sched: fix newly idle load balance in case of SMT

In the presence of SMT, newly idle balance was never happening for multi-core and SMP domains(even when both the logical siblings are idle).

If thread 0 is already idle and when thread 1 is about to go to idle, newly idle load balance always think that one of the threads is not idle and skips doing the newly idle load balance for multi-core and SMP domains.

This is because of the idle_cpu() macro, which checks if the current process on a cpu is an idle process. But this is not the case for the thread doing the load_balance_newidle().

Fix this by using runqueue's nr_running field instead of idle_cpu(). And also skip the logic of 'only one idle cpu in the group will be doing load balancing' during newly idle case.

---

index 3332bbb..623cee9 100644
@@ -2226,7 +2226,7 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 
 			rq = cpu_rq(i);
 
-			if (*sd_idle && !idle_cpu(i))
+			if (*sd_idle && rq->nr_running)
 				*sd_idle = 0;
 
 			/* Bias balancing toward cpus of our domain */

@@ -2248,9 +2248,11 @@ find_busiest_group(struct sched_domain *sd, int this_cpu,
 		/*
 		 * First idle cpu or the first cpu(busiest) in this sched group
 		 * is eligible for doing load balancing at this and above
-		 * domains.
+		 * domains. In the newly idle case, we will allow all the cpu's
+		 * to do the newly idle load balance.
 		 */
-		if (local_group && balance_cpu != this_cpu && balance) {
+		if (idle != CPU_NEWLY_IDLE && local_group &&
+		    balance_cpu != this_cpu && balance) {
 			*balance = 0;
 			goto ret;
 		}

ah, indeed - good catch! I've checked the other uses of idle_cpu() and those seem to be fine. I've added your patch to my queue.

Ingo