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    Subject[PATCH -rt 2/5] hotplug: Call cpu_unplug_begin() a little early
    Date
    cpu_unplug_begin() should be called before CPU_DOWN_PREPARE,
    because at CPU_DOWN_PREPARE cpu_active is cleared and sched_domain
    is rebuilt, it will finally lead to thread 'sync_unplug' will
    be running on the cpu on which it's created.

    I found by an incorrect warning on smp_processor_id() called by
    sync_unplug/1, and trace shows below:
    (echo 1 > /sys/device/system/cpu/cpu1/online)
    bash-1664 [000] 83.136620: _cpu_down: Bind sync_unplug to cpu 1
    bash-1664 [000] 83.136623: sched_wait_task: comm=sync_unplug/1 pid=1724 prio=120
    bash-1664 [000] 83.136624: _cpu_down: Wake sync_unplug
    bash-1664 [000] 83.136629: sched_wakeup: comm=sync_unplug/1 pid=1724 prio=120 success=1 target_cpu=000

    Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
    ---
    kernel/cpu.c | 16 +++++++---------
    1 files changed, 7 insertions(+), 9 deletions(-)

    diff --git a/kernel/cpu.c b/kernel/cpu.c
    index 83fb084..13066a3 100644
    --- a/kernel/cpu.c
    +++ b/kernel/cpu.c
    @@ -337,22 +337,20 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
    return -EBUSY;
    }

    - err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
    + cpu_hotplug_begin();
    + err = cpu_unplug_begin(cpu);
    if (err) {
    - nr_calls--;
    - __cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
    - printk("%s: attempt to take down CPU %u failed\n",
    - __func__, cpu);
    + printk("cpu_unplug_begin(%d) failed\n", cpu);
    goto out_cancel;
    }

    - cpu_hotplug_begin();
    - err = cpu_unplug_begin(cpu);
    + err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
    if (err) {
    nr_calls--;
    __cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
    - printk("cpu_unplug_begin(%d) failed\n", cpu);
    - goto out_cancel;
    + printk("%s: attempt to take down CPU %u failed\n",
    + __func__, cpu);
    + goto out_release;
    }

    err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
    --
    1.7.1


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