mm, page_alloc: reduce cost of fair zone allocation policy retry

The fair zone allocation policy is not without cost but it can be
reduced slightly.  This patch removes an unnecessary local variable,
checks the likely conditions of the fair zone policy first, uses a bool
instead of a flags check and falls through when a remote node is
encountered instead of doing a full restart.  The benefit is marginal
but it's there

                                             4.6.0-rc2                  4.6.0-rc2
                                         decstat-v1r20              optfair-v1r20
  Min      alloc-odr0-1               377.00 (  0.00%)           380.00 ( -0.80%)
  Min      alloc-odr0-2               273.00 (  0.00%)           273.00 (  0.00%)
  Min      alloc-odr0-4               226.00 (  0.00%)           227.00 ( -0.44%)
  Min      alloc-odr0-8               196.00 (  0.00%)           196.00 (  0.00%)
  Min      alloc-odr0-16              183.00 (  0.00%)           183.00 (  0.00%)
  Min      alloc-odr0-32              175.00 (  0.00%)           173.00 (  1.14%)
  Min      alloc-odr0-64              172.00 (  0.00%)           169.00 (  1.74%)
  Min      alloc-odr0-128             170.00 (  0.00%)           169.00 (  0.59%)
  Min      alloc-odr0-256             183.00 (  0.00%)           180.00 (  1.64%)
  Min      alloc-odr0-512             191.00 (  0.00%)           190.00 (  0.52%)
  Min      alloc-odr0-1024            199.00 (  0.00%)           198.00 (  0.50%)
  Min      alloc-odr0-2048            204.00 (  0.00%)           204.00 (  0.00%)
  Min      alloc-odr0-4096            210.00 (  0.00%)           209.00 (  0.48%)
  Min      alloc-odr0-8192            213.00 (  0.00%)           213.00 (  0.00%)
  Min      alloc-odr0-16384           214.00 (  0.00%)           214.00 (  0.00%)

The benefit is marginal at best but one of the most important benefits,
avoiding a second search when falling back to another node is not
triggered by this particular test so the benefit for some corner cases
is understated.

Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -2680,12 +2680,10 @@ get_page_from_freelist(gfp_t gfp_mask, unsigned int order, int alloc_flags,
 {
 	struct zoneref *z;
 	struct zone *zone;
-	bool fair_skipped;
-	bool zonelist_rescan;
+	bool fair_skipped = false;
+	bool apply_fair = (alloc_flags & ALLOC_FAIR);
 
 zonelist_scan:
-	zonelist_rescan = false;
-
 	/*
 	 * Scan zonelist, looking for a zone with enough free.
 	 * See also __cpuset_node_allowed() comment in kernel/cpuset.c.
@@ -2705,13 +2703,16 @@ zonelist_scan:
 		 * page was allocated in should have no effect on the
 		 * time the page has in memory before being reclaimed.
 		 */
-		if (alloc_flags & ALLOC_FAIR) {
-			if (!zone_local(ac->preferred_zone, zone))
-				break;
+		if (apply_fair) {
 			if (test_bit(ZONE_FAIR_DEPLETED, &zone->flags)) {
 				fair_skipped = true;
 				continue;
 			}
+			if (!zone_local(ac->preferred_zone, zone)) {
+				if (fair_skipped)
+					goto reset_fair;
+				apply_fair = false;
+			}
 		}
 		/*
 		 * When allocating a page cache page for writing, we
@@ -2800,18 +2801,13 @@ try_this_zone:
 	 * include remote zones now, before entering the slowpath and waking
 	 * kswapd: prefer spilling to a remote zone over swapping locally.
 	 */
-	if (alloc_flags & ALLOC_FAIR) {
-		alloc_flags &= ~ALLOC_FAIR;
-		if (fair_skipped) {
-			zonelist_rescan = true;
-			reset_alloc_batches(ac->preferred_zone);
-		}
-		if (nr_online_nodes > 1)
-			zonelist_rescan = true;
-	}
-
-	if (zonelist_rescan)
+	if (fair_skipped) {
+reset_fair:
+		apply_fair = false;
+		fair_skipped = false;
+		reset_alloc_batches(ac->preferred_zone);
 		goto zonelist_scan;
+	}
 
 	return NULL;
 }