Re: [Bug 172981] New: [bisected] SLAB: extreme load averages and over 2000 kworker threads

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Thu, Sep 29, 2016 at 11:00:50AM +0900, Joonsoo Kim wrote:
> On Wed, Sep 28, 2016 at 11:09:53AM +0300, Vladimir Davydov wrote:
> > On Tue, Sep 27, 2016 at 10:03:47PM -0400, Johannes Weiner wrote:
> > > [CC Vladimir]
> > > 
> > > These are the delayed memcg cache allocations, where in a fresh memcg
> > > that doesn't have per-memcg caches yet, every accounted allocation
> > > schedules a kmalloc work item in __memcg_schedule_kmem_cache_create()
> > > until the cache is finally available. It looks like those can be many
> > > more than the number of slab caches in existence, if there is a storm
> > > of slab allocations before the workers get a chance to run.
> > > 
> > > Vladimir, what do you think of embedding the work item into the
> > > memcg_cache_array? That way we make sure we have exactly one work per
> > > cache and not an unbounded number of them. The downside of course is
> > > that we'd have to keep these things around as long as the memcg is in
> > > existence, but that's the only place I can think of that allows us to
> > > serialize this.
> > 
> > We could set the entry of the root_cache->memcg_params.memcg_caches
> > array corresponding to the cache being created to a special value, say
> > (void*)1, and skip scheduling cache creation work on kmalloc if the
> > caller sees it. I'm not sure it's really worth it though, because
> > work_struct isn't that big (at least, in comparison with the cache
> > itself) to avoid embedding it at all costs.
> 
> Hello, Johannes and Vladimir.
> 
> I'm not familiar with memcg so have a question about this solution.
> This solution will solve the current issue but if burst memcg creation
> happens, similar issue would happen again. My understanding is correct?

Yes, I think you're right - embedding the work_struct responsible for
cache creation in kmem_cache struct won't help if a thousand of
different cgroups call kmem_cache_alloc() simultaneously for a cache
they haven't used yet.

Come to think of it, we could fix the issue by simply introducing a
special single-threaded workqueue used exclusively for cache creation
works - cache creation is done mostly under the slab_mutex, anyway. This
way, we wouldn't have to keep those used-once work_structs for the whole
kmem_cache life time.

> 
> I think that the other cause of the problem is that we call
> synchronize_sched() which is rather slow with holding a slab_mutex and
> it blocks further kmem_cache creation. Should we fix that, too?

Well, the patch you posted looks pretty obvious and it helps the
reporter, so personally I don't see any reason for not applying it.

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