[PATCH -mm 08/14] memcg: release memcg_cache_id on css offline

Vladimir Davydov
2014-09-21T15:14:40Z

The memcg_cache_id (mem_cgroup->kmemcg_id) is used as the index in root
cache's memcg_cache_params->memcg_caches array. Whenever a new kmem
active cgroup is created we must allocate an id for it. As a result, the
array size must always be greater than or equal to the number of memory
cgroups that have memcg_cache_id assigned to them.

Currently we release the id only on css free. This is bad, because css
can be zombieing around for quite a long time after css offline due to
pending charges, occupying an array slot and making the arrays grow
larger and larger. Although the number of arrays is limited - only root
kmem caches have them - we can still experience problems while creating
new kmem active cgroups, because they might require arrays relocation
and each array relocation will require costly high-order page
allocations if there are a lot of ids allocated. The situation will
become even worse when per-memcg list_lru's are introduced, because each
super block has a list_lru, and the number of super blocks is
practically unlimited.

This patch makes memcg release memcg_cache_id on css offline. The id of
a dead memcg is set to its parent cgroup's id. Currently ids are not
used after cgroup death so we could set it to -1, however, once per
memcg list_lru is introduced, we will have to deal with list_lru entries
accounted to the memcg somehow. I'm planning to move those entries to
parent cgroup's list_lru, so we have to set kmemcg_id appropriately.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
---
mm/memcontrol.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 56 insertions(+), 8 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 9ae2627bd3b1..d665d715090b 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -646,14 +646,10 @@ int memcg_limited_groups_array_size;
struct static_key memcg_kmem_enabled_key;
EXPORT_SYMBOL(memcg_kmem_enabled_key);

-static void memcg_free_cache_id(int id);
-
static void disarm_kmem_keys(struct mem_cgroup *memcg)
{
- if (memcg_kmem_is_active(memcg)) {
+ if (memcg_kmem_is_active(memcg))
static_key_slow_dec(&memcg_kmem_enabled_key);
- memcg_free_cache_id(memcg->kmemcg_id);
- }
/*
* This check can't live in kmem destruction function,
* since the charges will outlive the cgroup
@@ -2988,6 +2984,12 @@ static void memcg_register_cache(struct mem_cgroup *memcg,
lockdep_assert_held(&memcg_slab_mutex);

id = memcg_cache_id(memcg);
+ /*
+ * This might happen if the cgroup was taken offline while the create
+ * work was pending.
+ */
+ if (id < 0)
+ return;

/*
* Since per-memcg caches are created asynchronously on first
@@ -3036,8 +3038,15 @@ static void memcg_unregister_cache(struct kmem_cache *cachep)
memcg = cachep->memcg_params->memcg;
id = memcg_cache_id(memcg);

- BUG_ON(cache_from_memcg_idx(root_cache, id) != cachep);
- cache_install_at_memcg_idx(root_cache, id, NULL);
+ /*
+ * This function can be called both after and before css offline. If
+ * it's called before css offline, which happens on the root cache
+ * destruction, we should clear the slot corresponding to the cache in
+ * memcg_caches array. Otherwise the slot must have already been
+ * cleared in memcg_unregister_all_caches.
+ */
+ if (id >= 0 && cache_from_memcg_idx(root_cache, id) == cachep)
+ cache_install_at_memcg_idx(root_cache, id, NULL);

list_del(&cachep->memcg_params->list);

@@ -3093,19 +3102,49 @@ void __memcg_cleanup_cache_params(struct kmem_cache *s)
static void memcg_unregister_all_caches(struct mem_cgroup *memcg)
{
struct memcg_cache_params *params, *tmp;
+ int id = memcg_cache_id(memcg);
+ struct cgroup_subsys_state *iter;
+ struct mem_cgroup *parent;
+ int parent_id;

if (!memcg_kmem_is_active(memcg))
return;

+ /*
+ * Clear the slots corresponding to this cgroup in all root caches'
+ * memcg_params->memcg_caches arrays. If a cache is empty, remove it.
+ */
mutex_lock(&memcg_slab_mutex);
list_for_each_entry_safe(params, tmp, &memcg->memcg_slab_caches, list) {
struct kmem_cache *cachep = params->cachep;
+ struct kmem_cache *root_cache = params->root_cache;
+
+ BUG_ON(cache_from_memcg_idx(root_cache, id) != cachep);
+ cache_install_at_memcg_idx(root_cache, id, NULL);

kmem_cache_shrink(cachep);
if (atomic_read(&cachep->memcg_params->nr_pages) == 0)
memcg_unregister_cache(cachep);
}
mutex_unlock(&memcg_slab_mutex);
+
+ /*
+ * Change kmemcg_id of this cgroup and all its descendants (which are
+ * already dead) to the parent cgroup's id.
+ */
+ parent = parent_mem_cgroup(memcg);
+ if (parent && !mem_cgroup_is_root(parent)) {
+ BUG_ON(!memcg_kmem_is_active(parent));
+ parent_id = parent->kmemcg_id;
+ } else
+ parent_id = -1;
+
+ /* Safe, because we are holding the cgroup_mutex */
+ css_for_each_descendant_post(iter, &memcg->css)
+ mem_cgroup_from_css(iter)->kmemcg_id = parent_id;
+
+ /* The id is not used anymore, free it so that it could be reused. */
+ memcg_free_cache_id(id);
}

struct memcg_register_cache_work {
@@ -3204,6 +3243,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
{
struct mem_cgroup *memcg;
struct kmem_cache *memcg_cachep;
+ int id;

VM_BUG_ON(!cachep->memcg_params);
VM_BUG_ON(!cachep->memcg_params->is_root_cache);
@@ -3217,7 +3257,15 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
if (!memcg_can_account_kmem(memcg))
goto out;

- memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
+ id = memcg_cache_id(memcg);
+ /*
+ * This might happen if current was migrated to another cgroup and this
+ * cgroup was taken offline after we issued mem_cgroup_from_task above.
+ */
+ if (unlikely(id < 0))
+ goto out;
+
+ memcg_cachep = cache_from_memcg_idx(cachep, id);
if (likely(memcg_cachep)) {
cachep = memcg_cachep;
goto out;