linux-kernel - Re: [PATCH RFC -mm v2 0/3] kmemcg: simplify work-flow (was "memcg-vs-slab cleanup")

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Date:	Fri, 18 Apr 2014 20:04:38 +0400
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Johannes Weiner <hannes@...xchg.org>
CC:	<mhocko@...e.cz>, <akpm@...ux-foundation.org>, <glommer@...il.com>,
	<cl@...ux-foundation.org>, <penberg@...nel.org>,
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
	<devel@...nvz.org>
Subject: Re: [PATCH RFC -mm v2 0/3] kmemcg: simplify work-flow (was "memcg-vs-slab
 cleanup")

On 04/18/2014 05:23 PM, Johannes Weiner wrote:
>> First, it removes async per memcg cache destruction (see patches 1, 2).
>> Now caches are only destroyed on memcg offline. That means the caches
>> that are not empty on memcg offline will be leaked. However, they are
>> already leaked, because memcg_cache_params::nr_pages normally never
>> drops to 0 so the destruction work is never scheduled except
>> kmem_cache_shrink is called explicitly. In the future I'm planning
>> reaping such dead caches on vmpressure or periodically.
>
> I like the synchronous handling on css destruction, but the periodical
> reaping part still bothers me.  If there is absolutely 0 use for these
> caches remaining, they shouldn't hang around until we encounter memory
> pressure or a random time interval.

Agree.

> Would it be feasible to implement cache merging in both slub and slab,
> so that upon css destruction the child's cache's remaining slabs could
> be moved to the parent's cache?  If the parent doesn't have one, just
> reparent the whole cache.

Interesting idea. That would definitely look neater than periodic
reaping. But it's going to be an uneasy thing to do I guess, because
synchronization in sl[au]b is a subtle thing. I'll have a closer look at
slab's internals to understand if it's feasible.

>
>> Second, it substitutes per memcg slab_caches_mutex's with the global
>> memcg_slab_mutex, which should be taken during the whole per memcg cache
>> creation/destruction path before the slab_mutex (see patch 3). This
>> greatly simplifies synchronization among various per memcg cache
>> creation/destruction paths.
>
> This sounds reasonable.  I'll go look at the code.

Thank you!
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