napi_schedule

当需要对本设备进行poll的时候，调用__napi_schedule 将设备的napi 的poll_list 挂在每cpu变量 softnet_data的poll_list下面。
在net_rx_action函数中， 会轮讯softnet_data的poll_list，即调用设备的napi->poll 函数。

/* Called with irq disabled */
static inline void ____napi_schedule(struct softnet_data *sd,
         struct napi_struct *napi)
{
 list_add_tail(&napi->poll_list, &sd->poll_list); /*  napi 的poll_list 挂在 softnet_data的poll_list */
 __raise_softirq_irqoff(NET_RX_SOFTIRQ);       /*  触发softirq，rangsoftirq来执行poll 函数 */}

/**
 * __napi_schedule - schedule for receive
 * @n: entry to schedule
 *
 * The entry's receive function will be scheduled to run
 */
void __napi_schedule(struct napi_struct *n) /* 对____napi_schedule进行了一层封装，主要是关闭中断  */
{
 unsigned long flags;

 local_irq_save(flags);
 ____napi_schedule(&__get_cpu_var(softnet_data), n);
 local_irq_restore(flags);
}
EXPORT_SYMBOL(__napi_schedule);

/**
 * napi_schedule - schedule NAPI poll
 * @n: napi context
 *
 * Schedule NAPI poll routine to be called if it is not already
 * running.
 */
static inline void napi_schedule(struct napi_struct *n)
{
 if (napi_schedule_prep(n))  /*  判断是否可以调度  */
  __napi_schedule(n);
}


/**
 * napi_schedule_prep - check if napi can be scheduled
 * @n: napi context
 *
 * Test if NAPI routine is already running, and if not mark
 * it as running.  This is used as a condition variable
 * insure only one NAPI poll instance runs.  We also make
 * sure there is no pending NAPI disable.
 */
static inline bool napi_schedule_prep(struct napi_struct *n)
{
 return !napi_disable_pending(n) &&                                   /* 没有disable */
  !test_and_set_bit(NAPI_STATE_SCHED, &n->state);           /*  not running */
}