sched/numa: Prevent parallel updates to group stats during placement

Having multiple tasks in a group go through task_numa_placement
simultaneously can lead to a task picking a wrong node to run on, because
the group stats may be in the middle of an update. This patch avoids
parallel updates by holding the numa_group lock during placement
decisions.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1381141781-10992-52-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -1233,6 +1233,7 @@ static void task_numa_placement(struct task_struct *p)
 {
 	int seq, nid, max_nid = -1, max_group_nid = -1;
 	unsigned long max_faults = 0, max_group_faults = 0;
+	spinlock_t *group_lock = NULL;
 
 	seq = ACCESS_ONCE(p->mm->numa_scan_seq);
 	if (p->numa_scan_seq == seq)
@@ -1241,6 +1242,12 @@ static void task_numa_placement(struct task_struct *p)
 	p->numa_migrate_seq++;
 	p->numa_scan_period_max = task_scan_max(p);
 
+	/* If the task is part of a group prevent parallel updates to group stats */
+	if (p->numa_group) {
+		group_lock = &p->numa_group->lock;
+		spin_lock(group_lock);
+	}
+
 	/* Find the node with the highest number of faults */
 	for_each_online_node(nid) {
 		unsigned long faults = 0, group_faults = 0;
@@ -1279,20 +1286,24 @@ static void task_numa_placement(struct task_struct *p)
 		}
 	}
 
-	/*
-	 * If the preferred task and group nids are different,
-	 * iterate over the nodes again to find the best place.
-	 */
-	if (p->numa_group && max_nid != max_group_nid) {
-		unsigned long weight, max_weight = 0;
-
-		for_each_online_node(nid) {
-			weight = task_weight(p, nid) + group_weight(p, nid);
-			if (weight > max_weight) {
-				max_weight = weight;
-				max_nid = nid;
+	if (p->numa_group) {
+		/*
+		 * If the preferred task and group nids are different,
+		 * iterate over the nodes again to find the best place.
+		 */
+		if (max_nid != max_group_nid) {
+			unsigned long weight, max_weight = 0;
+
+			for_each_online_node(nid) {
+				weight = task_weight(p, nid) + group_weight(p, nid);
+				if (weight > max_weight) {
+					max_weight = weight;
+					max_nid = nid;
+				}
 			}
 		}
+
+		spin_unlock(group_lock);
 	}
 
 	/* Preferred node as the node with the most faults */