[PATCH]vmscan: add block plug for page reclaim

[Shaohua Li <shaohua.li@intel.com>]

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>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5ed24b9..8ec04b2 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1933,12 +1933,14 @@ 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) {
@@ -1962,6 +1964,7 @@ restart:
 		if (nr_reclaimed >= nr_to_reclaim && priority < DEF_PRIORITY)
 			break;
 	}
+	blk_finish_plug(&plug);
 	sc->nr_reclaimed += nr_reclaimed;
 
 	/*


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