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);