sched: pull only one task during NEWIDLE balancing to limit critical …

…section

git-id c4acb2c attempted to limit
newidle critical section length by stopping after at least one task
was moved.  Further investigation has shown that there are other
paths nested further inside the algorithm which still remain that allow
long latencies to occur with newidle balancing.  This patch applies
the same technique inside balance_tasks() to limit the duration of
this optional balancing operation.

Signed-off-by: Gregory Haskins <ghaskins@novell.com>
CC: Nick Piggin <npiggin@suse.de>

kernel/sched.c

@@ -2984,6 +2984,16 @@ balance_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
 	pulled++;
 	rem_load_move -= p->se.load.weight;
 
+#ifdef CONFIG_PREEMPT
+	/*
+	 * NEWIDLE balancing is a source of latency, so preemptible kernels
+	 * will stop after the first task is pulled to minimize the critical
+	 * section.
+	 */
+	if (idle == CPU_NEWLY_IDLE)
+		goto out;
+#endif
+
 	/*
 	 * We only want to steal up to the prescribed amount of weighted load.
 	 */
@@ -3030,9 +3040,15 @@ static int move_tasks(struct rq *this_rq, int this_cpu, struct rq *busiest,
 				sd, idle, all_pinned, &this_best_prio);
 		class = class->next;
 
+#ifdef CONFIG_PREEMPT
+		/*
+		 * NEWIDLE balancing is a source of latency, so preemptible
+		 * kernels will stop after the first task is pulled to minimize
+		 * the critical section.
+		 */
 		if (idle == CPU_NEWLY_IDLE && this_rq->nr_running)
 			break;
-
+#endif
 	} while (class && max_load_move > total_load_moved);
 
 	return total_load_moved > 0;