linux-kernel - Re: [PATCH -mm 7/8] slub: make dead caches discard free slabs immediately

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Date:	Mon, 2 Jun 2014 13:24:36 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Christoph Lameter <cl@...two.org>, akpm@...ux-foundation.org,
	hannes@...xchg.org, mhocko@...e.cz, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH -mm 7/8] slub: make dead caches discard free slabs
 immediately

On Sat, May 31, 2014 at 03:04:58PM +0400, Vladimir Davydov wrote:
> On Fri, May 30, 2014 at 09:57:10AM -0500, Christoph Lameter wrote:
> > On Fri, 30 May 2014, Vladimir Davydov wrote:
> > 
> > > (3) is a bit more difficult, because slabs are added to per-cpu partial
> > > lists lock-less. Fortunately, we only have to handle the __slab_free
> > > case, because, as there shouldn't be any allocation requests dispatched
> > > to a dead memcg cache, get_partial_node() should never be called. In
> > > __slab_free we use cmpxchg to modify kmem_cache_cpu->partial (see
> > > put_cpu_partial) so that setting ->partial to a special value, which
> > > will make put_cpu_partial bail out, will do the trick.
> > >
> > > Note, this shouldn't affect performance, because keeping empty slabs on
> > > per node lists as well as using per cpu partials are only worthwhile if
> > > the cache is used for allocations, which isn't the case for dead caches.
> > 
> > This all sounds pretty good to me but we still have some pretty extensive
> > modifications that I would rather avoid.
> > 
> > In put_cpu_partial you can simply check that the memcg is dead right? This
> > would avoid all the other modifications I would think and will not require
> > a special value for the per cpu partial pointer.
> 
> That would be racy. The check if memcg is dead and the write to per cpu
> partial ptr wouldn't proceed as one atomic operation. If we set the dead
> flag from another thread between these two operations, put_cpu_partial
> will add a slab to a per cpu partial list *after* the cache was zapped.

Hello, Vladimir.

I think that we can do (3) easily.
If we check memcg_cache_dead() in the end of put_cpu_partial() rather
than in the begin of put_cpu_partial(), we can avoid the race you 
mentioned. If someone do put_cpu_partial() before dead flag is set,
it can be zapped by who set dead flag. And if someone do
put_cpu_partial() after dead flag is set, it can be zapped by who
do put_cpu_partial().

Thanks.
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