在由内核执行的几个任务之间有些不是紧急的，在必要情况下他们可以延迟一段时间。一个中断处理程序的几个中断服务例程之间是串行执行的，并且通常在一个中断的处理程序结束前，不应该再次出现这个中断。相反，可延迟中断可以在开中断的情况下执行。
linux中所谓的可延迟函数，包括软中断和tasklet以及通过中作队列执行的函数（这个以后说），软中断的分配是静态的（即值编译时定义），而tasklet的分配和初始化可以在运行时进行。
软中断
软中断所使用的数据结构定义为
static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;
其中softirq_action类型为一个函数指针，从这里也可以看出，软中断的个数是有限的有NR_SOFTIRQS个，具体的定义如下:
/* PLEASE, avoid to allocate new softirqs, if you need not _really_ high frequency threaded job scheduling. For almost all the purposes tasklets are more than enough. F.e. all serial device BHs et al. should be converted to tasklets, not to softirqs.*/enum{ HI_SOFTIRQ=0, TIMER_SOFTIRQ, NET_TX_SOFTIRQ, NET_RX_SOFTIRQ, BLOCK_SOFTIRQ, TASKLET_SOFTIRQ, SCHED_SOFTIRQ,#ifdef CONFIG_HIGH_RES_TIMERS HRTIMER_SOFTIRQ,#endif RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */};
可以看出，一共10个软中断。
/* softirq mask and active fields moved to irq_cpustat_t in* asm/hardirq.h to get better cache usage. KAO*/struct softirq_action{ void (*action)(struct softirq_action *);};
软中断的初始化
void open_softirq(int nr, void (*action)(struct softirq_action *)){ softirq_vec[nr].action = action;}
上面函数中，参数nr为softirq_vec[]数组的下标，初始化就是初始化softirq_vec[]数组内容。
初始化了软中断后，要执行，接下来要做的是激活软中断，运用下面函数
void raise_softirq(unsigned int nr){ unsigned long flags; local_irq_save(flags); raise_softirq_irqoff(nr); local_irq_restore(flags);}
具体的激活工作由raise_softirq_irqoff函数实现
/** This function must run with irqs disabled!*/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();}
守护线程softirqd就是对软中断的处理
static int 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(); } 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;}
下面是软中断的执行
asmlinkage void do_softirq(void){ __u32 pending; unsigned long flags; if (in_interrupt()) return; local_irq_save(flags); pending = local_softirq_pending(); if (pending) __do_softirq(); local_irq_restore(flags);}
具体由__do_softirq函数实现
/*读取本地CPU的软中断掩码并执行与每个设置位相关的可延迟函数,__do_softirq只做固定次数的循环然后就返回。如果还有其余挂起的软中断，那么内核线程ksofirqd将会在预期的时间内处理他们*/
asmlinkage void __do_softirq(void){ struct softirq_action *h; __u32 pending; int max_restart = MAX_SOFTIRQ_RESTART; int cpu; pending = local_softirq_pending(); account_system_vtime(current); __local_bh_disable((unsigned long)__builtin_return_address(0)); trace_softirq_enter(); cpu = smp_processor_id();restart: /* Reset the pending bitmask before enabling irqs */ set_softirq_pending(0); local_irq_enable(); h = softirq_vec; do { if (pending & 1) { h->action(h); rcu_bh_qsctr_inc(cpu); } h++; pending >>= 1; } while (pending); local_irq_disable(); pending = local_softirq_pending(); if (pending && –max_restart) goto restart; if (pending) wakeup_softirqd(); trace_softirq_exit(); account_system_vtime(current); _local_bh_enable();}
到此，linux内核软中断的大致执行和实现基本上分析完了，中间有很多地方没有注释的，主要是考虑到需要别的实现机制以及有的比较易懂。能够自己看懂。
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