linux-kernel - Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE

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Message-Id: <20121119124327.20e008a0.akpm@linux-foundation.org>
Date:	Mon, 19 Nov 2012 12:43:27 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-mm@...ck.org, linaro-mm-sig@...ts.linaro.org,
	linux-kernel@...r.kernel.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Mel Gorman <mel@....ul.ie>,
	Michal Nazarewicz <mina86@...a86.com>,
	Minchan Kim <minchan@...nel.org>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [PATCH] mm: cma: allocate pages from CMA if NR_FREE_PAGES
 approaches low water mark

On Mon, 19 Nov 2012 16:38:18 +0100
Marek Szyprowski <m.szyprowski@...sung.com> wrote:

> Hello,
> 
> On 11/14/2012 11:58 PM, Andrew Morton wrote:
> > On Mon, 12 Nov 2012 09:59:42 +0100
> > Marek Szyprowski <m.szyprowski@...sung.com> wrote:
> >
> > > It has been observed that system tends to keep a lot of CMA free pages
> > > even in very high memory pressure use cases. The CMA fallback for movable
> > > pages is used very rarely, only when system is completely pruned from
> > > MOVABLE pages, what usually means that the out-of-memory even will be
> > > triggered very soon. To avoid such situation and make better use of CMA
> > > pages, a heuristics is introduced which turns on CMA fallback for movable
> > > pages when the real number of free pages (excluding CMA free pages)
> > > approaches low water mark.
>
> ...
>
> > erk, this is right on the page allocator hotpath.  Bad.
> 
> Yes, I know that it adds an overhead to allocation hot path, but I found 
> no other
> place for such change. Do You have any suggestion where such change can 
> be applied
> to avoid additional load on hot path?

Do the work somewhere else, not on a hot path?  Somewhere on the page
reclaim path sounds appropriate.  How messy would it be to perform some
sort of balancing at reclaim time?


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