1.和軟中斷相關的數據結構：

softing_vec數組（kernel/softirq.c）

1 static struct softirq_action softirq_vec[NR_SOFTIRQS] __cacheline_aligned_in_smp;

NR_SOFTIRQS值為10，說明內核支持10個軟中斷函數。

softirq_action結構體（include/linux/interrupt.h）

1 struct softirq_action2 {3     void    (*action)(struct softirq_action *);4 };

action是函數指針變量，指向了某個軟中斷函數。

irq_cpustat_t結構體（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;

每個cpu都有一個這樣的結構體變量，在軟中斷中，我們要使用的是第2行的成員，32位的軟中斷掩碼。當有一個軟中斷被掛起（將要被執行）的時候，會設置該掩碼中的相應位。

2.軟中斷的執行過程

首先使用open_softirq()函數注冊軟中斷函數，代碼如下（kernel/softirq.c）：

1 void open_softirq(int nr, void (*action)(struct softirq_action *))2 {3     softirq_vec[nr].action = action;4 }

將軟中斷函數指針action存入softirq_vec數組的對應元素中。

接著，使用raise_softirq()激活軟中斷，代碼如下（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行關閉本地中斷，第7行恢復中斷。第6行激活nr所對應的軟中斷函數。接著分析 raise_softirq_irqoff()，代碼如下（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函數設置了軟中斷掩碼的相應位，代碼如下（kernel/softirq.c）。然后第14行判斷軟中斷是否已經激活或者被禁用，如果沒有，那么在15行激活內核線程ksoftirq，去執行軟中斷。

1 void (unsigned int nr)2 {3     trace_softirq_raise(nr);4     or_softirq_pending(1UL << nr);5 }

具體而言，在第4行的函數中設置掩碼位。

3.下面分析下在哪些地方都可以進入軟中斷。

第一個地方，當然就是上邊所提到的，內核線程ksoftirq被激活的時候。下面看看ksoftirq線程（kernel/softirq.c）。

1 DEFINE_PER_CPU(struct task_struct *, ksoftirqd);

給每個cpu都定義一個指向struct task_struct類型的結構體變量，很顯然，該變量存的是ksoftirq線程的進程描述符。（由此也說明，linux的線程和進程是一個東西）

接著，我們要看看ksoftirq線程要執行的函數（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 }

該函數就是ksoftirq線程的線程體。第9行__do_softirq()中調用所有軟中斷函數。

我們回過頭來再分析下wakeup_softirqd()，看看ksoftirq線程怎樣被喚醒（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線程的描述符讀到tsk變量中，第6行中判斷ksoftirq線程如果沒有運行的話，第7行喚醒該線程。

第二個地方，中斷處理程序do_IRQ完成處理或者調用irq_exit函數時。下面看看irq_exit代碼（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 }

不用我說了吧，我覺得你一眼就能瞄見了第12行。看下invoke_softirq函數（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值為0，所以該函數也調用了__do_softirq()來執行軟中斷。

還有幾處地方暫時不分析了，以后有空補上。

4.下面來看下__do_softirq()函數（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 } 

在該函數中循環調用的所有的被激活的軟中斷函數。第5行MAX_SOFTIRQ_RESTART值為10，表示最多循環10次（不能讓其他進程等太久），第32行獲取pending表中第一被設置的比特位，第44行開始執行設置過的軟中斷函數。第53行對pending進行右移運算，然后進入下次循環。直到將本輪所有已設置的軟中斷函數全部執行完，退出循環。第59行重新獲得本地cpu的軟中斷掩碼，第61行如果時間沒有超出end而且沒有出現更高優先級的進程并且10次尋環未用完，那么跳回restart，重新for循環。否則，第65行喚醒softirqd內核線程。然后退出本函數。

至此，軟中斷的處理過程就分析完了。