[patch 3/8] mm: vmscan: save work scanning (almost) empty LRU lists

[Johannes Weiner <hannes@cmpxchg.org>]

In certain cases (kswapd reclaim, memcg target reclaim), a fixed
minimum amount of pages is scanned from the LRU lists on each
iteration, to make progress.

Do not make this minimum bigger than the respective LRU list size,
however, and save some busy work trying to isolate and reclaim pages
that are not there.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---
 include/linux/swap.h |  2 +-
 mm/vmscan.c          | 10 ++++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 68df9c1..8c66486 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -156,7 +156,7 @@ enum {
 	SWP_SCANNING	= (1 << 8),	/* refcount in scan_swap_map */
 };
 
-#define SWAP_CLUSTER_MAX 32
+#define SWAP_CLUSTER_MAX 32UL
 #define COMPACT_CLUSTER_MAX SWAP_CLUSTER_MAX
 
 /*
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 6e53446..1763e79 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1748,15 +1748,17 @@ static void get_scan_count(struct lruvec *lruvec, struct scan_control *sc,
 out:
 	for_each_evictable_lru(lru) {
 		int file = is_file_lru(lru);
+		unsigned long size;
 		unsigned long scan;
 
-		scan = get_lru_size(lruvec, lru);
+		size = get_lru_size(lruvec, lru);
 		if (sc->priority || noswap) {
-			scan >>= sc->priority;
+			scan = size >> sc->priority;
 			if (!scan && force_scan)
-				scan = SWAP_CLUSTER_MAX;
+				scan = min(size, SWAP_CLUSTER_MAX);
 			scan = div64_u64(scan * fraction[file], denominator);
-		}
+		} else
+			scan = size;
 		nr[lru] = scan;
 	}
 }
-- 
1.7.11.7

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