mm: vmscan: move logic from balance_pgdat() to kswapd_shrink_zone()

balance_pgdat() is very long and some of the logic can and should be
internal to kswapd_shrink_zone().  Move it so the flow of
balance_pgdat() is marginally easier to follow.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Rik van Riel <riel@redhat.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.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, 54 insertions, 56 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index a2d0c6842616..4a43c289b23a 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2709,18 +2709,53 @@ static bool prepare_kswapd_sleep(pg_data_t *pgdat, int order, long remaining,
  * This is used to determine if the scanning priority needs to be raised.
  */
 static bool kswapd_shrink_zone(struct zone *zone,
+                               int classzone_idx,
                                struct scan_control *sc,
                                unsigned long lru_pages,
                                unsigned long *nr_attempted)
 {
         unsigned long nr_slab;
+        int testorder = sc->order;
+        unsigned long balance_gap;
         struct reclaim_state *reclaim_state = current->reclaim_state;
         struct shrink_control shrink = {
                 .gfp_mask = sc->gfp_mask,
         };
+        bool lowmem_pressure;
 
         /* Reclaim above the high watermark. */
         sc->nr_to_reclaim = max(SWAP_CLUSTER_MAX, high_wmark_pages(zone));
+
+        /*
+         * Kswapd reclaims only single pages with compaction enabled. Trying
+         * too hard to reclaim until contiguous free pages have become
+         * available can hurt performance by evicting too much useful data
+         * from memory. Do not reclaim more than needed for compaction.
+         */
+        if (IS_ENABLED(CONFIG_COMPACTION) && sc->order &&
+                        compaction_suitable(zone, sc->order) !=
+                                COMPACT_SKIPPED)
+                testorder = 0;
+
+        /*
+         * We put equal pressure on every zone, unless one zone has way too
+         * many pages free already. The "too many pages" is defined as the
+         * high wmark plus a "gap" where the gap is either the low
+         * watermark or 1% of the zone, whichever is smaller.
+         */
+        balance_gap = min(low_wmark_pages(zone),
+                (zone->managed_pages + KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
+                KSWAPD_ZONE_BALANCE_GAP_RATIO);
+
+        /*
+         * If there is no low memory pressure or the zone is balanced then no
+         * reclaim is necessary
+         */
+        lowmem_pressure = (buffer_heads_over_limit && is_highmem(zone));
+        if (!lowmem_pressure && zone_balanced(zone, testorder,
+                                                balance_gap, classzone_idx))
+                return true;
+
         shrink_zone(zone, sc);
 
         reclaim_state->reclaimed_slab = 0;
@@ -2735,6 +2770,18 @@ static bool kswapd_shrink_zone(struct zone *zone,
 
         zone_clear_flag(zone, ZONE_WRITEBACK);
 
+        /*
+         * If a zone reaches its high watermark, consider it to be no longer
+         * congested. It's possible there are dirty pages backed by congested
+         * BDIs but as pressure is relieved, speculatively avoid congestion
+         * waits.
+         */
+        if (!zone->all_unreclaimable &&
+            zone_balanced(zone, testorder, 0, classzone_idx)) {
+                zone_clear_flag(zone, ZONE_CONGESTED);
+                zone_clear_flag(zone, ZONE_TAIL_LRU_DIRTY);
+        }
+
         return sc->nr_scanned >= sc->nr_to_reclaim;
 }
 
@@ -2870,8 +2917,6 @@ 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;
-                        int testorder;
-                        unsigned long balance_gap;
 
                         if (!populated_zone(zone))
                                 continue;
@@ -2892,61 +2937,14 @@ static unsigned long balance_pgdat(pg_data_t *pgdat, int order,
                         sc.nr_reclaimed += nr_soft_reclaimed;
 
                         /*
-                         * We put equal pressure on every zone, unless
-                         * one zone has way too many pages free
-                         * already. The "too many pages" is defined
-                         * as the high wmark plus a "gap" where the
-                         * gap is either the low watermark or 1%
-                         * of the zone, whichever is smaller.
-                         */
-                        balance_gap = min(low_wmark_pages(zone),
-                                (zone->managed_pages +
-                                        KSWAPD_ZONE_BALANCE_GAP_RATIO-1) /
-                                KSWAPD_ZONE_BALANCE_GAP_RATIO);
-                        /*
-                         * Kswapd reclaims only single pages with compaction
-                         * enabled. Trying too hard to reclaim until contiguous
-                         * free pages have become available can hurt performance
-                         * by evicting too much useful data from memory.
-                         * Do not reclaim more than needed for compaction.
+                         * There should be no need to raise the scanning
+                         * priority if enough pages are already being scanned
+                         * that that high watermark would be met at 100%
+                         * efficiency.
                          */
-                        testorder = order;
-                        if (IS_ENABLED(CONFIG_COMPACTION) && order &&
-                                        compaction_suitable(zone, order) !=
-                                                COMPACT_SKIPPED)
-                                testorder = 0;
-
-                        if ((buffer_heads_over_limit && is_highmem_idx(i)) ||
-                            !zone_balanced(zone, testorder,
-                                           balance_gap, end_zone)) {
-                                /*
-                                 * There should be no need to raise the
-                                 * scanning priority if enough pages are
-                                 * already being scanned that high
-                                 * watermark would be met at 100% efficiency.
-                                 */
-                                if (kswapd_shrink_zone(zone, &sc, lru_pages,
-                                                       &nr_attempted))
-                                        raise_priority = false;
-                        }
-
-                        if (zone->all_unreclaimable) {
-                                if (end_zone && end_zone == i)
-                                        end_zone--;
-                                continue;
-                        }
-
-                        if (zone_balanced(zone, testorder, 0, end_zone))
-                                /*
-                                 * If a zone reaches its high watermark,
-                                 * consider it to be no longer congested. It's
-                                 * possible there are dirty pages backed by
-                                 * congested BDIs but as pressure is relieved,
-                                 * speculatively avoid congestion waits
-                                 * or writing pages from kswapd context.
-                                 */
-                                zone_clear_flag(zone, ZONE_CONGESTED);
-                                zone_clear_flag(zone, ZONE_TAIL_LRU_DIRTY);
+                        if (kswapd_shrink_zone(zone, end_zone, &sc,
+                                        lru_pages, &nr_attempted))
+                                raise_priority = false;
                 }
 
                 /*