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软中断浅析
1.和软中断相关的数据结构：
softing_vec数组（kernel/softirq.c）
1 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）
1 struct softirq_action2 {3 void (*action)(struct softirq_action *);4 };
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）
1 typedef struct { 2 unsigned int __softirq_pending; 3 unsigned int __nmi_count; /* arch dependent */ 4 #ifdef CONFIG_X86_LOCAL_APIC 5 unsigned int apic_timer_irqs; /* arch dependent */ 6 unsigned int irq_spurious_count; 7 unsigned int icr_read_retry_count; 8 #endif 9 #ifdef CONFIG_HAVE_KVM10 unsigned int kvm_posted_intr_ipis;11 #endif12 unsigned int x86_platform_ipis; /* arch dependent */13 unsigned int apic_perf_irqs;14 unsigned int apic_irq_work_irqs;15 #ifdef CONFIG_SMP16 unsigned int irq_resched_count;17 unsigned int irq_call_count;18 /*19 * irq_tlb_count is double-counted in irq_call_count, so it must be20 * subtracted from irq_call_count when displaying irq_call_count21 */22 unsigned int irq_tlb_count;23 #endif24 #ifdef CONFIG_X86_THERMAL_VECTOR25 unsigned int irq_thermal_count;26 #endif27 #ifdef CONFIG_X86_MCE_THRESHOLD28 unsigned int irq_threshold_count;29 #endif30 #if IS_ENABLED(CONFIG_HYPERV) || defined(CONFIG_XEN)31 unsigned int irq_hv_callback_count;32 #endif33 } ____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）：
1 void open_softirq(int nr, void (*action)(struct softirq_action *))2 {3 softirq_vec[nr].action = action;4 }
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）：
1 void raise_softirq(unsigned int nr)2 {3 unsigned long flags;4 5 local_irq_save(flags);6 raise_softirq_irqoff(nr);7 local_irq_restore(flags);8 }
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）：
1 inline void raise_softirq_irqoff(unsigned int nr) 2 { 3 __raise_softirq_irqoff(nr); 4 5 /* 6 * If we‘re in an interrupt or softirq, we‘re done 7 * (this also catches softirq-disabled code). We will 8 * actually run the softirq once we return from 9 * the irq or softirq.10 *11 * Otherwise we wake up ksoftirqd to make sure we12 * schedule the softirq soon.13 */14 if (!in_interrupt())15 wakeup_softirqd();16 }
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。
1 void (unsigned int nr)2 {3 trace_softirq_raise(nr);4 or_softirq_pending(1UL << nr);5 }
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）。
1 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）。
1 static void run_ksoftirqd(unsigned int cpu) 2 { 3 local_irq_disable(); 4 if (local_softirq_pending()) { 5 /* 6 * We can safely run softirq on inline stack, as we are not deep 7 * in the task stack here. 8 */ 9 __do_softirq();10 rcu_note_context_switch(cpu);11 local_irq_enable();12 cond_resched();13 return;14 }15 local_irq_enable();16 }
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）。
1 static void wakeup_softirqd(void)2 {3 /* Interrupts are disabled: no need to stop preemption */4 struct task_struct *tsk = __this_cpu_read(ksoftirqd);5 6 if (tsk && tsk->state != TASK_RUNNING)7 wake_up_process(tsk);8 }
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）。
1 void irq_exit(void) 2 { 3 #ifndef __ARCH_IRQ_EXIT_IRQS_DISABLED 4 local_irq_disable(); 5 #else 6 WARN_ON_ONCE(!irqs_disabled()); 7 #endif 8 9 account_irq_exit_time(current);10 preempt_count_sub(HARDIRQ_OFFSET);11 if (!in_interrupt() && local_softirq_pending())12 invoke_softirq();13 14 tick_irq_exit();15 rcu_irq_exit();16 trace_hardirq_exit(); /* must be last! */17 }
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）。
1 static inline void invoke_softirq(void) 2 { 3 if (!force_irqthreads) { 4 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK 5 /* 6 * We can safely execute softirq on the current stack if 7 * it is the irq stack, because it should be near empty 8 * at this stage. 9 */10 __do_softirq();11 #else12 /*13 * Otherwise, irq_exit() is called on the task stack that can14 * be potentially deep already. So call softirq in its own stack15 * to prevent from any overrun.16 */17 do_softirq_own_stack();18 #endif19 } else {20 wakeup_softirqd();21 }22 }
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）。
1 asmlinkage __visible void __do_softirq(void) 2 { 3 unsigned long end = jiffies + MAX_SOFTIRQ_TIME; 4 unsigned long old_flags = current->flags; 5 int max_restart = MAX_SOFTIRQ_RESTART; 6 struct softirq_action *h; 7 bool in_hardirq; 8 __u32 pending; 9 int softirq_bit;10 11 /*12 * Mask out PF_MEMALLOC s current task context is borrowed for the13 * softirq. A softirq handled such as network RX might set PF_MEMALLOC14 * again if the socket is related to swap15 */16 current->flags &= ~PF_MEMALLOC;17 18 pending = local_softirq_pending();19 account_irq_enter_time(current);20 21 __local_bh_disable_ip(_RET_IP_, SOFTIRQ_OFFSET);22 in_hardirq = lockdep_softirq_start();23 24 restart:25 /* Reset the pending bitmask before enabling irqs */26 set_softirq_pending(0);27 28 local_irq_enable();29 30 h = softirq_vec;31 32 while ((softirq_bit = ffs(pending))) {33 unsigned int vec_nr;34 int prev_count;35 36 h += softirq_bit - 1;37 38 vec_nr = h - softirq_vec;39 prev_count = preempt_count();40 41 kstat_incr_softirqs_this_cpu(vec_nr);42 43 trace_softirq_entry(vec_nr);44 h->action(h);45 trace_softirq_exit(vec_nr);46 if (unlikely(prev_count != preempt_count())) {47 pr_err("huh, entered softirq %u %s %p with preempt_count %08x, exited with %08x?\n",48 vec_nr, softirq_to_name[vec_nr], h->action,49 prev_count, preempt_count());50 preempt_count_set(prev_count);51 }52 h++;53 pending >>= softirq_bit;54 }55 56 rcu_bh_qs(smp_processor_id());57 local_irq_disable();58 59 pending = local_softirq_pending();60 if (pending) {61 if (time_before(jiffies, end) && !need_resched() &&62 --max_restart)63 goto restart;64 65 wakeup_softirqd();66 }67 68 lockdep_softirq_end(in_hardirq);69 account_irq_exit_time(current);70 __local_bh_enable(SOFTIRQ_OFFSET);71 WARN_ON_ONCE(in_interrupt());72 tsk_restore_flags(current, old_flags, PF_MEMALLOC);73 }
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。