linux-kernel - Re: [PATCH -mm 1/3] slub: don't fail kmem_cache_shrink if slab placement optimization fails

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Date:	Mon, 26 Jan 2015 22:36:29 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Christoph Lameter <cl@...ux.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Pekka Enberg <penberg@...nel.org>,
	David Rientjes <rientjes@...gle.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Johannes Weiner <hannes@...xchg.org>,
	Michal Hocko <mhocko@...e.cz>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm 1/3] slub: don't fail kmem_cache_shrink if slab
 placement optimization fails

On Mon, Jan 26, 2015 at 12:24:49PM -0600, Christoph Lameter wrote:
> On Mon, 26 Jan 2015, Vladimir Davydov wrote:
> 
> > Anyways, I think that silently relying on the fact that the allocator
> > never fails small allocations is kind of unreliable. What if this
> 
> We are not doing that though. If the allocation fails we do nothing.

Yeah, that's correct, but memcg/kmem wants it to always free empty slabs
(see patch 3 for details), so I'm trying to be punctual and eliminate
any possibility of failure, because a failure (if it ever happened)
would result in a permanent memory leak (pinned mem_cgroup + its
kmem_caches).

> 
> > > > +			if (page->inuse < objects)
> > > > +				list_move(&page->lru,
> > > > +					  slabs_by_inuse + page->inuse);
> > > >  			if (!page->inuse)
> > > >  				n->nr_partial--;
> > > >  		}
> > >
> > > The condition is always true. A page that has page->inuse == objects
> > > would not be on the partial list.
> > >
> >
> > This is in case we failed to allocate the slabs_by_inuse array. We only
> > have a list for empty slabs then (on stack).
> 
> Ok in that case objects == 1. If you want to do this maybe do it in a more
> general way?
> 
> You could allocate an array on the stack to deal with the common cases. I
> believe an array of 32 objects would be fine to allocate and cover most of
> the slab caches on the system? Would eliminate most of the allocations in
> kmem_cache_shrink.

We could do that, but IMO that would only complicate the code w/o
yielding any real benefits. This function is slow and called rarely
anyway, so I don't think there is any point to optimize out a page
allocation here.

Thanks,
Vladimir
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