linux内核数据包转发流程（三）：网卡帧接收分析

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每个cpu都有队列来处理接收到的帧，都有其数据结构来处理入口和出口流量，因此，不同cpu之间没有必要使用上锁机制，。此队列数据结构为softnet_data(定义在include/linux/netdevice.h中):

/* * Incoming packets are placed on per-cpu queues so that * no locking is needed. */struct softnet_data{struct Qdisc *output_queue; struct sk_buff_headinput_pkt_queue;//有数据要传输的设备列表struct list_headpoll_list; //双向链表，其中的设备有输入帧等着被处理。struct sk_buff*completion_queue;//缓冲区列表，其中缓冲区已成功传输，可以释放掉struct napi_structbacklog;};

此结构字段可用于传输和接收。换而言之，NET_RX_SOFTIRQ和NET_TX_SOFTIRQ软IRQ都引用此结构。入口帧会排入input_pkt_queue(NAPI有所不同)。

softnet_data是在net_dev_init函数中初始化的：

/* *       This is called single threaded during boot, so no need *       to take the rtnl semaphore. */static int __init net_dev_init(void){int i, rc = -ENOMEM;....../** Initialise the packet receive queues.*/for_each_possible_cpu(i) {struct softnet_data *queue;queue = &per_cpu(softnet_data, i);skb_queue_head_init(&queue->input_pkt_queue);queue->completion_queue = NULL;INIT_LIST_HEAD(&queue->poll_list);queue->backlog.poll = process_backlog;queue->backlog.weight = weight_p;queue->backlog.gro_list = NULL;queue->backlog.gro_count = 0;}......open_softirq(NET_TX_SOFTIRQ, net_tx_action);open_softirq(NET_RX_SOFTIRQ, net_rx_action);......}

非NAPI设备驱动会为其所接收的每一个帧产生一个中断事件，在高流量负载下，会花掉大量时间处理中断事件，造成资源浪费。而NAPI驱动混合了中断事件和轮询，在高流量负载下其性能会比旧方法要好。
NAPI主要思想是混合使用中断事件和轮询，而不是仅仅使用中断事件驱动模型。当收到新的帧时，关中断，再一次处理完所有入口队列。从内核观点来看，NAPI方法因为中断事件少了，减少了cpu负载。
使用非NAPI的驱动程序的xx_rx()函数一般如下：

void xx_rx(){struct sk_buff *skb;skb = dev_alloc_skb(pkt_len + 5);if (skb != NULL) {skb_reserve(skb, 2);/* Align IP on 16 byte boundaries *//*memcpy(skb_put(skb, 2), pkt, pkt_len);*/ //copy data to skbskb->protocol = eth_type_trans(skb, dev);netif_rx(skb);}}

第一步是分配一个缓存区来保存报文。 注意缓存分配函数 (dev_alloc_skb) 需要知道数据长度。

第二步将报文数据被拷贝到缓存区; skb_put  函数更新缓存中的数据末尾指针并返回指向新建空间的指针。

第三步提取协议标识及获取其他信息。

最后调用netif_rx(skb)做进一步处理，该函数一般定义在net/core/dev.c中。

int netif_rx(struct sk_buff *skb){struct softnet_data *queue;unsigned long flags;/* if netpoll wants it, pretend we never saw it */if (netpoll_rx(skb))return NET_RX_DROP;if (!skb->tstamp.tv64)net_timestamp(skb);/** The code is rearranged so that the path is the most* short when CPU is congested, but is still operating.*/local_irq_save(flags);queue = &__get_cpu_var(softnet_data);__get_cpu_var(netdev_rx_stat).total++;if (queue->input_pkt_queue.qlen <= netdev_max_backlog) {//是否还有空间,netdev_max_backlog一般为300//只有当新缓冲区为空时，才会触发软中断（napi_schedule()）,如果缓冲区不为空，软中断已被触发，没有必要再去触发一次。if (queue->input_pkt_queue.qlen) {enqueue:__skb_queue_tail(&queue->input_pkt_queue, skb);//这里是关键之处，将skb加入input_pkt_queue之中。local_irq_restore(flags);return NET_RX_SUCCESS;}napi_schedule(&queue->backlog);//触发软中断goto enqueue;}__get_cpu_var(netdev_rx_stat).dropped++;local_irq_restore(flags);kfree_skb(skb);return NET_RX_DROP;}EXPORT_SYMBOL(netif_rx);

static inline void napi_schedule(struct napi_struct *n){if (napi_schedule_prep(n))__napi_schedule(n);}

void __napi_schedule(struct napi_struct *n){unsigned long flags;local_irq_save(flags);list_add_tail(&n->poll_list, &__get_cpu_var(softnet_data).poll_list);//将该设备加入轮询链表，等待该设备的帧被处理__raise_softirq_irqoff(NET_RX_SOFTIRQ);//最终触发软中断local_irq_restore(flags);}EXPORT_SYMBOL(__napi_schedule);

至此中断的上半部完成，其他的工作交由下半部来实现。napi_schedule(&queue->backlog)函数将有等待的接收数据包的NIC链入softnet_data的poll_list队列，然后触发软中断，让下半部去完成数据的处理工作。
而是用ＮＡＰＩ设备的接受数据时直接触发软中断，不需要通过netif_rx()函数设置好接收队列再触发软中断。比如e100硬中断处理函数为：

static irqreturn_t e100_intr(int irq, void *dev_id){struct net_device *netdev = dev_id;struct nic *nic = netdev_priv(netdev);u8 stat_ack = ioread8(&nic->csr->scb.stat_ack);DPRINTK(INTR, DEBUG, "stat_ack = 0x%02X\n", stat_ack);if (stat_ack == stat_ack_not_ours ||/* Not our interrupt */   stat_ack == stat_ack_not_present)/* Hardware is ejected */return IRQ_NONE;/* Ack interrupt(s) */iowrite8(stat_ack, &nic->csr->scb.stat_ack);/* We hit Receive No Resource (RNR); restart RU after cleaning */if (stat_ack & stat_ack_rnr)nic->ru_running = RU_SUSPENDED;if (likely(napi_schedule_prep(&nic->napi))) {e100_disable_irq(nic);__napi_schedule(&nic->napi);//此处触发软中断}return IRQ_HANDLED;}

