内核源码分析之软中断（基于3.16-rc4）

1.和软中断相关的数据结构：

softing_vec数组（kernel/softirq.c）

 static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;

NR_SOFTIRQS值为10，说明内核支持10个软中断函数。

softirq_action结构体（include/linux/interrupt.h）

 struct softirq_action
 {
     void    (*action)(struct softirq_action *);
 };

action是函数指针变量，指向了某个软中断函数。

irq_cpustat_t结构体（arch/x86/include/asm/hardirq.h）

 typedef struct {
     unsigned int __softirq_pending;
     unsigned int __nmi_count;    /* arch dependent */
 #ifdef CONFIG_X86_LOCAL_APIC
     unsigned int apic_timer_irqs;    /* arch dependent */
     unsigned int irq_spurious_count;
     unsigned int icr_read_retry_count;
 #endif
 #ifdef CONFIG_HAVE_KVM
     unsigned int kvm_posted_intr_ipis;
 #endif
     unsigned int x86_platform_ipis;    /* arch dependent */
     unsigned int apic_perf_irqs;
     unsigned int apic_irq_work_irqs;
 #ifdef CONFIG_SMP
     unsigned int irq_resched_count;
     unsigned int irq_call_count;
     /*
      * irq_tlb_count is double-counted in irq_call_count, so it must be
      * subtracted from irq_call_count when displaying irq_call_count
      */
     unsigned int irq_tlb_count;
 #endif
 #ifdef CONFIG_X86_THERMAL_VECTOR
     unsigned int irq_thermal_count;
 #endif
 #ifdef CONFIG_X86_MCE_THRESHOLD
     unsigned int irq_threshold_count;
 #endif
 #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)
     unsigned int irq_hv_callback_count;
 #endif
 } ____cacheline_aligned irq_cpustat_t;

每个cpu都有一个这样的结构体变量，在软中断中，我们要使用的是第2行的成员，32位的软中断掩码。当有一个软中断被挂起（将要被执行）的时候，会设置该掩码中的相应位。

2.软中断的执行过程

首先使用open_softirq()函数注册软中断函数，代码如下（kernel/softirq.c）：

 void open_softirq(int nr, void (*action)(struct softirq_action *))
 {
     softirq_vec[nr].action = action;
 }

将软中断函数指针action存入softirq_vec数组的对应元素中。

接着，使用raise_softirq()激活软中断，代码如下（kernel/softirq.c）：

 void raise_softirq(unsigned int nr)
 {
     unsigned long flags;

     local_irq_save(flags);
     raise_softirq_irqoff(nr);
     local_irq_restore(flags);
 }

第5行关闭本地中断，第7行恢复中断。第6行激活nr所对应的软中断函数。接着分析 raise_softirq_irqoff()，代码如下（kernel/softirq.c）：

 inline void raise_softirq_irqoff(unsigned int nr)
 {
     __raise_softirq_irqoff(nr);

     /*
      * If we're in an interrupt or softirq, we're done
      * (this also catches softirq-disabled code). We will
      * actually run the softirq once we return from
      * the irq or softirq.
      *
      * Otherwise we wake up ksoftirqd to make sure we
      * schedule the softirq soon.
      */
     if (!in_interrupt())
         wakeup_softirqd();
 }

第3行__raise_softirq_irqoff函数设置了软中断掩码的相应位，代码如下（kernel/softirq.c）。然后第14行判断软中断是否已经激活或者被禁用，如果没有，那么在15行激活内核线程ksoftirq，去执行软中断。

 void (unsigned int nr)
 {
     trace_softirq_raise(nr);
     or_softirq_pending(1UL << nr);
 }

具体而言，在第4行的函数中设置掩码位。

3.下面分析下在哪些地方都可以进入软中断。

第一个地方，当然就是上边所提到的，内核线程ksoftirq被激活的时候。下面看看ksoftirq线程（kernel/softirq.c）。

 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);

给每个cpu都定义一个指向struct task_struct类型的结构体变量，很显然，该变量存的是ksoftirq线程的进程描述符。（由此也说明，linux的线程和进程是一个东西）

接着，我们要看看ksoftirq线程要执行的函数（kernel/softirq.c）。

 static void run_ksoftirqd(unsigned int cpu)
 {
     local_irq_disable();
     if (local_softirq_pending()) {
         /*
          * We can safely run softirq on inline stack, as we are not deep
          * in the task stack here.
          */
         __do_softirq();
         rcu_note_context_switch(cpu);
         local_irq_enable();
         cond_resched();
         return;
     }
     local_irq_enable();
 }

