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[PATCHSET v2] slab: make memcg slab destruction scalable

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  [PATCHSET v2] slab: make memcg slab destruction scalable Tejun Heo <tj@kernel.org> - 2017-01-14 20:00 +0100
    [PATCH 7/8] slab: remove synchronous synchronize_sched() from memcg cache deactivation path Tejun Heo <tj@kernel.org> - 2017-01-14 20:00 +0100

#1559053 — [PATCHSET v2] slab: make memcg slab destruction scalable

This is v2.  Changes from the last version[L] are

* 0002-slab-remove-synchronous-rcu_barrier-call-in-memcg-ca.patch was
  incorrect and dropped.

* 0006-slab-don-t-put-memcg-caches-on-slab_caches-list.patch
  incorrectly converted places which needed to walk all caches.
  Replaced with 0005-slab-implement-slab_root_caches-list.patch which
  adds root-only list instead of converting slab_caches list to list
  only root caches.

* Misc fixes.

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.

I've seen machines which end up with hundred thousands of caches and
many millions of kernfs_nodes.  The current code is O(N^2) on the
total number of caches and has synchronous rcu_barrier() and
synchronize_sched() in cgroup offline / release path which is executed
while holding cgroup_mutex.  Combined, this leads to very expensive
and slow cache destruction operations which can easily keep running
for half a day.

This also messes up /proc/slabinfo along with other cache iterating
operations.  seq_file operates on 4k chunks and on each 4k boundary
tries to seek to the last position in the list.  With a huge number of
caches on the list, this becomes very slow and very prone to the list
content changing underneath it leading to a lot of missing and/or
duplicate entries.

This patchset addresses the scalability problem.

* Add root and per-memcg lists.  Update each user to use the
  appropriate list.

* Replace rcu_barrier() and synchronize_rcu() with call_rcu() and
  call_rcu_sched().

* For dying empty slub caches, remove the sysfs files after
  deactivation so that we don't end up with millions of sysfs files
  without any useful information on them.

This patchset contains the following nine patches.

 0001-Revert-slub-move-synchronize_sched-out-of-slab_mutex.patch
 0002-slab-remove-synchronous-rcu_barrier-call-in-memcg-ca.patch
 0003-slab-reorganize-memcg_cache_params.patch
 0004-slab-link-memcg-kmem_caches-on-their-associated-memo.patch
 0005-slab-implement-slab_root_caches-list.patch
 0006-slab-introduce-__kmemcg_cache_deactivate.patch
 0007-slab-remove-synchronous-synchronize_sched-from-memcg.patch
 0008-slab-remove-slub-sysfs-interface-files-early-for-emp.patch

0001 reverts an existing optimization to prepare for the following
changes.  0002 replaces rcu_barrier() in release path with call_rcu().
0004-0005 separate out the lists.  0006-0007 replace
synchronize_sched() in slub destruction path with call_rcu_sched().
0008 removes sysfs files early for empty dying caches.

This patchset is on top of the current linus#master a121103c9228 and
also available in the following git branch.

 git://git.kernel.org/pub/scm/linux/kernel/git/tj/misc.git review-kmemcg-scalability

diffstat follows.  Thanks.

 include/linux/memcontrol.h |    1 
 include/linux/slab.h       |   45 ++++++-
 include/linux/slab_def.h   |    5 
 include/linux/slub_def.h   |    9 +
 mm/memcontrol.c            |    7 -
 mm/slab.c                  |    7 +
 mm/slab.h                  |   32 ++++-
 mm/slab_common.c           |  269 +++++++++++++++++++++++++++------------------
 mm/slub.c                  |   55 +++++++++
 9 files changed, 302 insertions(+), 128 deletions(-)

--
tejun

[L] http://lkml.kernel.org/r/<20170114055449.11044-1-tj@kernel.org>

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#1559054 — [PATCH 7/8] slab: remove synchronous synchronize_sched() from memcg cache deactivation 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.

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>
Acked-by: 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 41c49cc..5ca8778 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -582,6 +582,12 @@ struct memcg_cache_params {
 			struct mem_cgroup *memcg;
 			struct list_head children_node;
 			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 0946d97..2fe07d7 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -304,6 +304,8 @@ static __always_inline void memcg_uncharge_slab(struct page *page, int order,
 
 extern void slab_init_memcg_params(struct kmem_cache *);
 extern void memcg_link_cache(struct kmem_cache *s);
+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 cd81cad..4a0605c 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -592,6 +592,66 @@ 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 c754ea0..184f80b 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

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