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<中断处理函数>
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;
- }
