in an endless loop because it expects that at
- * least one valid node will be returned. Root cannot disappear
- * because caller of the iterator should hold it already so
- * skipping css reference should be safe.
- */
- if (next_css) {
- struct mem_cgroup *memcg = mem_cgroup_from_css(next_css);
-
- if (next_css == &root->css)
- return memcg;
-
- if (css_tryget_online(next_css)) {
- /*
- * Make sure the memcg is initialized:
- * mem_cgroup_css_online() orders the the
- * initialization against setting the flag.
- */
- if (smp_load_acquire(&memcg->initialized))
- return memcg;
- css_put(next_css);
- }
-
- prev_css = next_css;
- goto skip_node;
- }
-
- return NULL;
-}
-
-static void mem_cgroup_iter_invalidate(struct mem_cgroup *root)
-{
- /*
- * When a group in the hierarchy below root is destroyed, the
- * hierarchy iterator can no longer be trusted since it might
- * have pointed to the destroyed group. Invalidate it.
- */
- atomic_inc(&root->dead_count);
-}
-
-static struct mem_cgroup *
-mem_cgroup_iter_load(struct mem_cgroup_reclaim_iter *iter,
- struct mem_cgroup *root,
- int *sequence)
-{
- struct mem_cgroup *position = NULL;
- /*
- * A cgroup destruction happens in two stages: offlining and
- * release. They are separated by a RCU grace period.
- *
- * If the iterator is valid, we may still race with an
- * offlining. The RCU lock ensures the object won't be
- * released, tryget will fail if we lost the race.
- */
- *sequence = atomic_read(&root->dead_count);
- if (iter->last_dead_count == *sequence) {
- smp_rmb();
- position = iter->last_visited;
-
- /*
- * We cannot take a reference to root because we might race
- * with root removal and returning NULL would end up in
- * an endless loop on the iterator user level when root
- * would be returned all the time.
- */
- if (position && position != root &&
- !css_tryget_online(&position->css))
- position = NULL;
- }
- return position;
-}
-
-static void mem_cgroup_iter_update(struct mem_cgroup_reclaim_iter *iter,
- struct mem_cgroup *last_visited,
- struct mem_cgroup *new_position,
- struct mem_cgroup *root,
- int sequence)
-{
- /* root reference counting symmetric to mem_cgroup_iter_load */
- if (last_visited && last_visited != root)
- css_put(&last_visited->css);
- /*
- * We store the sequence count from the time @last_visited was
- * loaded successfully instead of rereading it here so that we
- * don't lose destruction events in between. We could have
- * raced with the destruction of @new_position after all.
- */
- iter->last_visited = new_position;
- smp_wmb();
- iter->last_dead_count = sequence;
-}
-
/**
* mem_cgroup_iter - iterate over memory cgroup hierarchy
* @root: hierarchy root
@@ -1204,8 +1081,10 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
struct mem_cgroup *prev,
struct mem_cgroup_reclaim_cookie *reclaim)
{
+ struct reclaim_iter *uninitialized_var(iter);
+ struct cgroup_subsys_state *css = NULL;
struct mem_cgroup *memcg = NULL;
- struct mem_cgroup *last_visited = NULL;
+ struct mem_cgroup *pos = NULL;
if (mem_cgroup_disabled())
return NULL;
@@ -1214,50 +1093,93 @@ struct mem_cgroup *mem_cgroup_iter(struct mem_cgroup *root,
root = root_mem_cgroup;
if (prev && !reclaim)
- last_visited = prev;
+ pos = prev;
if (!root->use_hierarchy && root != root_mem_cgroup) {
if (prev)
- goto out_css_put;
+ goto out;
return root;
}
rcu_read_lock();
- while (!memcg) {
- struct mem_cgroup_reclaim_iter *uninitialized_var(iter);
- int uninitialized_var(seq);
-
- if (reclaim) {
- struct mem_cgroup_per_zone *mz;
-
- mz = mem_cgroup_zone_zoneinfo(root, reclaim->zone);
- iter = &mz->reclaim_iter[reclaim->priority];
- if (prev && reclaim->generation != iter->generation) {
- iter->last_visited = NULL;
- goto out_unlock;
- }
- last_visited = mem_cgroup_iter_load(iter, root, &seq);
+ if (reclaim) {
+ struct mem_cgroup_per_zone *mz;
+
+ mz = mem_cgroup_zone_zoneinfo(root, reclaim->zone);
+ iter = &mz->iter[reclaim->priority];
+
+ if (prev && reclaim->generation != iter->generation)
+ goto out_unlock;
+
+ do {
+ pos = ACCESS_ONCE(iter->position);
+ /*
+ * A racing update may change the position and
+ * put the last reference, hence css_tryget(),
+ * or retry to see the updated position.
+ */
+ } while (pos && !css_tryget(&pos->css));
+ }
+
+ if (pos)
+ css = &pos->css;
+
+ for (;;) {
+ css = css_next_descendant_pre(css, &root->css);
+ if (!css) {
+ /*
+ * Reclaimers share the hierarchy walk, and a
+ * new one might jump in right at the end of
+ * the hierarchy - make sure they see at least
+ * one group and restart from the beginning.
+ */
+ if (!prev)
+ continue;
+ break;
}
- memcg = __mem_cgroup_iter_next(root, last_visited);
+ /*
+ * Verify the css and acquire a reference. The root
+ * is provided by the caller, so we know it's alive
+ * and kicking, and don't take an extra reference.
+ */
+ memcg = mem_cgroup_from_css(css);
- if (reclaim) {
- mem_cgroup_iter_update(iter, last_visited, memcg, root,
- seq);
+ if (css == &root->css)
+ break;
+
+ if (css_tryget_online(css)) {
+ /*
+ * Make sure the memcg is initialized:
+ * mem_cgroup_css_online() orders the the
+ * initialization against setting the flag.
+ */
+ if (smp_load_acquire(&memcg->initialized))
+ break;
- if (!memcg)
- iter->generation++;
- else if (!prev && memcg)
- reclaim->generation = iter->generation;
+ css_put(css);
}
- if (prev && !memcg)
- goto out_unlock;
+ memcg = NULL;
+ }
+
+ if (reclaim) {
+ if (cmpxchg(&iter->position, pos, memcg) == pos && memcg)
+ css_get(&memcg->css);
+
+ if (pos)
+ css_put(&pos->css);
+
+ if (!memcg)
+ iter->generation++;
+ else if (!prev)
+ reclaim->generation = iter->generation;
}
+
out_unlock:
rcu_read_unlock();
-out_css_put:
+out:
if (prev && prev != root)
css_put(&prev->css);
@@ -5438,24 +5360,6 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
return 0;
}
-/*
- * Announce all parents that a group from their hierarchy is gone.
- */
-static void mem_cgroup_invalidate_reclaim_iterators(struct mem_cgroup *memcg)
-{
- struct mem_cgroup *parent = memcg;
-
- while ((parent = parent_mem_cgroup(parent)))
- mem_cgroup_iter_invalidate(parent);
-
- /*
- * if the root memcg is not hierarchical we have to check it
- * explicitely.
- */
- if (!root_mem_cgroup->use_hierarchy)
- mem_cgroup_iter_invalidate(root_mem_cgroup);
-}
-
static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
{
struct mem_cgroup *memcg = mem_cgroup_from_css(css);
@@ -5476,8 +5380,6 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css)
kmem_cgroup_css_offline(memcg);
- mem_cgroup_invalidate_reclaim_iterators(memcg);
-
/*
* This requires that offlining is serialized. Right now that is
* guaranteed because css_killed_work_fn() holds the cgroup_mutex.
--
2.1.2
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