sched: Fix sign under-flows in wake_affine

While care is taken around the zero-point in effective_load to not exceed
the instantaneous rq->weight, it's still possible (e.g. using wake_idx != 0)
for (load + effective_load) to underflow.

In this case the comparing the unsigned values can result in incorrect balanced
decisions.

Signed-off-by: Paul Turner <pjt@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20110122044851.734245014@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_fair.c

@@ -1432,7 +1432,7 @@ static inline unsigned long effective_load(struct task_group *tg, int cpu,
 
 static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
 {
-	unsigned long this_load, load;
+	s64 this_load, load;
 	int idx, this_cpu, prev_cpu;
 	unsigned long tl_per_task;
 	struct task_group *tg;
@@ -1471,8 +1471,8 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
 	 * Otherwise check if either cpus are near enough in load to allow this
 	 * task to be woken on this_cpu.
 	 */
-	if (this_load) {
-		unsigned long this_eff_load, prev_eff_load;
+	if (this_load > 0) {
+		s64 this_eff_load, prev_eff_load;
 
 		this_eff_load = 100;
 		this_eff_load *= power_of(prev_cpu);