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    SubjectRe: [PATCH] cpu-hotplug: Register update_sched_domains() notifier with higher prio
    On Mon, 10 Mar 2008 18:43:55 +0530 Gautham R Shenoy <ego@in.ibm.com> wrote:

    > cpu-hotplug: Register update_sched_domains() notifier with higher prio
    > From: Gautham R Shenoy <ego@in.ibm.com>
    >
    > The current -rt wake_up logic uses the rq->rd->online_map which is
    > updated on cpu-hotplug events by update_sched_domains(). Currently
    > update_sched_domains() is registered with a priority 0. It is
    > preferable that update_sched_domains() be the first notifier called on
    > a CPU_DEAD or CPU_ONLINE events inorder to ensure that the
    > rq->rd->online_map is in sync with the cpu_online_map.
    >
    > This way on a CPU_DOWN_PREPARE,
    > we would destroy the sched_domains and reattach all the rq's to the
    > def_root_domain. This will be followed by a CPU_DOWN_PREPARE in
    > migration_call() which will clear the bit of the cpu going offline
    > from the corresponding rq.
    > This will ensure that no task will be woken up on the cpu which is
    > going to be offlined between CPU_DOWN_PREPARE and CPU_DEAD.
    >
    > Ditto for the CPU_ONLINE path.
    >
    > Tested on a 4-way Intel Xeon Box with cpu-hotplug tests running in parallel
    > with kernbench.
    >
    > Signed-off-by: Gautham R Shenoy <ego@in.ibm.com>
    > ---
    >
    > kernel/sched.c | 12 ++++++++++--
    > 1 files changed, 10 insertions(+), 2 deletions(-)
    >
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index 69ecb1a..374c46f 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -7079,8 +7079,16 @@ void __init sched_init_smp(void)
    > if (cpus_empty(non_isolated_cpus))
    > cpu_set(smp_processor_id(), non_isolated_cpus);
    > put_online_cpus();
    > - /* XXX: Theoretical race here - CPU may be hotplugged now */
    > - hotcpu_notifier(update_sched_domains, 0);
    > + /*
    > + * XXX: Theoretical race here - CPU may be hotplugged now
    > + *
    > + * We register the notifier with priority 11, which means that
    > + * update_sched_domains() will be called just before migration_call().
    > + *
    > + * This is necessary to ensure that the rt wake up logic works fine
    > + * and the rq->rd->online_map remains in sync with the cpu_online_map.
    > + */
    > + hotcpu_notifier(update_sched_domains, 11);
    >
    > /* Move init over to a non-isolated CPU */
    > if (set_cpus_allowed(current, non_isolated_cpus) < 0)

    This fixes my poweroff-doesn't-work post-2.6.24 regression.


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