1 软中断的概念

软中断是一种软件实现的机制，而非硬件实现的中断。软中断属于中断上下文，当软中断在执行时，task无法打断软中断执行。

• 软中断的类型是静态定义的，内核不建议新增加软中断类型
• 软中断的回调函数是在开中断的情况下执行的
• 软中断的执行点：在硬中断处理函数返回之前irq_exit()
• 软中断属于中断上下文，软中断可以抢占进程上下文
• 同一类型的软中断可以在多个处理器上并行执行
• tasklet属于一种特殊的软中断，相同的tasklet在整个系统上只有一个可以执行，但是不同的tasklet可以同时在不同的处理器上运行(tasklet
  is running only on one CPU simultaneously，different tasklets
  may be run simultaneously on different CPUs.)

2 软中断执行的时机

2.1 软中断执行时机

中断处理函数执行完成，返回中断中断现场之前(irq_exit)会去检查：

• 检查当前是否处于中断上下文in_interrupt()
• 检查系统是否有待处理的软中断local_softirq_pending()
• 调用invoke_softirq()去处理软中断
  如果当前不处于中断上下文并且有待处理的软中断，则会调用invoke_softirq()去处理软中断。

2.2 irq_exit

代码路径：kernel/softirq.c

/** Exit an interrupt context. Process softirqs if needed and possible:*/
void irq_exit(void)
{#ifndef __ARCH_IRQ_EXIT_IRQS_DISABLEDlocal_irq_disable();
#elseWARN_ON_ONCE(!irqs_disabled());
#endifaccount_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! */
}

2.3 invoke_softirq

代码路径：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();}
}

3 软中断关键函数和数据结构

3.1 软中断的类型

代码路径：include/linux/interrupt.h

/* PLEASE, avoid to allocate new softirqs, if you need not _really_ highfrequency threaded job scheduling. For almost all the purposestasklets are more than enough. F.e. all serial device BHs etal. should be converted to tasklets, not to softirqs.*/enum
{HI_SOFTIRQ=0,TIMER_SOFTIRQ,NET_TX_SOFTIRQ,NET_RX_SOFTIRQ,BLOCK_SOFTIRQ,BLOCK_IOPOLL_SOFTIRQ,TASKLET_SOFTIRQ,SCHED_SOFTIRQ,HRTIMER_SOFTIRQ, /* Unused, but kept as tools rely on thenumbering. Sigh! */RCU_SOFTIRQ,    /* Preferable RCU should always be the last softirq */NR_SOFTIRQS
};

3.2 注册软中断

代码路径：kernel/softirq.c

3.2.1 软中断注册函数定义

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

3.2.2 软中断注册样例

示例代码路径：net/core/dev.c

/**      Initialize the DEV module. At boot time this walks the device list and*      unhooks any devices that fail to initialise (normally hardware not*      present) and leaves us with a valid list of present and active devices.**//**       This is called single threaded during boot, so no need*       to take the rtnl semaphore.*/
static int __init net_dev_init(void)
{...open_softirq(NET_TX_SOFTIRQ, net_tx_action);open_softirq(NET_RX_SOFTIRQ, net_rx_action);...
}

3.3 触发软中断

触发软中断其实是在中断中通过调用raise_softirq_irqoff或者raise_softirq设置_softirq_pending位图，当从异常返回执行到irq_exit()函数时，会检查_softirq_pending是否有设置的软中断待处理。

3.3.1 触发软中断处理的接口

代码路径：kernel/softirq.c

/** 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();
}void raise_softirq(unsigned int nr)
{unsigned long flags;local_irq_save(flags);raise_softirq_irqoff(nr);local_irq_restore(flags);
}

