mm: vmscan: decide whether to compact the pgdat based on reclaim progress

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@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
Cc: dormando <dormando@rydia.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 30 insertions, 29 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 1c10ee512215..cd0980393bac 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2661,7 +2661,8 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
  */
 static bool kswapd_shrink_zone(struct zone *zone,
                                struct scan_control *sc,
-                               unsigned long lru_pages)
+                               unsigned long lru_pages,
+                               unsigned long *nr_attempted)
 {
         unsigned long nr_slab;
         struct reclaim_state *reclaim_state = current->reclaim_state;
@@ -2677,6 +2678,9 @@ static bool kswapd_shrink_zone(struct zone *zone,
         nr_slab = shrink_slab(&shrink, sc->nr_scanned, lru_pages);
         sc->nr_reclaimed += reclaim_state->reclaimed_slab;
 
+        /* Account for the number of pages attempted to reclaim */
+        *nr_attempted += sc->nr_to_reclaim;
+
         if (nr_slab == 0 && !zone_reclaimable(zone))
                 zone->all_unreclaimable = 1;
 
@@ -2724,7 +2728,9 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
 
         do {
                 unsigned long lru_pages = 0;
+                unsigned long nr_attempted = 0;
                 bool raise_priority = true;
+                bool pgdat_needs_compaction = (order > 0);
 
                 sc.nr_reclaimed = 0;
 
@@ -2774,7 +2780,21 @@ 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);
+
+                        /*
+                         * If any zone is currently balanced then kswapd will
+                         * not call compaction as it is expected that the
+                         * necessary pages are already available.
+                         */
+                        if (pgdat_needs_compaction &&
+                                        zone_watermark_ok(zone, order,
+                                                low_wmark_pages(zone),
+                                                *classzone_idx, 0))
+                                pgdat_needs_compaction = false;
                 }
 
                 /*
@@ -2843,7 +2863,8 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                                  * already being scanned that high
                                  * watermark would be met at 100% efficiency.
                                  */
-                                if (kswapd_shrink_zone(zone, &sc, lru_pages))
+                                if (kswapd_shrink_zone(zone, &sc, lru_pages,
+                                                       &nr_attempted))
                                         raise_priority = false;
                         }
 
@@ -2896,6 +2917,13 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                         break;
 
                 /*
+                 * Compact if necessary and kswapd is reclaiming at least the
+                 * high watermark number of pages as requsted
+                 */
+                if (pgdat_needs_compaction && sc.nr_reclaimed > nr_attempted)
+                        compact_pgdat(pgdat, order);
+
+                /*
                  * Raise priority if scanning rate is too low or there was no
                  * progress in reclaiming pages
                  */
@@ -2904,33 +2932,6 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
         } 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()