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)。
/* * 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 skb skb->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(); } 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。
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