Commit 4f92e258 authored by Mel Gorman's avatar Mel Gorman Committed by Linus Torvalds
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mm: compaction: defer compaction using an exponential backoff when compaction fails 



The fragmentation index may indicate that a failure is due to external
fragmentation but after a compaction run completes, it is still possible
for an allocation to fail.  There are two obvious reasons as to why

  o Page migration cannot move all pages so fragmentation remains
  o A suitable page may exist but watermarks are not met

In the event of compaction followed by an allocation failure, this patch
defers further compaction in the zone (1 << compact_defer_shift) times.
If the next compaction attempt also fails, compact_defer_shift is
increased up to a maximum of 6.  If compaction succeeds, the defer
counters are reset again.

The zone that is deferred is the first zone in the zonelist - i.e.  the
preferred zone.  To defer compaction in the other zones, the information
would need to be stored in the zonelist or implemented similar to the
zonelist_cache.  This would impact the fast-paths and is not justified at
this time.

Signed-off-by: default avatarMel Gorman <mel@csn.ul.ie>
Cc: Rik van Riel <riel@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 5e771905

include/linux/compaction.h

+39 −0
Original line number Diff line number Diff line
@@ -22,6 +22,36 @@ extern int sysctl_extfrag_handler(struct ctl_table *table, int write, 
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);



/* Do not skip compaction more than 64 times */

#define COMPACT_MAX_DEFER_SHIFT 6



/*

 * Compaction is deferred when compaction fails to result in a page

 * allocation success. 1 << compact_defer_limit compactions are skipped up

 * to a limit of 1 << COMPACT_MAX_DEFER_SHIFT

 */

static inline void defer_compaction(struct zone *zone)

{

	zone->compact_considered = 0;

	zone->compact_defer_shift++;



	if (zone->compact_defer_shift > COMPACT_MAX_DEFER_SHIFT)

		zone->compact_defer_shift = COMPACT_MAX_DEFER_SHIFT;

}



/* Returns true if compaction should be skipped this time */

static inline bool compaction_deferred(struct zone *zone)

{

	unsigned long defer_limit = 1UL << zone->compact_defer_shift;



	/* Avoid possible overflow */

	if (++zone->compact_considered > defer_limit)

		zone->compact_considered = defer_limit;



	return zone->compact_considered < (1UL << zone->compact_defer_shift);

}



#else

static inline unsigned long try_to_compact_pages(struct zonelist *zonelist,

			int order, gfp_t gfp_mask, nodemask_t *nodemask)
@@ -29,6 +59,15 @@ static inline unsigned long try_to_compact_pages(struct zonelist *zonelist, 
	return COMPACT_CONTINUE;

}



static inline void defer_compaction(struct zone *zone)

{

}



static inline bool compaction_deferred(struct zone *zone)

{

	return 1;

}



#endif /* CONFIG_COMPACTION */



#if defined(CONFIG_COMPACTION) && defined(CONFIG_SYSFS) && defined(CONFIG_NUMA)

include/linux/mmzone.h

+9 −0
Original line number Diff line number Diff line
@@ -321,6 +321,15 @@ struct zone { 
	unsigned long		*pageblock_flags;

#endif /* CONFIG_SPARSEMEM */



#ifdef CONFIG_COMPACTION

	/*

	 * On compaction failure, 1<<compact_defer_shift compactions

	 * are skipped before trying again. The number attempted since

	 * last failure is tracked with compact_considered.

	 */

	unsigned int		compact_considered;

	unsigned int		compact_defer_shift;

#endif



	ZONE_PADDING(_pad1_)

mm/page_alloc.c

+4 −1
Original line number Diff line number Diff line
@@ -1769,7 +1769,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, 
{

	struct page *page;



	if (!order)

	if (!order || compaction_deferred(preferred_zone))

		return NULL;



	*did_some_progress = try_to_compact_pages(zonelist, order, gfp_mask,
@@ -1785,6 +1785,8 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, 
				alloc_flags, preferred_zone,

				migratetype);

		if (page) {

			preferred_zone->compact_considered = 0;

			preferred_zone->compact_defer_shift = 0;

			count_vm_event(COMPACTSUCCESS);

			return page;

		}
@@ -1795,6 +1797,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order, 
		 * but not enough to satisfy watermarks.

		 */

		count_vm_event(COMPACTFAIL);

		defer_compaction(preferred_zone);



		cond_resched();

	}

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