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>

1 files changed, 17 insertions, 2 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);