On Thu 20-10-16 20:44:35, Andrew Morton wrote:

On Tue, 4 Oct 2016 16:14:17 +0300 Vladimir Davydov [off-list ref] wrote:

quoted

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

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