linux-kernel - Re: [PATCH] mm: memmap_init_zone() performance improvement

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Date:	Tue, 30 Oct 2012 08:14:22 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	Mike Yoknis <mike.yoknis@...com>, mingo@...hat.com,
	akpm@...ux-foundation.org, linux-arch@...r.kernel.org,
	mmarek@...e.cz, tglx@...utronix.de, hpa@...or.com, arnd@...db.de,
	sam@...nborg.org, minchan@...nel.org,
	kamezawa.hiroyu@...fujitsu.com, mhocko@...e.cz,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: memmap_init_zone() performance improvement

On 10/20/2012 01:29 AM, Mel Gorman wrote:
> I'm travelling at the moment so apologies that I have not followed up on
> this. My problem is still the same with the patch - it changes more
> headers than is necessary and it is sparsemem specific. At minimum, try
> the suggestion of 
> 
> if (!early_pfn_valid(pfn)) {
>       pfn = ALIGN(pfn + MAX_ORDER_NR_PAGES, MAX_ORDER_NR_PAGES) - 1;
>       continue;
> }

Sorry I didn't catch this until v2...

Is that ALIGN() correct?  If pfn=3, then it would expand to:

(3+MAX_ORDER_NR_PAGES+MAX_ORDER_NR_PAGES-1) & ~(MAX_ORDER_NR_PAGES-1)

You would end up skipping the current MAX_ORDER_NR_PAGES area, and then
one _extra_ because ALIGN() aligns up, and you're adding
MAX_ORDER_NR_PAGES too.  It doesn't matter unless you run in to a
!early_valid_pfn() in the middle of a MAX_ORDER area, I guess.

I think this would work, plus be a bit smaller:

	pfn = ALIGN(pfn + 1, MAX_ORDER_NR_PAGES) - 1;

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