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    SubjectRe: [PATCH v2] NOHZ: fix nohz on cpu unplug
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    On Tue, 2009-02-03 at 22:19 -0800, Christian Borntraeger wrote:
    > Am Tuesday 03 February 2009 13:28:47 schrieb Ingo Molnar:
    > > > > + if (!cpu_active(cpu) {
    > > >
    > > > That does not compile...I used the following:
    > > > if (!cpu_active(cpu)) {
    > >
    > > Could you please resubmit an updated patch?
    >
    > Suresh,
    >
    > can you send the updated patch? After all, its your work and we need your
    > Signed-off-by anyway.
    >
    > You can add my
    > Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
    > Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>

    Thanks Christian for testing the fix. Ingo, appended is the updated patch. Thanks.
    ---

    From: Suresh Siddha <suresh.b.siddha@intel.com>
    Subject: sched: fix nohz load balancer on cpu offline

    Christian Borntraeger reports:

    > After a logical cpu offline, even on a complete idle system, there
    > is one cpu with full ticks. It turns out that nohz.cpu_mask has the
    > the offlined cpu still set.
    >
    > 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.

    Fix the select_nohz_load_balancer() to not set the nohz.cpu_mask
    while a cpu is going offline.

    Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
    Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
    Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
    ---

    diff --git a/kernel/sched.c b/kernel/sched.c
    index 96439a4..8a0419b 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -4026,19 +4026,24 @@ int select_nohz_load_balancer(int stop_tick)
    int cpu = smp_processor_id();

    if (stop_tick) {
    - cpumask_set_cpu(cpu, nohz.cpu_mask);
    cpu_rq(cpu)->in_nohz_recently = 1;

    - /*
    - * If we are going offline and still the leader, give up!
    - */
    - if (!cpu_active(cpu) &&
    - atomic_read(&nohz.load_balancer) == cpu) {
    + if (!cpu_active(cpu)) {
    + if (atomic_read(&nohz.load_balancer) != cpu)
    + return 0;
    +
    + /*
    + * If we are going offline and still the leader,
    + * give up!
    + */
    if (atomic_cmpxchg(&nohz.load_balancer, cpu, -1) != cpu)
    BUG();
    +
    return 0;
    }

    + cpumask_set_cpu(cpu, nohz.cpu_mask);
    +
    /* time for ilb owner also to sleep */
    if (cpumask_weight(nohz.cpu_mask) == num_online_cpus()) {
    if (atomic_read(&nohz.load_balancer) == cpu)



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