[PATCH/RFC] NOHZ: fix nohz on cpu unplug

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

This is a reminder mail. I still dont know if my fix is correct, although
it seems to work..
I also CCed: Siddha, Suresh B <suresh.b.siddha@intel.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;
>
>  		/*