sched/numa: Fix placement of workloads spread across multiple nodes

The load balancer will spread workloads across multiple NUMA nodes,
in order to balance the load on the system. This means that sometimes
a task's preferred node has available capacity, but moving the task
there will not succeed, because that would create too large an imbalance.

In that case, other NUMA nodes need to be considered.

Signed-off-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
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/1381141781-10992-42-git-send-email-mgorman@suse.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/sched/fair.c

@@ -1104,13 +1104,12 @@ static int task_numa_migrate(struct task_struct *p)
 	imp = task_faults(env.p, env.dst_nid) - faults;
 	update_numa_stats(&env.dst_stats, env.dst_nid);
 
-	/*
-	 * If the preferred nid has capacity then use it. Otherwise find an
-	 * alternative node with relatively better statistics.
-	 */
-	if (env.dst_stats.has_capacity) {
+	/* If the preferred nid has capacity, try to use it. */
+	if (env.dst_stats.has_capacity)
 		task_numa_find_cpu(&env, imp);
-	} else {
+
+	/* No space available on the preferred nid. Look elsewhere. */
+	if (env.best_cpu == -1) {
 		for_each_online_node(nid) {
 			if (nid == env.src_nid || nid == p->numa_preferred_nid)
 				continue;