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

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Date:   Fri, 21 Oct 2016 08:39:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm: memcontrol: use special workqueue for creating
 per-memcg caches

On Thu 20-10-16 20:44:35, Andrew Morton wrote:
> On Tue, 4 Oct 2016 16:14:17 +0300 Vladimir Davydov <vdavydov.dev@...il.com> 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. 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.
> 
> I'm having trouble working out the urgency of this patch?

Seeing thousands of kernel threads is certainly annoying so I think we
want to merge it sooner rather than later and have it backported to
stable as well.

-- 
Michal Hocko
SUSE Labs