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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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      Last update: 2009-01-30 17:31    [W:0.021 / U:33.972 seconds]
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