ARM Linux 如何--注册和触发--软中断

1. 注册软中断当然是通过open_softirq

例子如下：

1. void __init init_timers(void)
2. {
3. int err = timer_cpu_notify(&timers_nb, (unsigned long)CPU_UP_PREPARE,
4. (void *)(long)smp_processor_id());
5. init_timer_stats();
6. BUG_ON(err == NOTIFY_BAD);
7. register_cpu_notifier(&timers_nb);
8. open_softirq(TIMER_SOFTIRQ, run_timer_softirq);
9. }
10. void open_softirq(int nr, void (*action)(struct softirq_action *))
11. {
12. softirq_vec[nr].action = action;
13. }

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

1. static int run_ksoftirqd(void * __bind_cpu)
2. {
3. set_current_state(TASK_INTERRUPTIBLE);
4. while (!kthread_should_stop()) {
5. preempt_disable();
6. if (!local_softirq_pending()) {
7. preempt_enable_no_resched();
8. schedule();
9. preempt_disable();
10. }
11. __set_current_state(TASK_RUNNING);
12. while (local_softirq_pending()) {
13. /* Preempt disable stops cpu going offline.
14. If already offline, we'll be on wrong CPU:
15. don't process */
16. if (cpu_is_offline((long)__bind_cpu))
17. goto wait_to_die;
18. do_softirq();
19. preempt_enable_no_resched();
20. cond_resched();
21. preempt_disable();
22. rcu_sched_qs((long)__bind_cpu);
23. }
24. preempt_enable();
25. set_current_state(TASK_INTERRUPTIBLE);
26. }
27. __set_current_state(TASK_RUNNING);
28. return 0;
29. wait_to_die:
30. preempt_enable();
31. /* Wait for kthread_stop */
32. set_current_state(TASK_INTERRUPTIBLE);
33. while (!kthread_should_stop()) {
34. schedule();
35. set_current_state(TASK_INTERRUPTIBLE);
36. }
37. __set_current_state(TASK_RUNNING);
38. return 0;
39. }