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Subject[RFC] NOHZ: fix nohz on cpu unplug
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

Signed-off-by: Christian Borntraeger <>
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;


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