linux-kernel - Re: [PATCH] mm: cma: free cma page to buddy instead of being cpu hot page

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Date:	Wed, 6 Nov 2013 15:41:20 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	Mel Gorman <mgorman@...e.de>, zhang.mingjun@...aro.org,
	m.szyprowski@...sung.com, haojian.zhuang@...aro.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Mingjun Zhang <troy.zhangmingjun@...aro.org>
Subject: Re: [PATCH] mm: cma: free cma page to buddy instead of being cpu
 hot page

On Wed, 6 Nov 2013 15:43:02 +0900 Minchan Kim <minchan@...nel.org> wrote:

> > The added overhead is pretty small - just a comparison of a local with
> > a constant.  And that cost is not incurred for MIGRATE_UNMOVABLE,
> > MIGRATE_RECLAIMABLE and MIGRATE_MOVABLE, which are the common cases
> > (yes?).
> 
> True but bloat code might affect icache so we should be careful.
> And what Mel has a concern is about zone->lock, which would be more contended.
> I agree his opinion.
> 
> In addition, I think the gain is marginal because normally CMA is big range
> so free_contig_range in dma release path will fill per_cpu_pages with freed pages
> easily so it could drain per_cpu_pages frequently so race which steal page from
> per_cpu_pages is not big, I guess.
> 
> Morever, we could change free_contig_range with batch_free_page which would
> be useful for other cases if they want to free many number of pages
> all at once.
> 
> The bottom line is we need *number and real scenario* for that.

Well yes, quantitative results are always good to have with a patch like
this.

It doesn't actually compile (missing a "}"), which doesn't inspire
confidence.  I'll make the patch go away for now

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