Linux内核硬中断 / 软中断的原理和实现

/**
 * irq: 要分配的中断号
 * handler: 要注册的中断处理函数
 * flags: 标志(一般为0)
 * name: 设备名(dev->name)
 * dev: 设备(struct net_device *dev)，作为中断处理函数的参数
 * 成功返回0
 */
 
int request_irq(unsigned int irq, irq_handler_t handler, unsigned long flags, 
    const char *name, void *dev);

typedef irqreturn_t (*irq_handler_t) (int, void *);
 
/**
 * enum irqreturn
 * @IRQ_NONE: interrupt was not from this device
 * @IRQ_HANDLED: interrupt was handled by this device
 * @IRQ_WAKE_THREAD: handler requests to wake the handler thread
 */
enum irqreturn {
    IRQ_NONE,
    IRQ_HANDLED,
    IRQ_WAKE_THREAD,
};
typedef enum irqreturn irqreturn_t;
#define IRQ_RETVAL(x) ((x) != IRQ_NONE)

/**
 * free_irq - free an interrupt allocated with request_irq
 * @irq: Interrupt line to free
 * @dev_id: Device identity to free
 *
 * Remove an interrupt handler. The handler is removed and if the
 * interrupt line is no longer in use by any driver it is disabled.
 * On a shared IRQ the caller must ensure the interrupt is disabled
 * on the card it drives before calling this function. The function does
 * not return until any executing interrupts for this IRQ have completed.
 * This function must not be called from interrupt context.
 */
 
void free_irq(unsigned int irq, void *dev_id);

struct softirq_action {
    void (*action) (struct softirq_action *); /* 软中断的处理函数 */
};

static struct softirq_action softirq_vec[NR_SOFTIRQS];
 
enum {
    HI_SOFTIRQ = 0, /* 优先级高的tasklets */
    TIMER_SOFTIRQ, /* 定时器的下半部 */
    NET_TX_SOFTIRQ, /* 发送网络数据包 */
    NET_RX_SOFTIRQ, /* 接收网络数据包 */
    BLOCK_SOFTIRQ, /* BLOCK装置 */
    BLOCK_IOPOLL_SOFTIRQ,
    TASKLET_SOFTIRQ, /* 正常优先级的tasklets */
    SCHED_SOFTIRQ, /* 调度程序 */
    HRTIMER_SOFTIRQ, /* 高分辨率定时器 */
    RCU_SOFTIRQ, /* RCU锁定 */
    NR_SOFTIRQS /* 10 */
};

/**
 * @nr: 软中断的索引号
 * @action: 软中断的处理函数
 */
 
void open_softirq(int nr, void (*action) (struct softirq_action *))
{
    softirq_vec[nr].action = action;
}

void raise_softirq(unsigned int nr)
{
    unsigned long flags;
    local_irq_save(flags);
    raise_softirq_irqoff(nr);
    local_irq_restore(flags);
}
 
/* This function must run with irqs disabled */
inline void rasie_softirq_irqsoff(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()) /* 如果不处于硬中断或软中断 */
        wakeup_softirqd(void); /* 唤醒ksoftirqd/n进程 */
}

static inline void __rasie_softirq_irqoff(unsigned int nr)
{
    trace_softirq_raise(nr);
    or_softirq_pending(1UL << nr);
}
 
typedef struct {
    unsigned int __softirq_pending;
    unsigned int __nmi_count; /* arch dependent */
} irq_cpustat_t;
 
irq_cpustat_t irq_stat[];
#define __IRQ_STAT(cpu, member) (irq_stat[cpu].member)
#define or_softirq_pending(x) percpu_or(irq_stat.__softirq_pending, (x))
#define local_softirq_pending() percpu_read(irq_stat.__softirq_pending)

static void wakeup_softirqd(void)
{
    /* Interrupts are disabled: no need to stop preemption */
    struct task_struct *tsk = __get_cpu_var(ksoftirqd);
 
    if (tsk && tsk->state != TASK_RUNNING)
        wake_up_process(tsk);
}

asmlinkage void do_softirq(void)
{
    __u32 pending;
    unsigned long flags;
 
    /* 如果当前已处于硬中断或软中断中，直接返回 */
    if (in_interrupt()) 
        return;
 
    local_irq_save(flags);
    pending = local_softirq_pending();
    if (pending) /* 如果有激活的软中断 */
        __do_softirq(); /* 处理函数 */
    local_irq_restore(flags);
}

/* We restart softirq processing MAX_SOFTIRQ_RESTART times,
 * and we fall back to softirqd after that.
 * This number has been established via experimentation.
 * The two things to balance is latency against fairness - we want
 * to handle softirqs as soon as possible, but they should not be
 * able to lock up the box.
 */
asmlinkage void __do_softirq(void)
{
    struct softirq_action *h;
    __u32 pending;
    /* 本函数能重复触发执行的次数，防止占用过多的cpu时间 */
    int max_restart = MAX_SOFTIRQ_RESTART;
    int cpu;
 
    pending = local_softirq_pending(); /* 激活的软中断位图 */
    account_system_vtime(current);
    /* 本地禁止当前的软中断 */
    __local_bh_disable((unsigned long)__builtin_return_address(0), SOFTIRQ_OFFSET);
    lockdep_softirq_enter(); /* current->softirq_context++ */
    cpu = smp_processor_id(); /* 当前cpu编号 */
 
restart:
    /* Reset the pending bitmask before enabling irqs */
    set_softirq_pending(0); /* 重置位图 */
    local_irq_enable();
    h = softirq_vec;
    do {
        if (pending & 1) {
            unsigned int vec_nr = h - softirq_vec; /* 软中断索引 */
            int prev_count = preempt_count();
            kstat_incr_softirqs_this_cpu(vec_nr);
 
            trace_softirq_entry(vec_nr);
            h->action(h); /* 调用软中断的处理函数 */
            trace_softirq_exit(vec_nr);
 
            if (unlikely(prev_count != preempt_count())) {
                printk(KERN_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());
            }
            rcu_bh_qs(cpu);
        }
        h++;
        pending >>= 1;
    } while(pending);
 
    local_irq_disable();
    pending = local_softirq_pending();
    if (pending & --max_restart) /* 重复触发 */
        goto restart;
 
    /* 如果重复触发了10次了，接下来唤醒ksoftirqd/n内核线程来处理 */
    if (pending)
        wakeup_softirqd(); 
 
    lockdep_softirq_exit();
    account_system_vtime(current);
    __local_bh_enable(SOFTIRQ_OFFSET);
}

static int run_ksoftirqd(void *__bind_cpu)
{
    set_current_state(TASK_INTERRUPTIBLE);
    current->flags |= PF_KSOFTIRQD; /* I am ksoftirqd */
 
    while(! kthread_should_stop()) {
        preempt_disable();
 
        if (! local_softirq_pending()) { /* 如果没有要处理的软中断 */
            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))/* 被要求释放cpu */
                 goto wait_to_die;
 
            do_softirq(); /* 软中断的统一处理函数 */
 
            preempt_enable_no_resched();
            cond_resched();
            preempt_disable();
            rcu_note_context_switch((long)__bind_cpu);
        }
 
        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;
}