mm: numa: limit scope of lock for NUMA migrate rate limiting

NUMA migrate rate limiting protects a migration counter and window using a lock but in some cases this can be a contended lock. It is not critical that the number of pages be perfect, lost updates are acceptable. Reduce the importance of this lock. Signed-off-by: Mel Gorman <[email protected]> Reviewed-by: Rik van Riel <[email protected]> Cc: Alex Thorlton <[email protected]> Signed-off-by: Andrew Morton <[email protected]> Signed-off-by: Linus Torvalds <[email protected]>
chewitt · Jan 22, 2014 · 1c5e9c2 · 1c5e9c2
1 parent 1c30e01
commit 1c5e9c2
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Showing 2 changed files with 13 additions and 13 deletions.
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
@@ -764,10 +764,7 @@ typedef struct pglist_data {
 	int kswapd_max_order;
 	enum zone_type classzone_idx;
 #ifdef CONFIG_NUMA_BALANCING
-	/*
-	 * Lock serializing the per destination node AutoNUMA memory
-	 * migration rate limiting data.
-	 */
+	/* Lock serializing the migrate rate limiting window */
 	spinlock_t numabalancing_migrate_lock;
 
 	/* Rate limiting time interval */

diff --git a/mm/migrate.c b/mm/migrate.c
@@ -1602,26 +1602,29 @@ bool migrate_ratelimited(int node)
 static bool numamigrate_update_ratelimit(pg_data_t *pgdat,
 					unsigned long nr_pages)
 {
-	bool rate_limited = false;
-
 	/*
 	 * 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!
 	 */
-	spin_lock(&pgdat->numabalancing_migrate_lock);
 	if (time_after(jiffies, pgdat->numabalancing_migrate_next_window)) {
+		spin_lock(&pgdat->numabalancing_migrate_lock);
 		pgdat->numabalancing_migrate_nr_pages = 0;
 		pgdat->numabalancing_migrate_next_window = jiffies +
 			msecs_to_jiffies(migrate_interval_millisecs);
+		spin_unlock(&pgdat->numabalancing_migrate_lock);
 	}
 	if (pgdat->numabalancing_migrate_nr_pages > ratelimit_pages)
-		rate_limited = true;
-	else
-		pgdat->numabalancing_migrate_nr_pages += nr_pages;
-	spin_unlock(&pgdat->numabalancing_migrate_lock);
-
-	return rate_limited;
+		return true;
+
+	/*
+	 * This is an unlocked non-atomic update so errors are possible.
+	 * The consequences are failing to migrate when we potentiall should
+	 * have which is not severe enough to warrant locking. If it is ever
+	 * a problem, it can be converted to a per-cpu counter.
+	 */
+	pgdat->numabalancing_migrate_nr_pages += nr_pages;
+	return false;
 }
 
 static int numamigrate_isolate_page(pg_data_t *pgdat, struct page *page)