Re: [PATCH 1/2] mm: memcontrol: use special workqueue for creating per-memcg caches

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Mon, Oct 03, 2016 at 03:19:31PM +0200, Michal Hocko wrote:
> On Mon 03-10-16 15:35:06, Vladimir Davydov wrote:
> > On Mon, Oct 03, 2016 at 02:06:42PM +0200, Michal Hocko wrote:
> > > On Sat 01-10-16 16:56:47, Vladimir Davydov wrote:
> > > > 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. To prevent that from happening, let's use
> > > > a special workqueue for kmem cache creation with max in-flight work
> > > > items equal to 1.
> > > > 
> > > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=172981
> > > 
> > > This looks like a regression but I am not really sure I understand what
> > > has caused it. We had the WQ based cache creation since kmem was
> > > introduced more or less. So is it 801faf0db894 ("mm/slab: lockless
> > > decision to grow cache") which was pointed by bisection that changed the
> > > timing resp. relaxed the cache creation to the point that would allow
> > > this runaway?
> > 
> > It is in case of SLAB. For SLUB the issue was caused by commit
> > 81ae6d03952c ("mm/slub.c: replace kick_all_cpus_sync() with
> > synchronize_sched() in kmem_cache_shrink()").
> 
> OK, thanks for the confirmation. This would be useful in the changelog
> imho.
> 
> > > This would be really useful for the stable backport
> > > consideration.
> > > 
> > > Also, if I understand the fix correctly, now we do limit the number of
> > > workers to 1 thread. Is this really what we want? Wouldn't it be
> > > possible that few memcgs could starve others fromm having their cache
> > > created? What would be the result, missed charges?
> > 
> > Now kmem caches are created in FIFO order, i.e. if one memcg called
> > kmem_cache_alloc on a non-existent cache before another, it will be
> > served first.
> 
> I do not see where this FIFO is guaranteed.
> __memcg_schedule_kmem_cache_create doesn't seem to be using ordered WQ.

Yeah, you're right - I thought max_active implies ordering, but it
doesn't. Then we can use an ordered workqueue. Here's the updated
patch:

>From 10f5f126800912c6a4b78a8b615138c1322694ad Mon Sep 17 00:00:00 2001
From: Vladimir Davydov <vdavydov.dev@gmail.com>
Date: Sat, 1 Oct 2016 16:39:09 +0300
Subject: [PATCH] mm: memcontrol: use special workqueue for creating per-memcg
 caches

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@gmail.com>
Reported-by: Doug Smythies <dsmythies@telus.net>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>

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)