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

[Vladimir Davydov]

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()").

> 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. Since the number of caches that can be created by a single
memcg is obviously limited, I don't see any possibility of starvation.
Actually, this patch doesn't introduce any functional changes regarding
the order in which kmem caches are created, as the work function holds
the global slab_mutex during its whole runtime anyway. We only avoid
creating a thread per each work by making the queue single-threaded.