linux-kernel - Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

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Date:	Thu, 8 Nov 2012 11:21:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Glauber Costa <glommer@...allels.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, kamezawa.hiroyu@...fujitsu.com,
	Johannes Weiner <hannes@...xchg.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Suleiman Souhlal <suleiman@...gle.com>
Subject: Re: [PATCH v6 25/29] memcg/sl[au]b: shrink dead caches

On Thu, 8 Nov 2012 17:15:36 +0000
Christoph Lameter <cl@...ux.com> wrote:

> On Wed, 7 Nov 2012, Andrew Morton wrote:
> 
> > What's up with kmem_cache_shrink?  It's global and exported to modules
> > but its only external caller is some weird and hopelessly poorly
> > documented site down in drivers/acpi/osl.c.  slab and slob implement
> > kmem_cache_shrink() *only* for acpi!  wtf?  Let's work out what acpi is
> > trying to actually do there, then do it properly, then killkillkill!
> 
> kmem_cache_shrink is also used internally. Its simply releasing unused
> cached objects.

Only in slub.  It could be removed outright from the others and
simplified in slub.

> > Secondly, as slab and slub (at least) have the ability to shed cached
> > memory, why aren't they hooked into the core cache-shinking machinery.
> > After all, it's called "shrink_slab"!
> 
> Because the core cache shrinking needs the slab caches to free up memory
> from inodes and dentries. We could call kmem_cache_shrink at the end of
> the shrink passes in vmscan. The price would be that the caches would have
> to be repopulated when new allocations occur.

Well, the shrinker shouldn't strips away all the cache.  It will perform
a partial trim, the magnitude of which increases with perceived
external memory pressure.

AFACIT, this is correct and desirable behaviour for shrinking
slab's internal caches.

> >
> > If we can fix all that up then I wonder whether this particular patch
> > needs to exist at all.  If the kmem_cache is no longer used then we
> > can simply leave it floating around in memory and the regular cache
> > shrinking code out of shrink_slab() will clean up any remaining pages.
> > The kmem_cache itself can be reclaimed via another shrinker, if
> > necessary?
> 
> The kmem_cache can only be released if all its objects (used and unused)
> are released.  kmem_cache_shrink drops the unused objects on some internal
> slab specific list. That may enable us to release the kmem_cache
> structure.
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