mm: avoid calling pgdat_balanced() needlessly

Now that balance_pgdat() is slightly tidied up, thanks to more capable
pgdat_balanced(), it's become obvious that pgdat_balanced() is called to
check the status, then break the loop if pgdat is balanced, just to be
immediately called again.  The second call is completely unnecessary, of
course.

The patch introduces pgdat_is_balanced boolean, which helps resolve the
above suboptimal behavior, with the added benefit of slightly better
documenting one other place in the function where we jump and skip lots
of code.
Signed-off-by: Zlatko Calusic <zlatko.calusic@iskon.hr>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2616,6 +2616,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
 static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 							int *classzone_idx)
 {
+	bool pgdat_is_balanced = false;
 	struct zone *unbalanced_zone;
 	int i;
 	int end_zone = 0;	/* Inclusive.  0 = ZONE_DMA */
@@ -2690,8 +2691,11 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 				zone_clear_flag(zone, ZONE_CONGESTED);
 			}
 		}
-		if (i < 0)
+
+		if (i < 0) {
+			pgdat_is_balanced = true;
 			goto out;
+		}
 
 		for (i = 0; i <= end_zone; i++) {
 			struct zone *zone = pgdat->node_zones + i;
@@ -2818,8 +2822,11 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 				pfmemalloc_watermark_ok(pgdat))
 			wake_up(&pgdat->pfmemalloc_wait);
 
-		if (pgdat_balanced(pgdat, order, *classzone_idx))
+		if (pgdat_balanced(pgdat, order, *classzone_idx)) {
+			pgdat_is_balanced = true;
 			break;		/* kswapd: all done */
+		}
+
 		/*
 		 * OK, kswapd is getting into trouble.  Take a nap, then take
 		 * another pass across the zones.
@@ -2840,9 +2847,9 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 		if (sc.nr_reclaimed >= SWAP_CLUSTER_MAX)
 			break;
 	} while (--sc.priority >= 0);
-out:
 
-	if (!pgdat_balanced(pgdat, order, *classzone_idx)) {
+out:
+	if (!pgdat_is_balanced) {
 		cond_resched();
 
 		try_to_freeze();