从veth看虚拟网络设备的qdisc

void dev_init_scheduler(struct net_device *dev)
{
	dev->qdisc = &noop_qdisc;
	netdev_for_each_tx_queue(dev, dev_init_scheduler_queue, &noop_qdisc);
	dev_init_scheduler_queue(dev, &dev->rx_queue, &noop_qdisc);

	setup_timer(&dev->watchdog_timer, dev_watchdog, (unsigned long)dev);
}

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);
...
}

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 {///multi queue
		qdisc = qdisc_create_dflt(dev, txq, &mq_qdisc_ops, TC_H_ROOT);
		if (qdisc) {
			qdisc->ops->attach(qdisc);
			dev->qdisc = qdisc;
		}
	}
}

static void attach_one_default_qdisc(struct net_device *dev,
				     struct netdev_queue *dev_queue,
				     void *_unused)
{
	struct Qdisc *qdisc;

	if (dev->tx_queue_len) {
		qdisc = qdisc_create_dflt(dev, dev_queue,
					  &pfifo_fast_ops, TC_H_ROOT);
		if (!qdisc) {
			printk(KERN_INFO "%s: activation failed\n", dev->name);
			return;
		}

		/* Can by-pass the queue discipline for default qdisc */
		qdisc->flags |= TCQ_F_CAN_BYPASS;
	} else {
		qdisc =  &noqueue_qdisc;
	}
	dev_queue->qdisc_sleeping = qdisc;
}