[PATCH 8/9] slab: remove synchronous synchronize_sched() from memcg cache deactivation path

Tejun Heo
2017-01-14T05:54:48Z

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.

slub uses synchronize_sched() to deactivate a memcg cache.
synchronize_sched() is an expensive and slow operation and doesn't
scale when a huge number of caches are destroyed back-to-back. While
there used to be a simple batching mechanism, the batching was too
restricted to be helpful.

This patch implements slab_deactivate_memcg_cache_rcu_sched() which
slub can use to schedule sched RCU callback instead of performing
synchronize_sched() synchronously while holding cgroup_mutex. While
this adds online cpus, mems and slab_mutex operations, operating on
these locks back-to-back from the same kworker, which is what's gonna
happen when there are many to deactivate, isn't expensive at all and
this gets rid of the scalability problem completely.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Jay Vana <jsvana@fb.com>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
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>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
include/linux/slab.h | 6 ++++++
mm/slab.h | 2 ++
mm/slab_common.c | 60 ++++++++++++++++++++++++++++++++++++++++++++++++++++
mm/slub.c | 12 +++++++----
4 files changed, 76 insertions(+), 4 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 63d543d..84701bb 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -576,6 +576,12 @@ struct memcg_cache_params {
struct {
struct mem_cgroup *memcg;
struct list_head kmem_caches_node;
+
+ void (*deact_fn)(struct kmem_cache *);
+ union {
+ struct rcu_head deact_rcu_head;
+ struct work_struct deact_work;
+ };
};
};
};
diff --git a/mm/slab.h b/mm/slab.h
index 73ed6b5..b67ac8f 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -299,6 +299,8 @@ static __always_inline void memcg_uncharge_slab(struct page *page, int order,
}

extern void slab_init_memcg_params(struct kmem_cache *);
+extern void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
+ void (*deact_fn)(struct kmem_cache *));

#else /* CONFIG_MEMCG && !CONFIG_SLOB */

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 87e5535..4730ef6 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -583,6 +583,66 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
put_online_cpus();
}

+static void kmemcg_deactivate_workfn(struct work_struct *work)
+{
+ struct kmem_cache *s = container_of(work, struct kmem_cache,
+ memcg_params.deact_work);
+
+ get_online_cpus();
+ get_online_mems();
+
+ mutex_lock(&slab_mutex);
+
+ s->memcg_params.deact_fn(s);
+
+ mutex_unlock(&slab_mutex);
+
+ put_online_mems();
+ put_online_cpus();
+
+ /* done, put the ref from slab_deactivate_memcg_cache_rcu_sched() */
+ css_put(&s->memcg_params.memcg->css);
+}
+
+static void kmemcg_deactivate_rcufn(struct rcu_head *head)
+{
+ struct kmem_cache *s = container_of(head, struct kmem_cache,
+ memcg_params.deact_rcu_head);
+
+ /*
+ * We need to grab blocking locks. Bounce to ->deact_work. The
+ * work item shares the space with the RCU head and can't be
+ * initialized eariler.
+ */
+ INIT_WORK(&s->memcg_params.deact_work, kmemcg_deactivate_workfn);
+ schedule_work(&s->memcg_params.deact_work);
+}
+
+/**
+ * slab_deactivate_memcg_cache_rcu_sched - schedule deactivation after a
+ * sched RCU grace period
+ * @s: target kmem_cache
+ * @deact_fn: deactivation function to call
+ *
+ * Schedule @deact_fn to be invoked with online cpus, mems and slab_mutex
+ * held after a sched RCU grace period. The slab is guaranteed to stay
+ * alive until @deact_fn is finished. This is to be used from
+ * __kmemcg_cache_deactivate().
+ */
+void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
+ void (*deact_fn)(struct kmem_cache *))
+{
+ if (WARN_ON_ONCE(is_root_cache(s)) ||
+ WARN_ON_ONCE(s->memcg_params.deact_fn))
+ return;
+
+ /* pin memcg so that @s doesn't get destroyed in the middle */
+ css_get(&s->memcg_params.memcg->css);
+
+ s->memcg_params.deact_fn = deact_fn;
+ call_rcu_sched(&s->memcg_params.deact_rcu_head, kmemcg_deactivate_rcufn);
+}
+
void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
{
int idx;
diff --git a/mm/slub.c b/mm/slub.c
index ef89a07..8621940 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3949,6 +3949,12 @@ int __kmem_cache_shrink(struct kmem_cache *s)
}

#ifdef CONFIG_MEMCG
+static void kmemcg_cache_deact_after_rcu(struct kmem_cache *s)
+{
+ /* called with all the locks held after a sched RCU grace period */
+ __kmem_cache_shrink(s);
+}
+
void __kmemcg_cache_deactivate(struct kmem_cache *s)
{
/*
@@ -3960,11 +3966,9 @@ void __kmemcg_cache_deactivate(struct kmem_cache *s)

/*
* s->cpu_partial is checked locklessly (see put_cpu_partial), so
- * we have to make sure the change is visible.
+ * we have to make sure the change is visible before shrinking.
*/
- synchronize_sched();
-
- __kmem_cache_shrink(s);
+ slab_deactivate_memcg_cache_rcu_sched(s, kmemcg_cache_deact_after_rcu);
}
#endif

--
2.9.3