Messages in this thread Patch in this message |  | | | From | Christian Borntraeger <> | | Subject | [RFC] NOHZ: fix nohz on cpu unplug | | Date | Fri, 30 Jan 2009 17:29:30 +0100 |
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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; /*
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