netif_rx

netif_rx相比NAPI是旧的接口, 在中断中进行调用。将skb放入softnet_data的input_queue中。
中断一次只收一个包。
/**
 * netif_rx - post buffer to the network code
 * @skb: buffer to post
 *
 * This function receives a packet from a device driver and queues it for
 * the upper (protocol) levels to process.  It always succeeds. The buffer
 * may be dropped during processing for congestion control or by the
 * protocol layers.
 *
 * return values:
 * NET_RX_SUCCESS (no congestion)
 * NET_RX_DROP     (packet was dropped)
 *
 */

int netif_rx(struct sk_buff *skb)
{
 int ret;

 /* if netpoll wants it, pretend we never saw it */
 if (netpoll_rx(skb))
  return NET_RX_DROP;

 trace_lttng_net_dev_receive(skb);

 net_timestamp_check(netdev_tstamp_prequeue, skb);

 trace_netif_rx(skb);
#ifdef CONFIG_RPS
 if (static_key_false(&rps_needed)) {
  struct rps_dev_flow voidflow, *rflow = &voidflow;
  int cpu;

  preempt_disable();
  rcu_read_lock();

  cpu = get_rps_cpu(skb->dev, skb, &rflow);
  if (cpu < 0)
   cpu = smp_processor_id();

  ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);

  rcu_read_unlock();
  preempt_enable();
 } else
#endif
 {
  unsigned int qtail;
  ret = enqueue_to_backlog(skb, get_cpu(), &qtail);//插入per_cpu(softnet_data, cpu)->input_pkt_queue
  put_cpu();
 }
 return ret;
}

/*
 * enqueue_to_backlog is called to queue an skb to a per CPU backlog
 * queue (may be a remote CPU queue).
 */
static int enqueue_to_backlog(struct sk_buff *skb, int cpu,
         unsigned int *qtail)
{
 struct softnet_data *sd;
 unsigned long flags;

 sd = &per_cpu(softnet_data, cpu);

 local_irq_save(flags);

 rps_lock(sd);

 if (unlikely(skb->dev->rx_hook != NULL)) {
  int ret;
   
  ret = skb->dev->rx_hook(skb);
  if (ret == NET_RX_DROP)
  goto drop;
 }

 if (skb_queue_len(&sd->input_pkt_queue) <= netdev_max_backlog) {   //默认值1000，/proc/sys/net/core/netdev_budget中设置
  if (skb_queue_len(&sd->input_pkt_queue)) { //如果队列不空，直接插入队列尾部，不需要调度softirq
enqueue:
   __skb_queue_tail(&sd->input_pkt_queue, skb);  //skb放入input_pkt_queue的tail
   input_queue_tail_incr_save(sd, qtail);
   rps_unlock(sd);
   local_irq_restore(flags);
   return NET_RX_SUCCESS;
  }

  if (unlikely(skb->dev->rx_hook != NULL)) {
   int ret;
   ret = skb->dev->rx_hook(skb);
   if (ret == NET_RX_DROP)
   goto drop;
  }

  /* Schedule NAPI for backlog device
   * We can use non atomic operation since we own the queue lock
   */
  if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state)) {
   if (!rps_ipi_queued(sd))
    ____napi_schedule(sd, &sd->backlog); // 调度backlog，触发软中断。 backlog是一个伪装的NAPI目的是让NAPI 和none_NAPI具备同样的接口。
  }
  goto enqueue; // 调度backlog之后，skb进入input_pkt_queue的tail
 }

drop:
 sd->dropped++;
 rps_unlock(sd);

 local_irq_restore(flags);

 atomic_long_inc(&skb->dev->rx_dropped);
 kfree_skb(skb);
 return NET_RX_DROP;
}