linux-kernel - Re: [PATCH -mm 8/8] slab: recharge slab pages to the allocating memory cgroup

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Date:	Thu, 6 Nov 2014 18:22:09 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 8/8] slab: recharge slab pages to the allocating
 memory cgroup

On Thu, Nov 06, 2014 at 09:01:52AM -0600, Christoph Lameter wrote:
> On Thu, 6 Nov 2014, Vladimir Davydov wrote:
> 
> > I call memcg_kmem_recharge_slab only on alloc path. Free path isn't
> > touched. The overhead added is one function call. The function only
> > reads and compares two pointers under RCU most of time. This is
> > comparable to the overhead introduced by memcg_kmem_get_cache, which is
> > called in slab_alloc/slab_alloc_node earlier.
> 
> Right maybe remove those too? Things seem to be accumulating in the hot
> path which is bad. There is a slow path where these things can be added
> and also a page based even slower path for statistics keeping.
> 
> The approach in SLUB is to do accounting on a slab page basis. Also memory
> policies are applied at page granularity not object granularity.
> 
> > Anyways, if you think this is unacceptable, I don't mind dropping the
> > whole patch set and thinking more on how to fix this per-memcg caches
> > trickery. What do you think?
> 
> Maybe its possible to just use slab page accounting instead of object
> accounting? Reduces overhead significantly. There may be some fuzz here
> with occasional object accounted in the wrong way (which is similar to how
> memory policies and other methods work) but it has been done before and
> works ok.

Actually, it's not about mis-accounting. The problem is a newly
allocated object can pin a charge of a dead cgroup that used the cache
before. May be, it wouldn't be a problem though.

Anyways, I think I need more time to brood over the whole approach, so
I've asked Andrew to drop the patch set.

Thank you for the feedback!
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