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    Subject[RFC PATCH v2 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_atomic_light(), or
    get/put_online_cpus_atomic_full(), 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 the new sync primitives for readers, in the changelog ]
    Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
    ---

    kernel/cpu.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index c71c723..00a1edc 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -418,7 +418,7 @@ 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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