Re: [PATCH -mm 0/8] memcg: reparent kmem on css offline

Johannes Weiner
2014-07-08T22:05:35Z

On Mon, Jul 07, 2014 at 07:40:08PM +0400, Vladimir Davydov wrote:
> On Mon, Jul 07, 2014 at 10:25:06AM -0400, Johannes Weiner wrote:
> > You could then reap dead slab caches as part of the regular per-memcg
> > slab scanning in reclaim, without having to resort to auxiliary lists,
> > vmpressure events etc.
>
> Do you mean adding a per memcg shrinker that will call kmem_cache_shrink
> for all memcg caches on memcg/global pressure?
>
> Actually I recently made dead caches self-destructive at the cost of
> slowing down kfrees to dead caches (see
> https://www.lwn.net/Articles/602330/, it's already in the mmotm tree) so
> no dead cache reaping is necessary. Do you think if we need it now?
>
> > I think it would save us a lot of code and complexity. You want
> > per-memcg slab scanning *anyway*, all we'd have to change in the
> > existing code would be to pin the css until the LRUs and kmem caches
> > are truly empty, and switch mem_cgroup_iter() to css_tryget().
> >
> > Would this make sense to you?
>
> Hmm, interesting. Thank you for such a thorough explanation.
>
> One question. Do we still need to free mem_cgroup->kmemcg_id on css
> offline so that it can be reused by new kmem-active cgroups (currently
> we don't)?
>
> If we won't free it the root_cache->memcg_params->memcg_arrays may
> become really huge due to lots of dead css holding the id.

We only need the O(1) access of the array for allocation - not frees
and reclaim, right?

So with your self-destruct code, can we prune caches of dead css and
then just remove them from the array? Or move them from the array to
a per-memcg linked list that can be scanned on memcg memory pressure?