3.3.2 触发软中断处理的示例

在触发软中断处理时，通常会使用raise_softirq_irqoff和raise_softirq函数

   6     51  block/blk-softirq.c <<<unknown>>>raise_softirq_irqoff(BLOCK_SOFTIRQ);7     94  block/blk-softirq.c <<<unknown>>>raise_softirq_irqoff(BLOCK_SOFTIRQ);8    148  block/blk-softirq.c <<<unknown>>>raise_softirq_irqoff(BLOCK_SOFTIRQ);12    784  drivers/irqchip/irq-gic.c <<<unknown>>>static void gic_raise_softirq(const struct cpumask *mask, unsigned int irq)13   1157  drivers/irqchip/irq-gic.c <<<unknown>>>set_smp_cross_call(gic_raise_softirq);14    279  drivers/irqchip/irq-hip04.c <<<unknown>>>static void hip04_raise_softirq(const struct cpumask *mask, unsigned int irq)16   2288  drivers/net/wireless/rockchip_wlan/cywdhd/bcmdhd/dhd_linux.c <<<unknown>>>* This function will essentially invoke __raise_softirq_irqoff(NET_RX_SOFTIRQ)17    131  drivers/net/wireless/rockchip_wlan/cywdhd/bcmdhd/include/linuxver.h <<<unknown>>>cpu_raise_softirq(smp_processor_id(), NET_RX_SOFTIRQ)18    123  drivers/net/wireless/rockchip_wlan/cywdhd/bcmdhd/wl_iw.c <<<unknown>>>cpu_raise_softirq(smp_processor_id(), NET_RX_SOFTIRQ)19   1025  drivers/net/wireless/rockchip_wlan/rkwifi/bcmdhd/dhd_linux_lb.c <<<unknown>>>* This function will essentially invoke __raise_softirq_irqoff(NET_RX_SOFTIRQ)20   1042  drivers/net/wireless/rockchip_wlan/rkwifi/bcmdhd/dhd_linux_lb.c <<<unknown>>>raise_softirq(NET_RX_SOFTIRQ);

3.4 软中断处理函数__do_softirq

__do_softirq函数需要重点关注一下几个部分：

• pending = local_softirq_pending();获取有哪些软中断被置位
• h = softirq_vec;
• h += softirq_bit - 1;
• h->action(h);
• 当中断不满足跳出条件时，会一直执行goto restart去执行软中断处理函数；软中断退出的条件如下所示：
  • time_before(jiffies, end) /* end = jiffies + MAX_SOFTIRQ_TIME 软中断允许的最长占用时间为2s */
  • !need_resched() /* 检查TIF_NEED_RESCHED*/
  • –max_restart /* max_restart = MAX_SOFTIRQ_RESTART； 调度次数最多为MAX_SOFTIRQ_RESTART */
• 当退出软中断时还有需要处理的软中断则会通过调用wakeup_softirqd()函数去唤醒softirqd线程去处理剩余的软中断。

asmlinkage __visible void __softirq_entry __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(0);local_irq_enable();h = softirq_vec;while ((softirq_bit = ffs(pending))) {unsigned int vec_nr;int prev_count;h += softirq_bit - 1;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();local_irq_disable();pending = local_softirq_pending();if (pending) {   /* 存在软中断需要处理 */if (time_before(jiffies, end) && !need_resched() &&  /* 软中断调度时间最长为2s， 当前系统不需要调度并且最大执行次数没有超过MAX_SOFTIRQ_RESTART的限制 */--max_restart)goto restart;   /* 继续处理剩余的软中断 */wakeup_softirqd();  /* 剩余尚未来的及处理的软中断通过唤醒softirqd线程去处理 */}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);
}

4 tasklet

tasklet是一种特殊的软中断TASKLET_SOFTIRQ

4.1 tasklet数据结构

代码路径：include/linux/interrupt.h

/* Tasklets --- multithreaded analogue of BHs.Main feature differing them of generic softirqs: taskletis running only on one CPU simultaneously.Main feature differing them of BHs: different taskletsmay be run simultaneously on different CPUs.Properties:* If tasklet_schedule() is called, then tasklet is guaranteedto be executed on some cpu at least once after this.* If the tasklet is already scheduled, but its execution is still notstarted, it will be executed only once.* If this tasklet is already running on another CPU (or schedule is calledfrom tasklet itself), it is rescheduled for later.* Tasklet is strictly serialized wrt itself, but notwrt another tasklets. If client needs some intertask synchronization,he makes it with spinlocks.*/struct tasklet_struct
{struct tasklet_struct *next;unsigned long state;atomic_t count;void (*func)(unsigned long);unsigned long data;
};

