vmscan: add block plug for page reclaim

per-task block plug can reduce block queue lock contention and increase
request merge.  Currently page reclaim doesn't support it.  I originally
thought page reclaim doesn't need it, because kswapd thread count is
limited and file cache write is done at flusher mostly.

When I test a workload with heavy swap in a 4-node machine, each CPU is
doing direct page reclaim and swap.  This causes block queue lock
contention.  In my test, without below patch, the CPU utilization is about
2% ~ 7%.  With the patch, the CPU utilization is about 1% ~ 3%.  Disk
throughput isn't changed.  This should improve normal kswapd write and
file cache write too (increase request merge for example), but might not
be so obvious as I explain above.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b68a9342d5a3..b1520b077858 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2005,12 +2005,14 @@ static void shrink_zone(int priority, struct zone *zone,
         enum lru_list l;
         unsigned long nr_reclaimed, nr_scanned;
         unsigned long nr_to_reclaim = sc->nr_to_reclaim;
+        struct blk_plug plug;
 
 restart:
         nr_reclaimed = 0;
         nr_scanned = sc->nr_scanned;
         get_scan_count(zone, sc, nr, priority);
 
+        blk_start_plug(&plug);
         while (nr[LRU_INACTIVE_ANON] || nr[LRU_ACTIVE_FILE] ||
                                         nr[LRU_INACTIVE_FILE]) {
                 for_each_evictable_lru(l) {
@@ -2034,6 +2036,7 @@ restart:
                 if (nr_reclaimed >= nr_to_reclaim && priority < DEF_PRIORITY)
                         break;
         }
+        blk_finish_plug(&plug);
         sc->nr_reclaimed += nr_reclaimed;
 
         /*