[patch] sched: fix newly idle load balance in case of SMT

[patch] sched: fix newly idle load balance in case of SMT

[Siddha, Suresh B]

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.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index 3332bbb..623cee9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -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;
 		}

Re: [patch] sched: fix newly idle load balance in case of SMT

[Ingo Molnar]

* Siddha, Suresh B <suresh.b.siddha@intel.com> wrote:

> 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.

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