4.2 声明一个tasklet

4.2.1 DECLARE_TASKLET

#define DECLARE_TASKLET(name, func, data) \
struct tasklet_struct name = { NULL, 0, ATOMIC_INIT(0), func, data }

4.2.2 tasklet_init

void tasklet_init(struct tasklet_struct *t,void (*func)(unsigned long), unsigned long data)
{t->next = NULL;t->state = 0;atomic_set(&t->count, 0);t->func = func;t->data = data;
}
EXPORT_SYMBOL(tasklet_init);

4.2.3 tasklet_init使用样例

   1   1843  drivers/atm/eni.c <<<unknown>>>tasklet_init(&eni_dev->task,eni_tasklet,(unsigned long) dev);2   2043  drivers/atm/fore200e.c <<<unknown>>>tasklet_init(&fore200e->tx_tasklet, fore200e_tx_tasklet, (unsigned long)fore200e);3   2044  drivers/atm/fore200e.c <<<unknown>>>tasklet_init(&fore200e->rx_tasklet, fore200e_rx_tasklet, (unsigned long)fore200e);4    386  drivers/atm/he.c <<<unknown>>>tasklet_init(&he_dev->tasklet, he_tasklet, (unsigned long) he_dev);5   1304  drivers/atm/solos-pci.c <<<unknown>>>tasklet_init(&card->tlet, solos_bh, (unsigned long)card);6    903  drivers/block/umem.c <<<unknown>>>tasklet_init(&card->tasklet, process_page, (unsigned long)card);7    986  drivers/block/xsysace.c <<<unknown>>>tasklet_init(&ace->fsm_tasklet, ace_fsm_tasklet, (unsigned long)ace);8   2820  drivers/char/ipmi/ipmi_msghandler.c <<<unknown>>>tasklet_init(&intf->recv_tasklet,9    838  drivers/char/mmtimer.c <<<unknown>>>tasklet_init(&timers[node].tasklet, mmtimer_tasklet,10   1198  drivers/crypto/amcc/crypto4xx_core.c <<<unknown>>>tasklet_init(&core_dev->tasklet, crypto4xx_bh_tasklet_cb,11   1357  drivers/crypto/atmel-aes.c <<<unknown>>>tasklet_init(&aes_dd->done_task, atmel_aes_done_task,12   1359  drivers/crypto/atmel-aes.c <<<unknown>>>tasklet_init(&aes_dd->queue_task, atmel_aes_queue_task,13   1370  drivers/crypto/atmel-sha.c <<<unknown>>>tasklet_init(&sha_dd->done_task, atmel_sha_done_task,14   1378  drivers/crypto/atmel-tdes.c <<<unknown>>>tasklet_init(&tdes_dd->done_task, atmel_tdes_done_task,15   1380  drivers/crypto/atmel-tdes.c <<<unknown>>>tasklet_init(&tdes_dd->queue_task, atmel_tdes_queue_task,16    587  drivers/crypto/bfin_crc.c <<<unknown>>>tasklet_init(&crc->done_task, bfin_crypto_crc_done_task, (unsigned long)crc);

4.3 调度一个tasklet

__tasklet_schedule会通过调用raise_softirq_irqoff(TASKLET_SOFTIRQ);去设置_softirq_pending，

void __tasklet_schedule(struct tasklet_struct *t)
{unsigned long flags;local_irq_save(flags);t->next = NULL;*__this_cpu_read(tasklet_vec.tail) = t;__this_cpu_write(tasklet_vec.tail, &(t->next));raise_softirq_irqoff(TASKLET_SOFTIRQ);local_irq_restore(flags);
}
EXPORT_SYMBOL(__tasklet_schedule);

4.4 如何在驱动里面定义一个tasklet

• 首先要初始化一个tasklet，可以通过DECLARE_TASKLET或者tasklet_init去定义一个tasklet
• 实现tasklet的处理函数
• 调度自己的tasklet

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