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    Subject[RFC PATCH 10/10] cpu: No more __stop_machine() in _cpu_down()
    Date
    From: Paul E. McKenney <paul.mckenney@linaro.org>

    The _cpu_down() function invoked as part of the CPU-hotplug offlining
    process currently invokes __stop_machine(), which is slow and inflicts
    substantial real-time latencies on the entire system. This patch
    substitutes stop_cpus() for __stop_machine() in order to improve
    both performance and real-time latency.

    This is currently unsafe, because there are a number of uses of
    preempt_disable() that are intended to block CPU-hotplug offlining.
    These will be fixed by using get/put_online_cpus_stable_atomic(), but in the
    meantime, this commit is one way to help locate them.

    Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    [ Srivatsa: Refer to get/put_online_cpus_stable_atomic() in the changelog ]
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    kernel/cpu.c | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index aaf2393..59592e5 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -352,15 +352,20 @@ static int __ref take_cpu_down(void *_param)
    {
    struct take_cpu_down_param *param = _param;
    int err, cpu = (long)(param->hcpu);
    + unsigned long flags;

    prepare_cpu_take_down(cpu);

    /* Ensure this CPU doesn't handle any more interrupts. */
    + local_irq_save(flags);
    err = __cpu_disable();
    - if (err < 0)
    + if (err < 0) {
    + local_irq_restore(flags);
    return err;
    + }

    cpu_notify(CPU_DYING | param->mod, param->hcpu);
    + local_irq_restore(flags);
    return 0;
    }

    @@ -393,7 +398,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
    }
    smpboot_park_threads(cpu);

    - err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
    + err = stop_cpus(cpumask_of(cpu), take_cpu_down, &tcd_param);
    if (err) {
    /* CPU didn't die: tell everyone. Can't complain. */
    smpboot_unpark_threads(cpu);


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