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

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Message-Id: <20161020204435.5e0ffca43c7b6ab5f69d692a@linux-foundation.org>
Date:   Thu, 20 Oct 2016 20:44:35 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     Michal Hocko <mhocko@...nel.org>, 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 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?