mm: move zone->pages_scanned into a vmstat counter

zone->pages_scanned is a write-intensive cache line during page reclaim
and it's also updated during page free.  Move the counter into vmstat to
take advantage of the per-cpu updates and do not update it in the free
paths unless necessary.

On a small UMA machine running tiobench the difference is marginal.  On
a 4-node machine the overhead is more noticable.  Note that automatic
NUMA balancing was disabled for this test as otherwise the system CPU
overhead is unpredictable.

          3.16.0-rc3  3.16.0-rc3  3.16.0-rc3
             vanillarearrange-v5   vmstat-v5
User          746.94      759.78      774.56
System      65336.22    58350.98    32847.27
Elapsed     27553.52    27282.02    27415.04

Note that the overhead reduction will vary depending on where exactly
pages are allocated and freed.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 15 insertions, 7 deletions

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b7381d11f021..daa016063793 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -680,9 +680,12 @@ static void free_pcppages_bulk(struct zone *zone, int count,
         int migratetype = 0;
         int batch_free = 0;
         int to_free = count;
+        unsigned long nr_scanned;
 
         spin_lock(&zone->lock);
-        zone->pages_scanned = 0;
+        nr_scanned = zone_page_state(zone, NR_PAGES_SCANNED);
+        if (nr_scanned)
+                __mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);
 
         while (to_free) {
                 struct page *page;
@@ -731,8 +734,11 @@ static void free_one_page(struct zone *zone,
                                 unsigned int order,
                                 int migratetype)
 {
+        unsigned long nr_scanned;
         spin_lock(&zone->lock);
-        zone->pages_scanned = 0;
+        nr_scanned = zone_page_state(zone, NR_PAGES_SCANNED);
+        if (nr_scanned)
+                __mod_zone_page_state(zone, NR_PAGES_SCANNED, -nr_scanned);
 
         __free_one_page(page, pfn, zone, order, migratetype);
         if (unlikely(!is_migrate_isolate(migratetype)))
@@ -3248,7 +3254,7 @@ void show_free_areas(unsigned int filter)
                         K(zone_page_state(zone, NR_BOUNCE)),
                         K(zone_page_state(zone, NR_FREE_CMA_PAGES)),
                         K(zone_page_state(zone, NR_WRITEBACK_TEMP)),
-                        zone->pages_scanned,
+                        K(zone_page_state(zone, NR_PAGES_SCANNED)),
                         (!zone_reclaimable(zone) ? "yes" : "no")
                         );
                 printk("lowmem_reserve[]:");
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5fec1ba9951f..9c8222b499b4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -174,7 +174,8 @@ static unsigned long zone_reclaimable_pages(struct zone *zone)
 
 bool zone_reclaimable(struct zone *zone)
 {
-        return zone->pages_scanned < zone_reclaimable_pages(zone) * 6;
+        return zone_page_state(zone, NR_PAGES_SCANNED) <
+                zone_reclaimable_pages(zone) * 6;
 }
 
 static unsigned long get_lru_size(struct lruvec *lruvec, enum lru_list lru)
@@ -1508,7 +1509,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
         __mod_zone_page_state(zone, NR_ISOLATED_ANON + file, nr_taken);
 
         if (global_reclaim(sc)) {
-                zone->pages_scanned += nr_scanned;
+                __mod_zone_page_state(zone, NR_PAGES_SCANNED, nr_scanned);
                 if (current_is_kswapd())
                         __count_zone_vm_events(PGSCAN_KSWAPD, zone, nr_scanned);
                 else
@@ -1698,7 +1699,7 @@ static void shrink_active_list(unsigned long nr_to_scan,
         nr_taken = isolate_lru_pages(nr_to_scan, lruvec, &l_hold,
                                      &nr_scanned, sc, isolate_mode, lru);
         if (global_reclaim(sc))
-                zone->pages_scanned += nr_scanned;
+                __mod_zone_page_state(zone, NR_PAGES_SCANNED, nr_scanned);
 
         reclaim_stat->recent_scanned[file] += nr_taken;
 
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 8267f77d1875..e574e883fa70 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -763,6 +763,7 @@ const char * const vmstat_text[] = {
         "nr_shmem",
         "nr_dirtied",
         "nr_written",
+        "nr_pages_scanned",
 
 #ifdef CONFIG_NUMA
         "numa_hit",
@@ -1067,7 +1068,7 @@ static void zoneinfo_show_print(struct seq_file *m, pg_data_t *pgdat,
                    min_wmark_pages(zone),
                    low_wmark_pages(zone),
                    high_wmark_pages(zone),
-                   zone->pages_scanned,
+                   zone_page_state(zone, NR_PAGES_SCANNED),
                    zone->spanned_pages,
                    zone->present_pages,
                    zone->managed_pages);