[PATCH v2] NOHZ: fix nohz on cpu unplug

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

After some testing, I think this patch is better:
[PATCH] NOHZ: fix nohz on cpu unplug

From: Christian Borntraeger <borntraeger@de.ibm.com>

After using cpu unplug I have seen one cpu with full ticks, even
on a idle systems. It turns 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 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.

This patch clear the nohz bits in the migration_call, a function that
is already called by the cpu hotplug notifier.

Opinions?

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

Index: kvm/kernel/sched.c
===================================================================
--- kvm.orig/kernel/sched.c
+++ kvm/kernel/sched.c
@@ -6696,6 +6696,7 @@ migration_call(struct notifier_block *nf
 		rq->idle->sched_class = &idle_sched_class;
 		migrate_dead_tasks(cpu);
 		spin_unlock_irq(&rq->lock);
+		cpumask_clear_cpu(cpu, nohz.cpu_mask);
 		cpuset_unlock();
 		migrate_nr_uninterruptible(rq);
 		BUG_ON(rq->nr_running != 0);