linux-kernel - Re: [PATCH 2/5] mm, compaction: simplify deferred compaction

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Date:	Wed, 15 Oct 2014 15:32:12 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Minchan Kim <minchan@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH 2/5] mm, compaction: simplify deferred compaction

On Tue,  7 Oct 2014 17:33:36 +0200 Vlastimil Babka <vbabka@...e.cz> wrote:

> Since commit ("mm, compaction: defer each zone individually instead of
> preferred zone"), compaction is deferred for each zone where sync direct
> compaction fails, and reset where it succeeds. However, it was observed
> that for DMA zone compaction often appeared to succeed while subsequent
> allocation attempt would not, due to different outcome of watermark check.
> In order to properly defer compaction in this zone, the candidate zone has
> to be passed back to __alloc_pages_direct_compact() and compaction deferred
> in the zone after the allocation attempt fails.
> 
> The large source of mismatch between watermark check in compaction and
> allocation was the lack of alloc_flags and classzone_idx values in compaction,
> which has been fixed in the previous patch. So with this problem fixed, we
> can simplify the code by removing the candidate_zone parameter and deferring
> in __alloc_pages_direct_compact().
> 
> After this patch, the compaction activity during stress-highalloc benchmark is
> still somewhat increased, but it's negligible compared to the increase that
> occurred without the better watermark checking. This suggests that it is still
> possible to apparently succeed in compaction but fail to allocate, possibly
> due to parallel allocation activity.
> 
> ...
>
> --- a/include/linux/compaction.h
> +++ b/include/linux/compaction.h
> @@ -33,8 +33,7 @@ extern int fragmentation_index(struct zone *zone, unsigned int order);
>  extern unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  			int order, gfp_t gfp_mask, nodemask_t *mask,
>  			enum migrate_mode mode, int *contended,
> -			int alloc_flags, int classzone_idx,
> -			struct zone **candidate_zone);
> +			int alloc_flags, int classzone_idx);
>  extern void compact_pgdat(pg_data_t *pgdat, int order);
>  extern void reset_isolation_suitable(pg_data_t *pgdat);
>  extern unsigned long compaction_suitable(struct zone *zone, int order,
> @@ -105,8 +104,7 @@ static inline bool compaction_restarting(struct zone *zone, int order)
>  static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
>  			int order, gfp_t gfp_mask, nodemask_t *nodemask,
>  			enum migrate_mode mode, int *contended,
> -			int alloc_flags, int classzone_idx,
> -			struct zone **candidate_zone)
> +			int alloc_flags, int classzone_idx);
>  {
>  	return COMPACT_CONTINUE;
>  }

--- a/include/linux/compaction.h~mm-compaction-simplify-deferred-compaction-fix
+++ a/include/linux/compaction.h
@@ -104,7 +104,7 @@ static inline bool compaction_restarting
 static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,
 			int order, gfp_t gfp_mask, nodemask_t *nodemask,
 			enum migrate_mode mode, int *contended,
-			int alloc_flags, int classzone_idx);
+			int alloc_flags, int classzone_idx)
 {
 	return COMPACT_CONTINUE;
 }

It clearly wasn't tested with this config.  Please do so and let us
know the result?

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