mm: vmscan: save work scanning (almost) empty LRU lists

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.

Empty LRU lists are quite common with memory cgroups in NUMA environments
because there exists a set of LRU lists for each zone for each memory
cgroup, while the memory of a single cgroup is expected to stay on just
one node.  The number of expected empty LRU lists is thus

  memcgs * (nodes - 1) * lru types

Each attempt to reclaim from an empty LRU list does expensive size
comparisons between lists, acquires the zone's lru lock etc.  Avoid that.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Satoru Moriya <satoru.moriya@hds.com>
Cc: Simon Jeons <simon.jeons@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 68df9c17fbbb49c213f12cf8a0684071000c63c5..8c66486a8ca80cd4cd7e295c905914712d82b1b2 100644 (file)
--- 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 ff842d9a7714b1434d27345a0acbde6d4b9a88e1..e4521ba1ddd047998a062ba92a88f9ba76cc113c 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1761,15 +1761,17 @@ 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 || !vmscan_swappiness(sc)) {
-                       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;
        }
 }