[PATCH 04/10] mm: vmscan: Decide whether to compact the pgdat based on reclaim progress

[Mel Gorman]

In the past, kswapd makes a decision on whether to compact memory after the
pgdat was considered balanced. This more or less worked but it is late to
make such a decision and does not fit well now that kswapd makes a decision
whether to exit the zone scanning loop depending on reclaim progress.

This patch will compact a pgdat if at least  the requested number of pages
were reclaimed from unbalanced zones for a given priority. If any zone is
currently balanced, kswapd will not call compaction as it is expected the
necessary pages are already available.

Signed-off-by: Mel Gorman <mgorman@xxxxxxx>
---
 mm/vmscan.c | 52 +++++++++++++++++++++-------------------------------
 1 file changed, 21 insertions(+), 31 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 279d0c2..7513bd1 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2694,8 +2694,11 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 
 	do {
 		unsigned long lru_pages = 0;
+		unsigned long nr_to_reclaim = 0;
 		unsigned long nr_reclaimed = sc.nr_reclaimed;
+		unsigned long this_reclaimed;
 		bool raise_priority = true;
+		bool pgdat_needs_compaction = true;
 
 		/*
 		 * Scan in the highmem->dma direction for the highest
@@ -2743,7 +2746,17 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 		for (i = 0; i <= end_zone; i++) {
 			struct zone *zone = pgdat->node_zones + i;
 
+			if (!populated_zone(zone))
+				continue;
+
 			lru_pages += zone_reclaimable_pages(zone);
+
+			/* Check if the memory needs to be defragmented */
+			if (order && pgdat_needs_compaction &&
+					zone_watermark_ok(zone, order,
+						low_wmark_pages(zone),
+						*classzone_idx, 0))
+				pgdat_needs_compaction = false;
 		}
 
 		/*
@@ -2814,6 +2827,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 				 */
 				if (kswapd_shrink_zone(zone, &sc, lru_pages))
 					raise_priority = false;
+
+				nr_to_reclaim += sc.nr_to_reclaim;
 			}
 
 			/*
@@ -2864,46 +2879,21 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 		if (try_to_freeze() || kthread_should_stop())
 			break;
 
-		/* If no reclaim progress then increase scanning priority */
-		if (sc.nr_reclaimed - nr_reclaimed == 0)
-			raise_priority = true;
+		/* Compact if necessary and kswapd is reclaiming efficiently */
+		this_reclaimed = sc.nr_reclaimed - nr_reclaimed;
+		if (order && pgdat_needs_compaction &&
+				this_reclaimed > nr_to_reclaim)
+			compact_pgdat(pgdat, order);
 
 		/*
 		 * Raise priority if scanning rate is too low or there was no
 		 * progress in reclaiming pages
 		 */
-		if (raise_priority || sc.nr_reclaimed - nr_reclaimed == 0)
+		if (raise_priority || !this_reclaimed)
 			sc.priority--;
 	} while (sc.priority >= 0 &&
 		 !pgdat_balanced(pgdat, order, *classzone_idx));
 
-	/*
-	 * If kswapd was reclaiming at a higher order, it has the option of
-	 * sleeping without all zones being balanced. Before it does, it must
-	 * ensure that the watermarks for order-0 on *all* zones are met and
-	 * that the congestion flags are cleared. The congestion flag must
-	 * be cleared as kswapd is the only mechanism that clears the flag
-	 * and it is potentially going to sleep here.
-	 */
-	if (order) {
-		int zones_need_compaction = 1;
-
-		for (i = 0; i <= end_zone; i++) {
-			struct zone *zone = pgdat->node_zones + i;
-
-			if (!populated_zone(zone))
-				continue;
-
-			/* Check if the memory needs to be defragmented. */
-			if (zone_watermark_ok(zone, order,
-				    low_wmark_pages(zone), *classzone_idx, 0))
-				zones_need_compaction = 0;
-		}
-
-		if (zones_need_compaction)
-			compact_pgdat(pgdat, order);
-	}
-
 out:
 	/*
 	 * Return the order we were reclaiming at so prepare_kswapd_sleep()
-- 
1.8.1.4

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