mm, vmscan: remove balance gap

The balance gap was introduced to apply equal pressure to all zones when
reclaiming for a higher zone.  With node-based LRU, the need for the
balance gap is removed and the code is dead so remove it.

[vbabka@suse.cz: Also remove KSWAPD_ZONE_BALANCE_GAP_RATIO]
Link: http://lkml.kernel.org/r/1467970510-21195-9-git-send-email-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2518,7 +2518,7 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc,
  */
 static inline bool compaction_ready(struct zone *zone, int order, int classzone_idx)
 {
-	unsigned long balance_gap, watermark;
+	unsigned long watermark;
 	bool watermark_ok;
 
 	/*
@@ -2527,9 +2527,7 @@ static inline bool compaction_ready(struct zone *zone, int order, int classzone_
 	 * there is a buffer of free pages available to give compaction
 	 * a reasonable chance of completing and allocating the page
 	 */
-	balance_gap = min(low_wmark_pages(zone), DIV_ROUND_UP(
-			zone->managed_pages, KSWAPD_ZONE_BALANCE_GAP_RATIO));
-	watermark = high_wmark_pages(zone) + balance_gap + (2UL << order);
+	watermark = high_wmark_pages(zone) + (2UL << order);
 	watermark_ok = zone_watermark_ok_safe(zone, 0, watermark, classzone_idx);
 
 	/*
@@ -3000,10 +2998,9 @@ static void age_active_anon(struct pglist_data *pgdat,
 	} while (memcg);
 }
 
-static bool zone_balanced(struct zone *zone, int order,
-			unsigned long balance_gap, int classzone_idx)
+static bool zone_balanced(struct zone *zone, int order, int classzone_idx)
 {
-	unsigned long mark = high_wmark_pages(zone) + balance_gap;
+	unsigned long mark = high_wmark_pages(zone);
 
 	return zone_watermark_ok_safe(zone, order, mark, classzone_idx);
 }
@@ -3045,7 +3042,7 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
 		if (!populated_zone(zone))
 			continue;
 
-		if (zone_balanced(zone, order, 0, classzone_idx))
+		if (zone_balanced(zone, order, classzone_idx))
 			return true;
 	}
 
@@ -3148,7 +3145,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 				break;
 			}
 
-			if (!zone_balanced(zone, order, 0, 0)) {
+			if (!zone_balanced(zone, order, 0)) {
 				classzone_idx = i;
 				break;
 			} else {
@@ -3216,7 +3213,7 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
 			if (!populated_zone(zone))
 				continue;
 
-			if (zone_balanced(zone, sc.order, 0, classzone_idx)) {
+			if (zone_balanced(zone, sc.order, classzone_idx)) {
 				clear_bit(PGDAT_CONGESTED, &pgdat->flags);
 				clear_bit(PGDAT_DIRTY, &pgdat->flags);
 				goto out;
@@ -3427,7 +3424,7 @@ void wakeup_kswapd(struct zone *zone, int order, enum zone_type classzone_idx)
 	}
 	if (!waitqueue_active(&pgdat->kswapd_wait))
 		return;
-	if (zone_balanced(zone, order, 0, 0))
+	if (zone_balanced(zone, order, 0))
 		return;
 
 	trace_mm_vmscan_wakeup_kswapd(pgdat->node_id, zone_idx(zone), order);