qdisc的创建过程

register_netdevice会初始化netdev的Tx调度discipline, 缺省使用noop_qdisc

register_netdevice
 --->dev_init_scheduler

void dev_init_scheduler(struct net_device *dev)
{
 dev->qdisc = &noop_qdisc; //缺省为设备配置noop_qdisc
 netdev_for_each_tx_queue(dev, dev_init_scheduler_queue, &noop_qdisc); //缺省为每个队列配置noop_qdisc
  if (dev_ingress_queue(dev))
  dev_init_scheduler_queue(dev, dev_ingress_queue(dev), &noop_qdisc);

 setup_timer(&dev->watchdog_timer, dev_watchdog, (unsigned long)dev);
}
 
dev_open
--->__dev_open
---->dev_activate
---->attach_default_qdiscs
 ---->attach_one_default_qdisc
 为单队列的设备创建pfifo_fast的qdisc
 static void attach_one_default_qdisc(struct net_device *dev,
         struct netdev_queue *dev_queue,
         void *_unused)
{
 struct Qdisc *qdisc = &noqueue_qdisc;

 if (dev->tx_queue_len) {
  qdisc = qdisc_create_dflt(dev_queue,
       &pfifo_fast_ops, TC_H_ROOT);
  if (!qdisc) {
   netdev_info(dev, "activation failed\n");
   return;
  }
 }
 dev_queue->qdisc_sleeping = qdisc;
}
  
dev_open
--->__dev_open
---->dev_activate
---->attach_default_qdiscs
---->qdisc_create_dflt 
 为多队列的设备创建mq_qdisc, 创建完mq_qdisc， 接着调用mq_qdisc_ops->mq_init函数为每个队列创建pfifo_fast_ops的qdisc

struct Qdisc *qdisc_create_dflt(struct netdev_queue *dev_queue,
    struct Qdisc_ops *ops, unsigned int parentid)
{
 struct Qdisc *sch;

 sch = qdisc_alloc(dev_queue, ops);
 if (IS_ERR(sch))
  goto errout;
 sch->parent = parentid;

 if (!ops->init || ops->init(sch, NULL) == 0)
  return sch;

 qdisc_destroy(sch);
errout:
 return NULL;
}
EXPORT_SYMBOL(qdisc_create_dflt);

 dev_open
--->__dev_open
---->dev_activate
---->attach_default_qdiscs
static void attach_default_qdiscs(struct net_device *dev)
{
 struct netdev_queue *txq;
 struct Qdisc *qdisc;

 txq = netdev_get_tx_queue(dev, 0);

 if (!netif_is_multiqueue(dev) || dev->tx_queue_len == 0) {
  netdev_for_each_tx_queue(dev, attach_one_default_qdisc, NULL);
  dev->qdisc = txq->qdisc_sleeping;
  atomic_inc(&dev->qdisc->refcnt);
 } else {
  qdisc = qdisc_create_dflt(txq, &mq_qdisc_ops, TC_H_ROOT);
  if (qdisc) {
   qdisc->ops->attach(qdisc);
   dev->qdisc = qdisc;
  }
 }
}

dev_open函数会调用dev_activate：
a. 为单队列的设备创建pfifo_fast的qdisc
b. 为多队列的设备创建mq_qdisc, 创建完mq_qdisc， 接着调用mq_qdisc_ops->mq_init函数为每个队列创建pfifo_fast_ops的qdisc
dev_open
 --->__dev_open
 ---->dev_activate

void dev_activate(struct net_device *dev)
{
 int need_watchdog;

 /* No queueing discipline is attached to device;
    create default one i.e. pfifo_fast for devices,
    which need queueing and noqueue_qdisc for
    virtual interfaces
  */

 if (dev->qdisc == &noop_qdisc)
  attach_default_qdiscs(dev);

 if (!netif_carrier_ok(dev))
  /* Delay activation until next carrier-on event */
  return;

 need_watchdog = 0;
 netdev_for_each_tx_queue(dev, transition_one_qdisc, &need_watchdog);
 if (dev_ingress_queue(dev))
  transition_one_qdisc(dev, dev_ingress_queue(dev), NULL);

 if (need_watchdog) {
  dev->trans_start = jiffies;
  dev_watchdog_up(dev);
 }
}