关于软中断和系统调用的一点分析

来源：互联网发布：六场预测法知乎编辑：程序博客网时间：2024/06/06 03:50

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_VECTOR0x80ENTRY(system_call)RING0_INT_FRAME# can't unwind into user space anywaypushl_cfi %eax# save orig_eaxSAVE_ALLGET_THREAD_INFO(%ebp)# system call tracing in operation / emulationtestl $_TIF_WORK_SYSCALL_ENTRY,TI_flags(%ebp)jnz syscall_trace_entrycmpl $(nr_syscalls), %eaxjae syscall_badsyssyscall_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]

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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]

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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。