linux-kernel - Re: 2.6.22 -mm merge plans: slub on PowerPC

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Message-Id: <1178238344.6353.79.camel@localhost.localdomain>
Date:	Fri, 04 May 2007 10:25:44 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Paul Mackerras <paulus@...ba.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: 2.6.22 -mm merge plans: slub on PowerPC

On Thu, 2007-05-03 at 22:04 +0100, Hugh Dickins wrote:
> On Thu, 3 May 2007, Hugh Dickins wrote:
> > 
> > Seems we're all wrong in thinking Christoph's Kconfiggery worked
> > as intended: maybe it just works some of the time.  I'm not going
> > to hazard a guess as to how to fix it up, will resume looking at
> > the powerpc's quicklist potential later.
> 
> Here's the patch I've been testing on G5, with 4k and with 64k pages,
> with SLAB and with SLUB.  But, though it doesn't crash, the pgd
> kmem_cache in the 4k-page SLUB case is revealing SLUB's propensity
> for using highorder allocations where SLAB would stick to order 0:
> under load, exec's mm_init gets page allocation failure on order 4
> - SLUB's calculate_order may need some retuning.  (I'd expect it to
> be going for order 3 actually, I'm not sure how order 4 comes about.)
> 
> I don't know how offensive Ben and Paulus may find this patch:
> the kmem_cache use was nicely done and this messes it up a little.
> 
> 
> The SLUB allocator relies on struct page fields first_page and slab,
> overwritten by ptl when SPLIT_PTLOCK: so the SLUB allocator cannot then
> be used for the lowest level of pagetable pages.  This was obstructing
> SLUB on PowerPC, which uses kmem_caches for its pagetables.  So convert
> its pte level to use quicklist pages (whereas pmd, pud and 64k-page pgd
> want partpages, so continue to use kmem_caches for pmd, pud and pgd).
> But to keep up appearances for pgtable_free, we still need PTE_CACHE_NUM.

Interesting... I'll have a look asap.

Ben.


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