[PATCH 2/6] mm/migrate: Use trylock while resetting rate limit

[Srikar Dronamraju]

Since this spinlock will only serialize migrate rate limiting,
convert the spinlock to a trylock. If another task races ahead of this task
then this task can simply move on.

While here, add correct two abnormalities.
- Avoid time being stretched for every interval.
- Use READ/WRITE_ONCE with next window.

specjbb2005 / bops/JVM / higher bops are better
on 2 Socket/2 Node Intel
JVMS  Prev    Current  %Change
4     206350  200892   -2.64502
1     319963  325766   1.81365


on 2 Socket/2 Node Power9 (PowerNV)
JVMS  Prev    Current  %Change
4     186539  190261   1.99529
1     220344  195305   -11.3636


on 4 Socket/4 Node Power7
JVMS  Prev    Current  %Change
8     56836   57651.1  1.43413
1     112970  111351   -1.43312


dbench / transactions / higher numbers are better
on 2 Socket/2 Node Intel
count  Min      Max      Avg      Variance  %Change
5      13136.1  13170.2  13150.2  14.7482
5      12254.7  12331.9  12297.8  28.1846   -6.48203


on 2 Socket/4 Node Power8 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      4319.79  4998.19  4836.53  261.109
5      4997.83  5030.14  5015.54  12.947    3.70121


on 2 Socket/2 Node Power9 (PowerNV)
count  Min      Max      Avg      Variance  %Change
5      9325.56  9402.7   9362.49  25.9638
5      9331.84  9375.11  9352.04  16.0703   -0.111616


on 4 Socket/4 Node Power7
count  Min      Max      Avg      Variance  %Change
5      132.581  191.072  170.554  21.6444
5      147.55   181.605  168.963  11.3513   -0.932842

Signed-off-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
---
Changelog v1->v2:
Fix stretch every interval pointed by Peter Zijlstra.
Verified that some of the regression is due to fixing interval stretch.

 mm/migrate.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 8c0af0f..dbc2cb7 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1868,16 +1868,24 @@ static struct page *alloc_misplaced_dst_page(struct page *page,
 static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
 					unsigned long nr_pages)
 {
+	unsigned long next_window, interval;
+
+	next_window = READ_ONCE(pgdat->numabalancing_migrate_next_window);
+	interval = msecs_to_jiffies(migrate_interval_millisecs);
+
 	/*
 	 * Rate-limit the amount of data that is being migrated to a node.
 	 * Optimal placement is no good if the memory bus is saturated and
 	 * all the time is being spent migrating!
 	 */
-	if (time_after(jiffies, next_window)) {
-		spin_lock(&pgdat->numabalancing_migrate_lock);
+	if (time_after(jiffies, next_window) &&
+			spin_trylock(&pgdat->numabalancing_migrate_lock)) {
 		pgdat->numabalancing_migrate_nr_pages = 0;
-		pgdat->numabalancing_migrate_next_window = jiffies +
-			msecs_to_jiffies(migrate_interval_millisecs);
+		do {
+			next_window += interval;
+		} while (unlikely(time_after(jiffies, next_window)));
+
+		WRITE_ONCE(pgdat->numabalancing_migrate_next_window, next_window);
 		spin_unlock(&pgdat->numabalancing_migrate_lock);
 	}
 	if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages) {
-- 
1.8.3.1