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
......@@ -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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