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    Subject[PATCH RFC] cpu: No more __stop_machine() in _cpu_down()
    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, but in the meantime, this commit is one way to help
    locate them. It nevertheless passes light rcutorture/hotplug stress
    testing. Meaning that we should not be relying on pure testing to find
    places where people are relying on preemption disabling to block CPUs
    from going offline. ;-)

    Not-yet-signed-off-by: Paul E. McKenney <>

    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index a4eb522..47e63a0 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -243,13 +243,18 @@ static int __ref take_cpu_down(void *_param)
    struct take_cpu_down_param *param = _param;
    int err;
    + unsigned long flags;

    /* 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;

    @@ -281,7 +286,7 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
    goto out_release;

    - 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. */
    cpu_notify_nofail(CPU_DOWN_FAILED | mod, hcpu);

     \ /
      Last update: 2012-07-27 01:21    [W:0.041 / U:2.364 seconds]
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