[RFC] NOHZ: fix nohz on cpu unplug

[Christian Borntraeger <borntraeger@de.ibm.com>]

Ingo, Thomas,

After using cpu unplug I have seen one cpu with enabled ticks on s390,
even on a idle systems. It turned out that nohz.cpu_mask is not
updated on cpu unplug. 
In select_nohz_load_balancer we check if the system is completely
idle to turn of load balancing. We compare the weights of
cpu_online_map with nohz.cpu_mask.
Since cpu_online_map is updated on cpu unplug, but nohz.cpu_mask is
not, the check fails and the scheduler believes that we need an 
"idle load balancer" even on a fully idle system. Since the ilb 
cpu does not deactivate the timer tick this "breaks" NOHZ.

One possible fix is to only add the cpu to the nohz.cpu_mask,
if the cpu is active.

The alternative is to add nohz.cpu_mask in the cpu hotplug path.

This patch seems to work on s390 (Shortly tested with kvm guests
with cpu hotplug and kvm_stat on the host), but I want to hear your
opinion.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
---
 kernel/sched.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: kvm/kernel/sched.c
===================================================================
--- kvm.orig/kernel/sched.c
+++ kvm/kernel/sched.c
@@ -3880,7 +3880,8 @@ int select_nohz_load_balancer(int stop_t
 	int cpu = smp_processor_id();
 
 	if (stop_tick) {
-		cpumask_set_cpu(cpu, nohz.cpu_mask);
+		if (cpu_active(cpu))
+			cpumask_set_cpu(cpu, nohz.cpu_mask);
 		cpu_rq(cpu)->in_nohz_recently = 1;
 
 		/*