linux-kernel - Re: [PATCH 07/50] mm: Account for a THP NUMA hinting update as one PTE update

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Date:	Mon, 16 Sep 2013 16:54:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Mel Gorman <mgorman@...e.de>,
	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 07/50] mm: Account for a THP NUMA hinting update as one
 PTE update

On Mon, Sep 16, 2013 at 09:39:59AM -0400, Rik van Riel wrote:
> On 09/16/2013 08:36 AM, Peter Zijlstra wrote:
> > On Tue, Sep 10, 2013 at 10:31:47AM +0100, Mel Gorman wrote:
> >> A THP PMD update is accounted for as 512 pages updated in vmstat.  This is
> >> large difference when estimating the cost of automatic NUMA balancing and
> >> can be misleading when comparing results that had collapsed versus split
> >> THP. This patch addresses the accounting issue.
> >>
> >> Signed-off-by: Mel Gorman <mgorman@...e.de>
> >> ---
> >>  mm/mprotect.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/mm/mprotect.c b/mm/mprotect.c
> >> index 94722a4..2bbb648 100644
> >> --- a/mm/mprotect.c
> >> +++ b/mm/mprotect.c
> >> @@ -145,7 +145,7 @@ static inline unsigned long change_pmd_range(struct vm_area_struct *vma,
> >>  				split_huge_page_pmd(vma, addr, pmd);
> >>  			else if (change_huge_pmd(vma, pmd, addr, newprot,
> >>  						 prot_numa)) {
> >> -				pages += HPAGE_PMD_NR;
> >> +				pages++;
> > 
> > But now you're not counting pages anymore..
> 
> The migrate statistics still count pages. That makes sense, since the
> amount of work scales with the amount of memory moved.

Right.

> It is just the "number of faults" counters that actually count the
> number of faults again, instead of the number of pages represented
> by each fault.

So you're suggesting s/pages/faults/ or somesuch?

> IMHO this change makes sense.

I never said the change didn't make sense as such. Just that we're no
longer counting pages in change_*_range().
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