Commit 657dc2f9 authored by Tejun Heo's avatar Tejun Heo Committed by Linus Torvalds
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slab: remove synchronous rcu_barrier() call in memcg cache release path 

With kmem cgroup support enabled, kmem_caches can be created and
destroyed frequently and a great number of near empty kmem_caches can
accumulate if there are a lot of transient cgroups and the system is not
under memory pressure.  When memory reclaim starts under such
conditions, it can lead to consecutive deactivation and destruction of
many kmem_caches, easily hundreds of thousands on moderately large
systems, exposing scalability issues in the current slab management
code.  This is one of the patches to address the issue.

SLAB_DESTORY_BY_RCU caches need to flush all RCU operations before
destruction because slab pages are freed through RCU and they need to be
able to dereference the associated kmem_cache.  Currently, it's done
synchronously with rcu_barrier().  As rcu_barrier() is expensive
time-wise, slab implements a batching mechanism so that rcu_barrier()
can be done for multiple caches at the same time.

Unfortunately, the rcu_barrier() is in synchronous path which is called
while holding cgroup_mutex and the batching is too limited to be
actually helpful.

This patch updates the cache release path so that the batching is
asynchronous and global.  All SLAB_DESTORY_BY_RCU caches are queued
globally and a work item consumes the list.  The work item calls
rcu_barrier() only once for all caches that are currently queued.

* release_caches() is removed and shutdown_cache() now either directly
  release the cache or schedules a RCU callback to do that.  This
  makes the cache inaccessible once shutdown_cache() is called and
  makes it impossible for shutdown_memcg_caches() to do memcg-specific
  cleanups afterwards.  Move memcg-specific part into a helper,
  unlink_memcg_cache(), and make shutdown_cache() call it directly.

Link: http://lkml.kernel.org/r/20170117235411.9408-4-tj@kernel.org


Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reported-by: default avatarJay Vana <jsvana@fb.com>
Acked-by: default avatarVladimir Davydov <vdavydov@tarantool.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent bf5eb3de

mm/slab_common.c

+60 −42
Original line number Diff line number Diff line
@@ -30,6 +30,11 @@ LIST_HEAD(slab_caches); 
DEFINE_MUTEX(slab_mutex);

struct kmem_cache *kmem_cache;



static LIST_HEAD(slab_caches_to_rcu_destroy);

static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work);

static DECLARE_WORK(slab_caches_to_rcu_destroy_work,

		    slab_caches_to_rcu_destroy_workfn);



/*

 * Set of flags that will prevent slab merging

 */
@@ -215,6 +220,11 @@ int memcg_update_all_caches(int num_memcgs) 
	mutex_unlock(&slab_mutex);

	return ret;

}



static void unlink_memcg_cache(struct kmem_cache *s)

{

	list_del(&s->memcg_params.list);

}

#else

static inline int init_memcg_params(struct kmem_cache *s,

		struct mem_cgroup *memcg, struct kmem_cache *root_cache)
@@ -225,6 +235,10 @@ static inline int init_memcg_params(struct kmem_cache *s, 
static inline void destroy_memcg_params(struct kmem_cache *s)

{

}



static inline void unlink_memcg_cache(struct kmem_cache *s)

{

}

#endif /* CONFIG_MEMCG && !CONFIG_SLOB */



/*
@@ -461,33 +475,59 @@ out_unlock: 
}

EXPORT_SYMBOL(kmem_cache_create);



static int shutdown_cache(struct kmem_cache *s,

		struct list_head *release, bool *need_rcu_barrier)

static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)

{

	if (__kmem_cache_shutdown(s) != 0)

		return -EBUSY;

	LIST_HEAD(to_destroy);

	struct kmem_cache *s, *s2;



	if (s->flags & SLAB_DESTROY_BY_RCU)

		*need_rcu_barrier = true;

	/*

	 * On destruction, SLAB_DESTROY_BY_RCU kmem_caches are put on the

	 * @slab_caches_to_rcu_destroy list.  The slab pages are freed

	 * through RCU and and the associated kmem_cache are dereferenced

	 * while freeing the pages, so the kmem_caches should be freed only

	 * after the pending RCU operations are finished.  As rcu_barrier()

	 * is a pretty slow operation, we batch all pending destructions

	 * asynchronously.

