linux-kernel - Re: [PATCH 04/18] mm: numa: Do not migrate or account for hinting faults on the zero page

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Date:	Wed, 31 Jul 2013 09:11:30 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/18] mm: numa: Do not migrate or account for hinting
 faults on the zero page

On Wed, Jul 17, 2013 at 01:00:53PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 15, 2013 at 04:20:06PM +0100, Mel Gorman wrote:
> > The zero page is not replicated between nodes and is often shared
> > between processes. The data is read-only and likely to be cached in
> > local CPUs if heavily accessed meaning that the remote memory access
> > cost is less of a concern. This patch stops accounting for numa hinting
> > faults on the zero page in both terms of counting faults and scheduling
> > tasks on nodes.
> > 
> > Signed-off-by: Mel Gorman <mgorman@...e.de>
> > ---
> >  mm/huge_memory.c | 9 +++++++++
> >  mm/memory.c      | 7 ++++++-
> >  2 files changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index e4a79fa..ec938ed 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1302,6 +1302,15 @@ int do_huge_pmd_numa_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >  
> >  	page = pmd_page(pmd);
> >  	get_page(page);
> > +
> > +	/*
> > +	 * Do not account for faults against the huge zero page. The read-only
> > +	 * data is likely to be read-cached on the local CPUs and it is less
> > +	 * useful to know about local versus remote hits on the zero page.
> > +	 */
> > +	if (is_huge_zero_pfn(page_to_pfn(page)))
> > +		goto clear_pmdnuma;
> > +
> >  	src_nid = numa_node_id();
> >  	count_vm_numa_event(NUMA_HINT_FAULTS);
> >  	if (src_nid == page_to_nid(page))
> 
> And because of:
> 
>   5918d10 thp: fix huge zero page logic for page with pfn == 0
> 

Yes. Thanks.

-- 
Mel Gorman
SUSE Labs
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