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 <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Alex Thorlton <athorlton@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 12 insertions, 9 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 41eba21f10ba..4612bb2e3677 100644
--- 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)