[PATCH] mm: memcontrol: use special workqueue for creating per-memcg caches

[Vladimir Davydov]

Creating a lot of cgroups at the same time might stall all worker
threads with kmem cache creation works, because kmem cache creation is
done with the slab_mutex held. The problem was amplified by commits
801faf0db894 ("mm/slab: lockless decision to grow cache") in case of
SLAB and 81ae6d03952c ("mm/slub.c: replace kick_all_cpus_sync() with
synchronize_sched() in kmem_cache_shrink()") in case of SLUB, which
increased the maximal time the slab_mutex can be held.

To prevent that from happening, let's use a special ordered single
threaded workqueue for kmem cache creation. This shouldn't introduce any
functional changes regarding how kmem caches are created, as the work
function holds the global slab_mutex during its whole runtime anyway,
making it impossible to run more than one work at a time. By using a
single threaded workqueue, we just avoid creating a thread per each
work. Ordering is required to avoid a situation when a cgroup's work is
put off indefinitely because there are other cgroups to serve, in other
words to guarantee fairness.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=172981
Signed-off-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx>
Reported-by: Doug Smythies <dsmythies@xxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxxxx>
Cc: Pekka Enberg <penberg@xxxxxxxxxx>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4be518d4e68a..8d753d87ca37 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2175,6 +2175,8 @@ struct memcg_kmem_cache_create_work {
 	struct work_struct work;
 };
 
+static struct workqueue_struct *memcg_kmem_cache_create_wq;
+
 static void memcg_kmem_cache_create_func(struct work_struct *w)
 {
 	struct memcg_kmem_cache_create_work *cw =
@@ -2206,7 +2208,7 @@ static void __memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,
 	cw->cachep = cachep;
 	INIT_WORK(&cw->work, memcg_kmem_cache_create_func);
 
-	schedule_work(&cw->work);
+	queue_work(memcg_kmem_cache_create_wq, &cw->work);
 }
 
 static void memcg_schedule_kmem_cache_create(struct mem_cgroup *memcg,
@@ -5794,6 +5796,17 @@ static int __init mem_cgroup_init(void)
 {
 	int cpu, node;
 
+#ifndef CONFIG_SLOB
+	/*
+	 * Kmem cache creation is mostly done with the slab_mutex held,
+	 * so use a special workqueue to avoid stalling all worker
+	 * threads in case lots of cgroups are created simultaneously.
+	 */
+	memcg_kmem_cache_create_wq =
+		alloc_ordered_workqueue("memcg_kmem_cache_create", 0);
+	BUG_ON(!memcg_kmem_cache_create_wq);
+#endif
+
 	hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
 
 	for_each_possible_cpu(cpu)