mm: vmstat: use a single setter function and callback for adjusting percpu thresholds

reduce_pgdat_percpu_threshold() and restore_pgdat_percpu_threshold() exist
to adjust the per-cpu vmstat thresholds while kswapd is awake to avoid
errors due to counter drift.  The functions duplicate some code so this
patch replaces them with a single set_pgdat_percpu_threshold() that takes
a callback function to calculate the desired threshold as a parameter.

[akpm@linux-foundation.org: readability tweak]
[kosaki.motohiro@jp.fujitsu.com: set_pgdat_percpu_threshold(): don't use for_each_online_cpu]
Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: Christoph Lameter <cl@linux.com>
Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 24 insertions, 31 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 5da4295e7d67..86f8c3418795 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2448,9 +2448,24 @@ static int kswapd(void *p)
                                  */
                                 if (!sleeping_prematurely(pgdat, order, remaining)) {
                                         trace_mm_vmscan_kswapd_sleep(pgdat->node_id);
-                                        restore_pgdat_percpu_threshold(pgdat);
+
+                                        /*
+                                         * vmstat counters are not perfectly
+                                         * accurate and the estimated value
+                                         * for counters such as NR_FREE_PAGES
+                                         * can deviate from the true value by
+                                         * nr_online_cpus * threshold. To
+                                         * avoid the zone watermarks being
+                                         * breached while under pressure, we
+                                         * reduce the per-cpu vmstat threshold
+                                         * while kswapd is awake and restore
+                                         * them before going back to sleep.
+                                         */
+                                        set_pgdat_percpu_threshold(pgdat,
+                                                calculate_normal_threshold);
                                         schedule();
-                                        reduce_pgdat_percpu_threshold(pgdat);
+                                        set_pgdat_percpu_threshold(pgdat,
+                                                calculate_pressure_threshold);
                                 } else {
                                         if (remaining)
                                                 count_vm_event(KSWAPD_LOW_WMARK_HIT_QUICKLY);
diff --git a/mm/vmstat.c b/mm/vmstat.c
index bc0f095791b4..751a65e00aac 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -83,7 +83,7 @@ EXPORT_SYMBOL(vm_stat);
 
 #ifdef CONFIG_SMP
 
-static int calculate_pressure_threshold(struct zone *zone)
+int calculate_pressure_threshold(struct zone *zone)
 {
         int threshold;
         int watermark_distance;
@@ -107,7 +107,7 @@ static int calculate_pressure_threshold(struct zone *zone)
         return threshold;
 }
 
-static int calculate_threshold(struct zone *zone)
+int calculate_normal_threshold(struct zone *zone)
 {
         int threshold;
         int mem;        /* memory in 128 MB units */
@@ -166,7 +166,7 @@ static void refresh_zone_stat_thresholds(void)
         for_each_populated_zone(zone) {
                 unsigned long max_drift, tolerate_drift;
 
-                threshold = calculate_threshold(zone);
+                threshold = calculate_normal_threshold(zone);
 
                 for_each_online_cpu(cpu)
                         per_cpu_ptr(zone->pageset, cpu)->stat_threshold
@@ -185,46 +185,24 @@ static void refresh_zone_stat_thresholds(void)
         }
 }
 
-void reduce_pgdat_percpu_threshold(pg_data_t *pgdat)
+void set_pgdat_percpu_threshold(pg_data_t *pgdat,
+                                int (*calculate_pressure)(struct zone *))
 {
         struct zone *zone;
         int cpu;
         int threshold;
         int i;
 
-        get_online_cpus();
-        for (i = 0; i < pgdat->nr_zones; i++) {
-                zone = &pgdat->node_zones[i];
-                if (!zone->percpu_drift_mark)
-                        continue;
-
-                threshold = calculate_pressure_threshold(zone);
-                for_each_online_cpu(cpu)
-                        per_cpu_ptr(zone->pageset, cpu)->stat_threshold
-                                                        = threshold;
-        }
-        put_online_cpus();
-}
-
-void restore_pgdat_percpu_threshold(pg_data_t *pgdat)
-{
-        struct zone *zone;
-        int cpu;
-        int threshold;
-        int i;
-
-        get_online_cpus();
         for (i = 0; i < pgdat->nr_zones; i++) {
                 zone = &pgdat->node_zones[i];
                 if (!zone->percpu_drift_mark)
                         continue;
 
-                threshold = calculate_threshold(zone);
-                for_each_online_cpu(cpu)
+                threshold = (*calculate_pressure)(zone);
+                for_each_possible_cpu(cpu)
                         per_cpu_ptr(zone->pageset, cpu)->stat_threshold
                                                         = threshold;
         }
-        put_online_cpus();
 }
 
 /*