在前面我们已经知道在net_dev_init()函数中注册了收报软中断函数net_rx_action(),当软中断被触发之后，该函数将被调用。
net_rx_action()函数为：

static void net_rx_action(struct softirq_action *h){struct list_head *list = &__get_cpu_var(softnet_data).poll_list;unsigned long time_limit = jiffies + 2;int budget = netdev_budget;void *have;local_irq_disable();while (!list_empty(list)) {struct napi_struct *n;int work, weight;/* If softirq window is exhuasted then punt. * Allow this to run for 2 jiffies since which will allow * an average latency of 1.5/HZ. */if (unlikely(budget <= 0 || time_after(jiffies, time_limit)))//入口队列仍然有缓冲区，软IRQ再度被调度执行。goto softnet_break;local_irq_enable();/* Even though interrupts have been re-enabled, this * access is safe because interrupts can only add new * entries to the tail of this list, and only ->poll() * calls can remove this head entry from the list. */n = list_entry(list->next, struct napi_struct, poll_list);have = netpoll_poll_lock(n);weight = n->weight;/* This NAPI_STATE_SCHED test is for avoiding a race * with netpoll's poll_napi().  Only the entity which * obtains the lock and sees NAPI_STATE_SCHED set will * actually make the ->poll() call.  Therefore we avoid * accidently calling ->poll() when NAPI is not scheduled. */work = 0;if (test_bit(NAPI_STATE_SCHED, &n->state)) {work = n->poll(n, weight);//执行poll函数，返回已处理的帧trace_napi_poll(n);}WARN_ON_ONCE(work > weight);budget -= work;local_irq_disable();/* Drivers must not modify the NAPI state if they * consume the entire weight.  In such cases this code * still "owns" the NAPI instance and therefore can * move the instance around on the list at-will. */if (unlikely(work == weight)) {//队列被清空。调用napi_complete()负责此事。if (unlikely(napi_disable_pending(n))) {local_irq_enable();napi_complete(n);local_irq_disable();} elselist_move_tail(&n->poll_list, list);}netpoll_poll_unlock(have);}out:local_irq_enable();#ifdef CONFIG_NET_DMA/* * There may not be any more sk_buffs coming right now, so push * any pending DMA copies to hardware */dma_issue_pending_all();#endifreturn;softnet_break:__get_cpu_var(netdev_rx_stat).time_squeeze++;__raise_softirq_irqoff(NET_RX_SOFTIRQ);goto out;}

由上可见，下半部的主要工作是遍历有数据帧等待接收的设备链表，对于每个设备，执行它相应的poll函数。
对非NAPI设备来说，poll函数在net_dev_init()函数中初始化为process_backlog()。
process_backlog()函数定义为：

static int process_backlog(struct napi_struct *napi, int quota){int work = 0;struct softnet_data *queue = &__get_cpu_var(softnet_data);unsigned long start_time = jiffies;napi->weight = weight_p;do {struct sk_buff *skb;local_irq_disable();skb = __skb_dequeue(&queue->input_pkt_queue);if (!skb) {__napi_complete(napi);local_irq_enable();break;}local_irq_enable();netif_receive_skb(skb);} while (++work < quota && jiffies == start_time);return work;}

对NAPI设备来的说，驱动程序必须提供一个poll方法,poll 方法有下面原型:
int (*poll)(struct napi_struct *dev, int *budget); 
在初始化时需要添加该方法：
netif_napi_add(netdev, &nic->napi, xx_poll, XX_NAPI_WEIGHT);

NAPI驱动 的 poll 方法实现一般如下（借用《Linux设备驱动程序》中代码，内核有点没对上，懒得去写了）:

static int xx_poll(struct net_device *dev, int *budget){    int npackets = 0, quota = min(dev->quota, *budget);    struct sk_buff *skb;    struct xx_priv *priv = netdev_priv(dev);    struct xx_packet *pkt;    while (npackets < quota && priv->rx_queue) {        pkt = xx_dequeue_buf(dev);        skb = dev_alloc_skb(pkt->datalen + 2);        if (! skb) {            if (printk_ratelimit())                printk(KERN_NOTICE "xx: packet dropped\n"); priv->stats.rx_dropped++; xx_release_buffer(pkt); continue;        }        memcpy(skb_put(skb, pkt->datalen), pkt->data, pkt->datalen);        skb->dev = dev;        skb->protocol = eth_type_trans(skb, dev);        skb->ip_summed = CHECKSUM_UNNECESSARY; /* don't check it */        netif_receive_skb(skb);        /* Maintain stats */        npackets++;        priv->stats.rx_packets++;        priv->stats.rx_bytes += pkt->datalen;        xx_release_buffer(pkt);    }    /* If we processed all packets, we're done; tell the kernel and reenable ints */    *budget -= npackets;    dev->quota -= npackets;    if (! priv->rx_queue) {        netif_rx_complete(dev);        xx_rx_ints(dev, 1);        return 0;    }    /* We couldn't process everything. */    return 1;}

NAPI驱动提供自己的poll函数和私有队列。
不管是非NAPI或NAPI，他们的poll函数最后都会调用netif_receive_skb(skb)来处理接收到的帧。该函数会想各个已注册的协议例程发送一个skb，之后数据进入Linux内核协议栈处理。