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

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

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?

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?

Thanks.

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