	 */

	mutex_lock(&slab_mutex);

	list_splice_init(&slab_caches_to_rcu_destroy, &to_destroy);

	mutex_unlock(&slab_mutex);



	list_move(&s->list, release);

	return 0;

	if (list_empty(&to_destroy))

		return;



	rcu_barrier();



	list_for_each_entry_safe(s, s2, &to_destroy, list) {

#ifdef SLAB_SUPPORTS_SYSFS

		sysfs_slab_release(s);

#else

		slab_kmem_cache_release(s);

#endif

	}

}



static void release_caches(struct list_head *release, bool need_rcu_barrier)

static int shutdown_cache(struct kmem_cache *s)

{

	struct kmem_cache *s, *s2;

	if (__kmem_cache_shutdown(s) != 0)

		return -EBUSY;



	if (need_rcu_barrier)

		rcu_barrier();

	list_del(&s->list);

	if (!is_root_cache(s))

		unlink_memcg_cache(s);



	list_for_each_entry_safe(s, s2, release, list) {

	if (s->flags & SLAB_DESTROY_BY_RCU) {

		list_add_tail(&s->list, &slab_caches_to_rcu_destroy);

		schedule_work(&slab_caches_to_rcu_destroy_work);

	} else {

#ifdef SLAB_SUPPORTS_SYSFS

		sysfs_slab_release(s);

#else

		slab_kmem_cache_release(s);

#endif

	}



	return 0;

}



#if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
@@ -602,22 +642,8 @@ void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg) 
	put_online_cpus();

}



static int __shutdown_memcg_cache(struct kmem_cache *s,

		struct list_head *release, bool *need_rcu_barrier)

{

	BUG_ON(is_root_cache(s));



	if (shutdown_cache(s, release, need_rcu_barrier))

		return -EBUSY;



	list_del(&s->memcg_params.list);

	return 0;

}



void memcg_destroy_kmem_caches(struct mem_cgroup *memcg)

{

	LIST_HEAD(release);

	bool need_rcu_barrier = false;

	struct kmem_cache *s, *s2;



	get_online_cpus();
@@ -631,18 +657,15 @@ void memcg_destroy_kmem_caches(struct mem_cgroup *memcg) 
		 * The cgroup is about to be freed and therefore has no charges

		 * left. Hence, all its caches must be empty by now.

		 */

		BUG_ON(__shutdown_memcg_cache(s, &release, &need_rcu_barrier));

		BUG_ON(shutdown_cache(s));

	}

	mutex_unlock(&slab_mutex);



	put_online_mems();

	put_online_cpus();



	release_caches(&release, need_rcu_barrier);

}



static int shutdown_memcg_caches(struct kmem_cache *s,

		struct list_head *release, bool *need_rcu_barrier)

static int shutdown_memcg_caches(struct kmem_cache *s)

{

	struct memcg_cache_array *arr;

	struct kmem_cache *c, *c2;
@@ -661,7 +684,7 @@ static int shutdown_memcg_caches(struct kmem_cache *s, 
		c = arr->entries[i];

		if (!c)

			continue;

		if (__shutdown_memcg_cache(c, release, need_rcu_barrier))

		if (shutdown_cache(c))

			/*

			 * The cache still has objects. Move it to a temporary

			 * list so as not to try to destroy it for a second
@@ -684,7 +707,7 @@ static int shutdown_memcg_caches(struct kmem_cache *s, 
	 */

	list_for_each_entry_safe(c, c2, &s->memcg_params.list,

				 memcg_params.list)

		__shutdown_memcg_cache(c, release, need_rcu_barrier);

		shutdown_cache(c);



	list_splice(&busy, &s->memcg_params.list);
@@ -697,8 +720,7 @@ static int shutdown_memcg_caches(struct kmem_cache *s, 
	return 0;

}

#else

static inline int shutdown_memcg_caches(struct kmem_cache *s,

		struct list_head *release, bool *need_rcu_barrier)

static inline int shutdown_memcg_caches(struct kmem_cache *s)

{

	return 0;

}
@@ -714,8 +736,6 @@ void slab_kmem_cache_release(struct kmem_cache *s) 


void kmem_cache_destroy(struct kmem_cache *s)

{

	LIST_HEAD(release);

	bool need_rcu_barrier = false;

	int err;



	if (unlikely(!s))
@@ -731,9 +751,9 @@ void kmem_cache_destroy(struct kmem_cache *s) 
	if (s->refcount)

		goto out_unlock;



	err = shutdown_memcg_caches(s, &release, &need_rcu_barrier);

	err = shutdown_memcg_caches(s);

	if (!err)

		err = shutdown_cache(s, &release, &need_rcu_barrier);

		err = shutdown_cache(s);



	if (err) {

		pr_err("kmem_cache_destroy %s: Slab cache still has objects\n",
@@ -745,8 +765,6 @@ out_unlock: 


	put_online_mems();

	put_online_cpus();



	release_caches(&release, need_rcu_barrier);

}

EXPORT_SYMBOL(kmem_cache_destroy);

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