该函数就是ksoftirq线程的线程体。第9行__do_softirq()中调用所有软中断函数。

我们回过头来再分析下wakeup_softirqd()，看看ksoftirq线程怎样被唤醒（kernel/softirq.c）。

 static void wakeup_softirqd(void)
 {
     /* Interrupts are disabled: no need to stop preemption */
     struct task_struct *tsk = __this_cpu_read(ksoftirqd);

     if (tsk && tsk->state != TASK_RUNNING)
         wake_up_process(tsk);
 }

第4行把本地cpu的ksoftirq线程的描述符读到tsk变量中，第6行中判断ksoftirq线程如果没有运行的话，第7行唤醒该线程。

第二个地方，中断处理程序do_IRQ完成处理或者调用irq_exit函数时。下面看看irq_exit代码（kernel/softirq.c）。

 void irq_exit(void)
 {
 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED
     local_irq_disable();
 #else
     WARN_ON_ONCE(!irqs_disabled());
 #endif

     account_irq_exit_time(current);
     preempt_count_sub(HARDIRQ_OFFSET);
     if (!in_interrupt() && local_softirq_pending())
         invoke_softirq();

     tick_irq_exit();
     rcu_irq_exit();
     trace_hardirq_exit(); /* must be last! */
 }

不用我说了吧，我觉得你一眼就能瞄见了第12行。看下invoke_softirq函数（kernel/softirq.c）。

 static inline void invoke_softirq(void)
 {
     if (!force_irqthreads) {
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
         /*
          * We can safely execute softirq on the current stack if
          * it is the irq stack, because it should be near empty
          * at this stage.
          */
         __do_softirq();
 #else
         /*
          * Otherwise, irq_exit() is called on the task stack that can
          * be potentially deep already. So call softirq in its own stack
          * to prevent from any overrun.
          */
         do_softirq_own_stack();
 #endif
     } else {
         wakeup_softirqd();
     }
 }

第3行force_irqthreads值为0，所以该函数也调用了__do_softirq()来执行软中断。

还有几处地方暂时不分析了，以后有空补上。

4.下面来看下__do_softirq()函数（kernel/softirq.c）。

 asmlinkage __visible void __do_softirq(void)
 {
     unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
     unsigned long old_flags = current->flags;
     int max_restart = MAX_SOFTIRQ_RESTART;
     struct softirq_action *h;
     bool in_hardirq;
     __u32 pending;
     int softirq_bit;

     /*
      * Mask out PF_MEMALLOC s current task context is borrowed for the
      * softirq. A softirq handled such as network RX might set PF_MEMALLOC
      * again if the socket is related to swap
      */
     current->flags &= ~PF_MEMALLOC;

     pending = local_softirq_pending();
     account_irq_enter_time(current);

     __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);
     in_hardirq = lockdep_softirq_start();

 restart:
     /* Reset the pending bitmask before enabling irqs */
     set_softirq_pending();

     local_irq_enable();

     h = softirq_vec;

     while ((softirq_bit = ffs(pending))) {
         unsigned int vec_nr;
         int prev_count;

         h += softirq_bit - ;

         vec_nr = h - softirq_vec;
         prev_count = preempt_count();

         kstat_incr_softirqs_this_cpu(vec_nr);

         trace_softirq_entry(vec_nr);
         h->action(h);
         trace_softirq_exit(vec_nr);
         if (unlikely(prev_count != preempt_count())) {
             pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",
                    vec_nr, softirq_to_name[vec_nr], h->action,
                    prev_count, preempt_count());
             preempt_count_set(prev_count);
         }
         h++;
         pending >>= softirq_bit;
     }

     rcu_bh_qs(smp_processor_id());
     local_irq_disable();

     pending = local_softirq_pending();
     if (pending) {
         if (time_before(jiffies, end) && !need_resched() &&
             --max_restart)
             goto restart;

         wakeup_softirqd();
     }

     lockdep_softirq_end(in_hardirq);
     account_irq_exit_time(current);
     __local_bh_enable(SOFTIRQ_OFFSET);
     WARN_ON_ONCE(in_interrupt());
     tsk_restore_flags(current, old_flags, PF_MEMALLOC);
 } 

在该函数中循环调用的所有的被激活的软中断函数。第5行MAX_SOFTIRQ_RESTART值为10，表示最多循环10次（不能让其他进程等太久），第32行获取pending表中第一被设置的比特位，第44行开始执行设置过的软中断函数。第53行对pending进行右移运算，然后进入下次循环。直到将本轮所有已设置的软中断函数全部执行完，退出循环。第59行重新获得本地cpu的软中断掩码，第61行如果时间没有超出end而且没有出现更高优先级的进程并且10次寻环未用完，那么跳回restart，重新for循环。否则，第65行唤醒softirqd内核线程。然后退出本函数。

至此，软中断的处理过程就分析完了。