mm: memcg/slab: simplify memcg cache creation

Because the number of non-root kmem_caches doesn't depend on the number of
memory cgroups anymore and is generally not very big, there is no more
need for a dedicated workqueue.

Also, as there is no more need to pass any arguments to the
memcg_create_kmem_cache() except the root kmem_cache, it's possible to
just embed the work structure into the kmem_cache and avoid the dynamic
allocation of the work structure.

This will also simplify the synchronization: for each root kmem_cache
there is only one work.  So there will be no more concurrent attempts to
create a non-root kmem_cache for a root kmem_cache: the second and all
following attempts to queue the work will fail.

On the kmem_cache destruction path there is no more need to call the
expensive flush_workqueue() and wait for all pending works to be finished.
Instead, cancel_work_sync() can be used to cancel/wait for only one work.

Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Link: http://lkml.kernel.org/r/20200623174037.3951353-14-guro@fb.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/memcontrol.h

@@ -1418,7 +1418,6 @@ int obj_cgroup_charge(struct obj_cgroup *objcg, gfp_t gfp, size_t size);
 void obj_cgroup_uncharge(struct obj_cgroup *objcg, size_t size);
 
 extern struct static_key_false memcg_kmem_enabled_key;
-extern struct workqueue_struct *memcg_kmem_cache_wq;
 
 extern int memcg_nr_cache_ids;
 void memcg_get_cache_ids(void);

mm/memcontrol.c

@@ -399,8 +399,6 @@ void memcg_put_cache_ids(void)
  */
 DEFINE_STATIC_KEY_FALSE(memcg_kmem_enabled_key);
 EXPORT_SYMBOL(memcg_kmem_enabled_key);
-
-struct workqueue_struct *memcg_kmem_cache_wq;
 #endif
 
 static int memcg_shrinker_map_size;
@@ -2902,39 +2900,6 @@ static void memcg_free_cache_id(int id)
 	ida_simple_remove(&memcg_cache_ida, id);
 }
 
-struct memcg_kmem_cache_create_work {
-	struct kmem_cache *cachep;
-	struct work_struct work;
-};
-
-static void memcg_kmem_cache_create_func(struct work_struct *w)
-{
-	struct memcg_kmem_cache_create_work *cw =
-		container_of(w, struct memcg_kmem_cache_create_work, work);
-	struct kmem_cache *cachep = cw->cachep;
-
-	memcg_create_kmem_cache(cachep);
-
-	kfree(cw);
-}
-
-/*
- * Enqueue the creation of a per-memcg kmem_cache.
- */
-static void memcg_schedule_kmem_cache_create(struct kmem_cache *cachep)
-{
-	struct memcg_kmem_cache_create_work *cw;
-
-	cw = kmalloc(sizeof(*cw), GFP_NOWAIT | __GFP_NOWARN);
-	if (!cw)
-		return;
-
-	cw->cachep = cachep;
-	INIT_WORK(&cw->work, memcg_kmem_cache_create_func);
-
-	queue_work(memcg_kmem_cache_wq, &cw->work);
-}
-
 /**
  * memcg_kmem_get_cache: select memcg or root cache for allocation
  * @cachep: the original global kmem cache
@@ -2951,7 +2916,7 @@ struct kmem_cache *memcg_kmem_get_cache(struct kmem_cache *cachep)
 
 	memcg_cachep = READ_ONCE(cachep->memcg_params.memcg_cache);
 	if (unlikely(!memcg_cachep)) {
-		memcg_schedule_kmem_cache_create(cachep);
+		queue_work(system_wq, &cachep->memcg_params.work);
 		return cachep;
 	}
 
@@ -7022,17 +6987,6 @@ static int __init mem_cgroup_init(void)
 {
 	int cpu, node;
 
-#ifdef CONFIG_MEMCG_KMEM
-	/*
-	 * Kmem cache creation is mostly done with the slab_mutex held,
-	 * so use a workqueue with limited concurrency to avoid stalling
-	 * all worker threads in case lots of cgroups are created and
-	 * destroyed simultaneously.
-	 */
-	memcg_kmem_cache_wq = alloc_workqueue("memcg_kmem_cache", 0, 1);
-	BUG_ON(!memcg_kmem_cache_wq);
-#endif
-
 	cpuhp_setup_state_nocalls(CPUHP_MM_MEMCQ_DEAD, "mm/memctrl:dead", NULL,
 				  memcg_hotplug_cpu_dead);
 

mm/slab.h

@@ -45,12 +45,14 @@ struct kmem_cache {
  * @memcg_cache: pointer to memcg kmem cache, used by all non-root memory
  *		cgroups.
  * @root_caches_node: list node for slab_root_caches list.
+ * @work: work struct used to create the non-root cache.
  */
 struct memcg_cache_params {
 	struct kmem_cache *root_cache;
 
 	struct kmem_cache *memcg_cache;
 	struct list_head __root_caches_node;
+	struct work_struct work;
 };
 #endif /* CONFIG_SLOB */
 

mm/slab_common.c

@@ -134,10 +134,18 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
 
 LIST_HEAD(slab_root_caches);
 
+static void memcg_kmem_cache_create_func(struct work_struct *work)
+{
+	struct kmem_cache *cachep = container_of(work, struct kmem_cache,
+						 memcg_params.work);
+	memcg_create_kmem_cache(cachep);
+}
+
 void slab_init_memcg_params(struct kmem_cache *s)
 {
 	s->memcg_params.root_cache = NULL;
 	s->memcg_params.memcg_cache = NULL;
+	INIT_WORK(&s->memcg_params.work, memcg_kmem_cache_create_func);
 }
 
 static void init_memcg_params(struct kmem_cache *s,
@@ -586,23 +594,17 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
 	return 0;
 }
 
-static void flush_memcg_workqueue(struct kmem_cache *s)
+static void cancel_memcg_cache_creation(struct kmem_cache *s)
 {
-	/*
-	 * SLAB and SLUB create memcg kmem_caches through workqueue and SLUB
-	 * deactivates the memcg kmem_caches through workqueue. Make sure all
-	 * previous workitems on workqueue are processed.
-	 */
-	if (likely(memcg_kmem_cache_wq))
-		flush_workqueue(memcg_kmem_cache_wq);
+	cancel_work_sync(&s->memcg_params.work);
 }
 #else
 static inline int shutdown_memcg_caches(struct kmem_cache *s)
 {
 	return 0;
 }
 
-static inline void flush_memcg_workqueue(struct kmem_cache *s)
+static inline void cancel_memcg_cache_creation(struct kmem_cache *s)
 {
 }
 #endif /* CONFIG_MEMCG_KMEM */
@@ -621,7 +623,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	if (unlikely(!s))
 		return;
 
-	flush_memcg_workqueue(s);
+	cancel_memcg_cache_creation(s);
 
 	get_online_cpus();
 	get_online_mems();