关于软中断和系统调用的一点分析
感觉之前对于软中断一直有一些误解。
软中断的定义:
软中断是利用硬件中断的概念,用软件方式进行模拟,实现宏观上的异步执行效果。很多情况下,软中断和“信号”类似。同时,软中断又是和硬中断相对应的,“硬中断是外部设备对CPU的中断”,“软中断通常是硬件中断服务程序对内核的中断”
作为系统调用而言,对于i386则是通过软中断int80实现
对于其它的软中断,则是在硬件中断之后触发的软中断。
系统调用在Linux2.6中的实现
set_system_trap_gate(SYSCALL_VECTOR, &system_call); # define SYSCALL_VECTOR 0x80 ENTRY(system_call) RING0_INT_FRAME # can't unwind into user space anyway pushl_cfi %eax # save orig_eax SAVE_ALL GET_THREAD_INFO(%ebp) # system call tracing in operation / emulation testl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp) jnz syscall_trace_entry cmpl $(nr_syscalls), %eax jae syscall_badsys syscall_call: call *sys_call_table(,%eax,4) movl %eax,PT_EAX(%esp) # store the return value ENTRY(sys_call_table) .long sys_restart_syscall /* 0 - old "setup()" system call, used for restarting */ .long sys_exit .long ptregs_fork .long sys_read .long sys_write .long sys_open /* 5 */ .long sys_close .long sys_waitpid .long sys_creat .long sys_link .long sys_unlink /* 10 */ .long ptregs_execve .long sys_chdir .long sys_time .long sys_mknod .long sys_chmod /* 15 */ .long sys_lchown16 .long sys_ni_syscall /* old break syscall holder */ .long sys_stat .long sys_lseek .long sys_getpid /* 20 */ .long sys_mount .long sys_oldumount ...
对于软中断而言,则稍微复杂些
1.注册软中断当然是通过open_softirq
例子如下:
[cpp]
- 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 interrupthandler->
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
[cpp]
- 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;
- }
From <http://blog.csdn.net/myarrow/article/details/7064060>
从上面两种情况的对比可以看出,系统调用的中断由于是软件触发的中断,所以称为软中断,而对于后者的软中断,虽然也是软件触发,但是并不经过中断向量表。