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qdisc实现分析

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tc qdisc show
echo pfifo > /proc/sys/net/core/default_qdisc
tc qdisc add dev eth0 root pfifo
tc qdisc del dev eth0 root

https://github.com/liucimin/Learning/blob/master/linux%E7%BD%91%E7%BB%9C%E7%9B%B8%E5%85%B3/Tc%20%E7%BD%91%E5%8D%A1%E5%A4%9A%E9%98%9F%E5%88%97%E6%97%B6%E6%AF%8F%E4%B8%AA%E9%98%9F%E5%88%97%E9%85%8D%E7%BD%AE%E5%85%AC%E5%B9%B3%E9%98%9F%E5%88%97sfq.md

http://man7.org/linux/man-pages/man8/tc-fq_codel.8.html


https://blog.csdn.net/one_clouder/article/details/52685249

二层发送中,实现qdisc的主要函数是 __dev_xmit_skb 和 net_tx_action,本篇将分析qdisc实现的原理,但是不涉及qdisc内部的算法,仅对框架进行分析。

1、__dev_xmit_skb 函数

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static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
				 struct net_device *dev,
				 struct netdev_queue *txq)
{
	spinlock_t *root_lock = qdisc_lock(q);
	bool contended;
	int rc;
 
	qdisc_pkt_len_init(skb);
	qdisc_calculate_pkt_len(skb, q);
	/*
	 * Heuristic to force contended enqueues to serialize on a
	 * separate lock before trying to get qdisc main lock.
	 * This permits __QDISC___STATE_RUNNING owner to get the lock more
	 * often and dequeue packets faster.
	 */
	contended = qdisc_is_running(q);  //判断qdisc是否运行
	if (unlikely(contended))
		spin_lock(&q->busylock);
 
	spin_lock(root_lock);
	if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
		kfree_skb(skb);
		rc = NET_XMIT_DROP;
	} else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&  //qisc没有运行,且没有缓存报文,则直接可以发送报文
		   qdisc_run_begin(q)) {
		/*
		 * This is a work-conserving queue; there are no old skbs
		 * waiting to be sent out; and the qdisc is not running -
		 * xmit the skb directly.
		 */
 
		qdisc_bstats_update(q, skb);
 
		if (sch_direct_xmit(skb, q, dev, txq, root_lock, true)) {
			if (unlikely(contended)) {
				spin_unlock(&q->busylock);
				contended = false;
			}
			__qdisc_run(q);       //sch_direct_xmit返回为正值,说明qdisc中有报文待发送,尝试发送缓冲区报文
		} else
			qdisc_run_end(q); //正常发送完成,qdisc停止运行
 
		rc = NET_XMIT_SUCCESS;
	} else {
		rc = q->enqueue(skb, q) & NET_XMIT_MASK;   //qdisc running或者有缓存报文, 则把报文发动qdisc队列中
		if (qdisc_run_begin(q)) {         //尝试启动qdisc,如果qisc成功启动,则尝试发送报文
			if (unlikely(contended)) {
				spin_unlock(&q->busylock);
				contended = false;
			}
			__qdisc_run(q);       //发送qdisc缓冲队列中的报文
		}
	}
	spin_unlock(root_lock);
	if (unlikely(contended))
		spin_unlock(&q->busylock);
	return rc;
}

2、__qdisc_run 函数

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void __qdisc_run(struct Qdisc *q)
{
	int quota = weight_p;
	int packets;
 
	while (qdisc_restart(q, &packets)) {  //循环发送报文
		/*
		 * Ordered by possible occurrence: Postpone processing if
		 * 1. we've exceeded packet quota
		 * 2. another process needs the CPU;
		 */
		quota -= packets;
		if (quota <= 0 || need_resched()) {    //如果配额或需要调度,则触发软中断后退出
			__netif_schedule(q);
			break;
		}
	}
 
	qdisc_run_end(q); //qdisc停止
}

3、qdisc_restart函数

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static inline int qdisc_restart(struct Qdisc *q, int *packets)
{
	struct netdev_queue *txq;
	struct net_device *dev;
	spinlock_t *root_lock;
	struct sk_buff *skb;
	bool validate;
 
	/* Dequeue packet */
	skb = dequeue_skb(q, &validate, packets); //从缓存区中得到待发送的报文,因为流量限制原因,就算缓冲区有报文,也可能返回NULL
	if (unlikely(!skb))
		return 0;
 
	root_lock = qdisc_lock(q);
	dev = qdisc_dev(q);
	txq = skb_get_tx_queue(dev, skb);
 
	return sch_direct_xmit(skb, q, dev, txq, root_lock, validate);    //发送报文
}

4、dequeue_skb函数

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static struct sk_buff *dequeue_skb(struct Qdisc *q, bool *validate,
				   int *packets)
{
	struct sk_buff *skb = q->gso_skb;
	const struct netdev_queue *txq = q->dev_queue;
 
	*packets = 1;
	*validate = true;
	if (unlikely(skb)) {
		/* check the reason of requeuing without tx lock first */
		txq = skb_get_tx_queue(txq->dev, skb);
		if (!netif_xmit_frozen_or_stopped(txq)) {
			q->gso_skb = NULL;
			q->q.qlen--;
		} else
			skb = NULL;
		/* skb in gso_skb were already validated */
		*validate = false;
	} else {
		if (!(q->flags & TCQ_F_ONETXQUEUE) ||
			!netif_xmit_frozen_or_stopped(txq)) {
			skb = q->dequeue(q);           //调用qdisc的dequeue函数获取skb
			if (skb && qdisc_may_bulk(q))<span style="white-space:pre">     </span>//如果还能继续获取skb,则一次性获取多个skb
				try_bulk_dequeue_skb(q, skb, txq, packets);
		}
	}
	return skb;
}

net_tx_action

net_tx_action为报文发送软中断,在处理报文发送软中断时,尝试该CPU softnet_data上所有qdisc发送报文。

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static void net_tx_action(struct softirq_action *h)
{
	struct softnet_data *sd = this_cpu_ptr(&softnet_data);
 
	if (sd->completion_queue) {
		struct sk_buff *clist;
 
		local_irq_disable();
		clist = sd->completion_queue;
		sd->completion_queue = NULL;
		local_irq_enable();
 
		while (clist) {
			struct sk_buff *skb = clist;
			clist = clist->next;
 
			WARN_ON(atomic_read(&skb->users));
			if (likely(get_kfree_skb_cb(skb)->reason == SKB_REASON_CONSUMED))
				trace_consume_skb(skb);
			else
				trace_kfree_skb(skb, net_tx_action);
			__kfree_skb(skb);
		}
	}
 
	if (sd->output_queue) {
		struct Qdisc *head;
 
		local_irq_disable();
		head = sd->output_queue;
		sd->output_queue = NULL;
		sd->output_queue_tailp = &sd->output_queue;
		local_irq_enable();
 
		while (head) {
			struct Qdisc *q = head;
			spinlock_t *root_lock;
 
			head = head->next_sched;
 
			root_lock = qdisc_lock(q);
			if (spin_trylock(root_lock)) {
				smp_mb__before_atomic();
				clear_bit(__QDISC_STATE_SCHED,
					  &q->state);
				qdisc_run(q);         //尝试启动qdisc发送报文
				spin_unlock(root_lock);
			} else {
				if (!test_bit(__QDISC_STATE_DEACTIVATED,
						  &q->state)) {
					__netif_reschedule(q);
				} else {
					smp_mb__before_atomic();
					clear_bit(__QDISC_STATE_SCHED,
						  &q->state);
				}
			}
		}
	}
}