1. 注册软中断当然是通过open_softirq

例子如下：

void __init init_timers(void)  

{  

    int err = timer_cpu_notify(&timers_nb, (unsigned long)CPU_UP_PREPARE,  

                (void *)(long)smp_processor_id());  

    init_timer_stats();  

    BUG_ON(err == NOTIFY_BAD);  

    register_cpu_notifier(&timers_nb);  

    open_softirq(TIMER_SOFTIRQ, run_timer_softirq);  

}  

void open_softirq(int nr, void (*action)(struct softirq_action *))  

{  

    softirq_vec[nr].action = action;  

}  

软中断TIMER_SOFTIRQ的中断处理函数为：run_timer_softirq

之所以成为softirq，是因为这些中断是由硬件中断来间接触发的,如何间接触发的呢：

硬件中断处理函数-->对软中断的相应位置位-->唤醒ksoftirqd线程-->执行软中断的中断处理函数

2. 硬件中断如何通过置位唤醒ksoftirqd线程

timer interrupt handler->

timer_tick->

update_process_times->

run_local_timers->

hrtimer_run_queues()和raise_softirq(TIMER_SOFTIRQ)->

raise_softirq_irqoff->

__raise_softirq_irqoff { or_softirq_pending(1UL << (nr)); }

即(local_softirq_pending() |= (x))

3. 如何执行软中断的action

对于TIMER_SOFTIRQ来说，每次system clock产生中断时，即一个tick 到来时，在system clock的中断处理函数中会调用run_local_timers来设置TIMER_SOFTIRQ触发条件；也就是当前CPU对应的irq_cpustat_t结构体中的__softirq_pending成员的第TIMER_SOFTIRQ个BIT被置为1。 而当这个条件满足时，ksoftirqd线程(入口函数run_ksoftirqd,cpu_callback:kthread_create(run_ksoftirqd, hcpu, "ksoftirqd/%d", hotcpu);)会被唤醒，然后按照下面的流程调用TIMER_SOFTIRQ在数组softirq_vec中注册的action，即run_timer_softirq。

run_ksoftirqd--->do_softirq--->__do_softirq--->softirq_vec[TIMER_SOFTIRQ].action

static int run_ksoftirqd(void * __bind_cpu)  

{  

    set_current_state(TASK_INTERRUPTIBLE);  

    while (!kthread_should_stop()) {  

        preempt_disable();  

        if (!local_softirq_pending()) {  

            preempt_enable_no_resched();  

            schedule();  

            preempt_disable();  

        }  

        __set_current_state(TASK_RUNNING);  

        while (local_softirq_pending()) {  

            /* Preempt disable stops cpu going offline. 

               If already offline, we'll be on wrong CPU: 

               don't process */  

            if (cpu_is_offline((long)__bind_cpu))  

                goto wait_to_die;  

            do_softirq();  

            preempt_enable_no_resched();  

            cond_resched();  

            preempt_disable();  

            rcu_sched_qs((long)__bind_cpu);  

        }  

        preempt_enable();  

        set_current_state(TASK_INTERRUPTIBLE);  

    }  

    __set_current_state(TASK_RUNNING);  

    return 0;  

wait_to_die:  

    preempt_enable();  

    /* Wait for kthread_stop */  

    set_current_state(TASK_INTERRUPTIBLE);  

    while (!kthread_should_stop()) {  

        schedule();  

        set_current_state(TASK_INTERRUPTIBLE);  

    }  

    __set_current_state(TASK_RUNNING);  

    return 0;  

}