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authorVladimir Davydov <vdavydov@parallels.com>2014-06-05 03:07:40 +0400
committerLinus Torvalds <torvalds@linux-foundation.org>2014-06-05 03:54:01 +0400
commitbd67314586a3d5725e60f2f6587b4cb0f659bb67 (patch)
tree581a51f3f04f18bf3e8b906fa2d6f440ccd12055 /mm/slab_common.c
parentc67a8a685a6e9abbaf0235e084168f15a721ae39 (diff)
downloadlinux-bd67314586a3d5725e60f2f6587b4cb0f659bb67.tar.xz
memcg, slab: simplify synchronization scheme
At present, we have the following mutexes protecting data related to per memcg kmem caches: - slab_mutex. This one is held during the whole kmem cache creation and destruction paths. We also take it when updating per root cache memcg_caches arrays (see memcg_update_all_caches). As a result, taking it guarantees there will be no changes to any kmem cache (including per memcg). Why do we need something else then? The point is it is private to slab implementation and has some internal dependencies with other mutexes (get_online_cpus). So we just don't want to rely upon it and prefer to introduce additional mutexes instead. - activate_kmem_mutex. Initially it was added to synchronize initializing kmem limit (memcg_activate_kmem). However, since we can grow per root cache memcg_caches arrays only on kmem limit initialization (see memcg_update_all_caches), we also employ it to protect against memcg_caches arrays relocation (e.g. see __kmem_cache_destroy_memcg_children). - We have a convention not to take slab_mutex in memcontrol.c, but we want to walk over per memcg memcg_slab_caches lists there (e.g. for destroying all memcg caches on offline). So we have per memcg slab_caches_mutex's protecting those lists. The mutexes are taken in the following order: activate_kmem_mutex -> slab_mutex -> memcg::slab_caches_mutex Such a syncrhonization scheme has a number of flaws, for instance: - We can't call kmem_cache_{destroy,shrink} while walking over a memcg::memcg_slab_caches list due to locking order. As a result, in mem_cgroup_destroy_all_caches we schedule the memcg_cache_params::destroy work shrinking and destroying the cache. - We don't have a mutex to synchronize per memcg caches destruction between memcg offline (mem_cgroup_destroy_all_caches) and root cache destruction (__kmem_cache_destroy_memcg_children). Currently we just don't bother about it. This patch simplifies it by substituting per memcg slab_caches_mutex's with the global memcg_slab_mutex. It will be held whenever a new per memcg cache is created or destroyed, so it protects per root cache memcg_caches arrays and per memcg memcg_slab_caches lists. The locking order is following: activate_kmem_mutex -> memcg_slab_mutex -> slab_mutex This allows us to call kmem_cache_{create,shrink,destroy} under the memcg_slab_mutex. As a result, we don't need memcg_cache_params::destroy work any more - we can simply destroy caches while iterating over a per memcg slab caches list. Also using the global mutex simplifies synchronization between concurrent per memcg caches creation/destruction, e.g. mem_cgroup_destroy_all_caches vs __kmem_cache_destroy_memcg_children. The downside of this is that we substitute per-memcg slab_caches_mutex's with a hummer-like global mutex, but since we already take either the slab_mutex or the cgroup_mutex along with a memcg::slab_caches_mutex, it shouldn't hurt concurrency a lot. Signed-off-by: Vladimir Davydov <vdavydov@parallels.com> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Cc: Michal Hocko <mhocko@suse.cz> Cc: Glauber Costa <glommer@gmail.com> Cc: Pekka Enberg <penberg@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/slab_common.c')
-rw-r--r--mm/slab_common.c23
1 files changed, 8 insertions, 15 deletions
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 2dd920dc3776..7e348cff814d 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -160,7 +160,6 @@ do_kmem_cache_create(char *name, size_t object_size, size_t size, size_t align,
s->refcount = 1;
list_add(&s->list, &slab_caches);
- memcg_register_cache(s);
out:
if (err)
return ERR_PTR(err);
@@ -270,9 +269,10 @@ EXPORT_SYMBOL(kmem_cache_create);
* requests going from @memcg to @root_cache. The new cache inherits properties
* from its parent.
*/
-void kmem_cache_create_memcg(struct mem_cgroup *memcg, struct kmem_cache *root_cache)
+struct kmem_cache *kmem_cache_create_memcg(struct mem_cgroup *memcg,
+ struct kmem_cache *root_cache)
{
- struct kmem_cache *s;
+ struct kmem_cache *s = NULL;
char *cache_name;
get_online_cpus();
@@ -280,14 +280,6 @@ void kmem_cache_create_memcg(struct mem_cgroup *memcg, struct kmem_cache *root_c
mutex_lock(&slab_mutex);
- /*
- * Since per-memcg caches are created asynchronously on first
- * allocation (see memcg_kmem_get_cache()), several threads can try to
- * create the same cache, but only one of them may succeed.
- */
- if (cache_from_memcg_idx(root_cache, memcg_cache_id(memcg)))
- goto out_unlock;
-
cache_name = memcg_create_cache_name(memcg, root_cache);
if (!cache_name)
goto out_unlock;
@@ -296,14 +288,18 @@ void kmem_cache_create_memcg(struct mem_cgroup *memcg, struct kmem_cache *root_c
root_cache->size, root_cache->align,
root_cache->flags, root_cache->ctor,
memcg, root_cache);
- if (IS_ERR(s))
+ if (IS_ERR(s)) {
kfree(cache_name);
+ s = NULL;
+ }
out_unlock:
mutex_unlock(&slab_mutex);
put_online_mems();
put_online_cpus();
+
+ return s;
}
static int kmem_cache_destroy_memcg_children(struct kmem_cache *s)
@@ -348,11 +344,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
goto out_unlock;
list_del(&s->list);
- memcg_unregister_cache(s);
-
if (__kmem_cache_shutdown(s) != 0) {
list_add(&s->list, &slab_caches);
- memcg_register_cache(s);
printk(KERN_ERR "kmem_cache_destroy %s: "
"Slab cache still has objects\n", s->name);
dump_stack();