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@xxxxxxxxx> 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@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) -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. 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