Messages in this thread |  | | Date | Mon, 3 Oct 2016 15:19:31 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH 1/2] mm: memcontrol: use special workqueue for creating per-memcg caches |
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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.
> Since the number of caches that can be created by a single > memcg is obviously limited,
by the number of existing caches, right?
> I don't see any possibility of starvation.
What I meant was that while now workers can contend on the slab_mutex with the patch there will be a real ordering in place AFAIU and so an unlucky memcg can be waiting for N(memcgs) * N (caches) to be served. Not that the current implementation gives us anything because the ordering should be more or less scheduling and workers dependent. Or I am missing something. A per-cache memcg WQ would mitigate to some extent.
> 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.
OK please put this information into the changelog.
That being said I am not opposing the current solution I just wanted to understand all the consequences and would appreciate more information in the changelog as this seems like the stable material.
Thanks!
-- Michal Hocko SUSE Labs
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