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每一个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，当中的设备有输入帧等着被处理。
struct sk_buff*completion_queue;//缓冲区列表。当中缓冲区已成功传输，能够释放掉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。而不是只使用中断事件驱动模型。当收到新的帧时。关中断。再一次处理全然部入口队列。

从内核观点来看。NAPI方法由于中断事件少了。降低了cpu负载。

使用非NAPI的驱动程序的xx_rx()函数一般例如以下：

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) 须要知道数据长度。

第二步将报文数据被复制到缓存区; 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它信息。

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
//仅仅有当新缓冲区为空时。才会触发软中断（napi_schedule()）,假设缓冲区不为空，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增加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);//将该设备增加轮询链表，等待该设备的帧被处理__raise_softirq_irqoff(NET_RX_SOFTIRQ);//终于触发软中断local_irq_restore(flags);
}
EXPORT_SYMBOL(__napi_schedule);

至此中断的上半部完毕，其它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，让下半部去完毕数据的处理工作。
而是用ＮＡＰＩ设备的接受数据时直接触发软中断，不须要通过netif_rx()函数设置好接收队列再触发软中断。

比方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()函数中注冊了收报软中断函数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再度被调度运行。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);//运行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(); } else list_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(); #endif return; 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，对于每一个设备。运行它对应的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 方法有以下原型:
int (*poll)(struct napi_struct *dev, int *budget); 
在初始化时须要加入该方法：
netif_napi_add(netdev, &nic->napi, xx_poll, XX_NAPI_WEIGHT);

NAPI驱动 的 poll 方法实现一般例如以下（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。
无论是非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冊的协议例程发送一个skb。之后数据进入Linux内核协议栈处理。