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: Mel 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cd88a86..95ad42d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -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();
 	}