数据包接收系列 — 上半部实现(内核接口)

本文主要内容:网络数据包接收的上半部实现,主要分析内核接口相关部分。

内核版本:2.6.37

Author:zhangskd @ csdn blog

 

上半部的实现

 

接收数据包的上半部处理流程为:

el_interrupt() // 网卡驱动

    |--> el_receive() // 网卡驱动

                |--> netif_rx() // 内核接口

                           |--> enqueue_to_backlog() // 内核接口

 

我们已经分析了网卡驱动相关部分,现在来看下内核接口相关部分:)

 

netif_rx

 

netif_rx()是内核接收网络数据包的入口(目前多数网卡支持新的接口NAPI,后续文章会分析)。

netif_rx()主要调用enqueue_to_backlog()进行后续处理。

/**
 * netif_rx - post buffer to the network code
 * @skb: buffer to post
 * This function receives a packet from a device and queues it for the upper (protocol)
 * levels to process. It always succeeds. The buffer may be dropped during processing
 * for congestion control or by the protocol layers.
 * return values:
 * NET_RX_SUCCESS (no congestion)
 * NET_RX_DROP (packet was dropped)
 */

int netif_rx(struct sk_buff *skb)
{
    int ret;

    /* if netpoll wants it, pretend we never saw it */
    if (netpoll_rx(skb))
        return NET_RX_DROP;

    /* 记录接收时间到skb->tstamp */
    if (netdev_tstamp_prequeue)
       net_timestamp_check(skb);

   trace_netif_rx(skb);
#ifdef CONFIG_RPS
    /* 暂不考虑RPS,后续再分析 */
    ...
#else
    {
        unsigned int qtail;
        ret = enqueue_to_backlog(skb, get_cpu(), &qtail);
        put_cpu();
    }
#endif
    return ret;
}

 

softnet_data

 

每个cpu都有一个softnet_data实例,用于收发数据包。

/* Incoming packets are placed on per-cpu queues */

struct softnet_data {
    struct Qdisc *output_queue; /* 输出包队列 */
    struct Qdisc **output_queue_tailp;
 
     /* 其中设备是处于轮询状态的,即入口队列有新的帧等待处理 */
    struct list_head poll_list;

    struct sk_buff *completion_queue; /* 成功传输的数据包队列 */
    
    /* 处理队列,把input_pkt_queue接入 */
    struct sk_buff_head process_queue;

    /* stats */
    unsigned int processed; /* 处理过的数据包个数 */
    unsigned int time_squeeze; /* poll受限于允许的时间或数据包个数 */
    unsigned int cpu_collision;
    unsigned int received_rps;

#ifdef CONFIG_RPS
    /* 暂不研究RPS */
    ...
#endif

    unsigned dropped; /* 因输入队列满而丢包的个数 */

    /* 输入队列,保存接收到的数据包。
     * 非NAPI使用,支持NAPI的网卡驱动有自己的私有队列。
     */
    struct sk_buff_head input_pkt_queue;
    struct napi_struct backlog; /* 虚拟设备,非NAPI设备共用 */
};

 

定义

/* Device drivers call our routines to queue packets here.
 * We empty the queue in the local softnet handler.
 */
DEFINE_PER_CPU_ALIGNED(struct softnet_data, softnet_data);
EXPORT_PER_CPU_SYMBOL(softnet_data);

 

初始化

/* Initialize the DEV module. At boot time this walks the device list and
 * unhooks any devices that fail to initialise (normally hardware not present)
 * and leaves us with a valid list of present and active devices.
 *
 * This is called single threaded during boot, so no need to take the rtnl semaphore.
 */

static int __init net_dev_init(void)
{
    ...
    /* Initialise the packet receive queues.
     * 初始化每个cpu的softnet_data实例。
     */
    for_each_possible_cpu(i) {
        struct softnet_data *sd = &per_cpu(softnet_data, i);
        memset(sd, 0, sizeof(*sd));
        skb_queue_head_init(&sd->input_pkt_queue);
        skb_queue_head_init(&sd->process_queue);
        sd->completion_queue = NULL;
        INIT_LIST_HEAD(&sd->poll_list);
        sd->output_queue = NULL;
        sd->output_queue_tailp = &sd->output_queue;

#ifdef CONFIG_RPS
        ...
#endif

        sd->backlog.poll = process_backlog; /* 非NAPI的默认轮询函数 */
        sd->backlog.weight = weight_p; /* 64,每次轮询处理数据包个数上限 */
        sd->backlog.gro_list = NULL;
        sd->backlog.gro_count = 0;
    }
    ...
    /* 注册软中断处理函数 */
    open_softirq(NET_TX_SOFTIRQ, net_tx_action);
    open_softirq(NET_RX_SOFTIRQ, net_rx_action);
    ...
}

 

enqueue_to_backlog

 

netif_rx()调用enqueue_to_backlog()来处理。

首先获取当前cpu的softnet_data实例sd,然后:

1. 如果接收队列sd->input_pkt_queue不为空,说明已经有软中断在处理数据包了,

    则不需要再次触发软中断,直接将数据包添加到接收队列尾部即可。

2. 如果接收队列sd->input_pkt_queue为空,说明当前没有软中断在处理数据包,

    则把虚拟设备backlog添加到sd->poll_list中以便进行轮询,最后设置NET_RX_SOFTIRQ

    标志触发软中断。

3. 如果接收队列sd->input_pkt_queue满了,则直接丢弃数据包。

/* queue an skb to a per CPU backlog queue (may be a remote CPU queue). */

static int enqueue_to_backlog(struct sk_buff *skb, int cpu, unsigned int *qtail)
{
    struct softnet_data *sd;
    unsigned long flags;

    sd = &per_cpu(softnet_data, cpu); /* 获取当前cpu上的softnet_data实例 */

    local_irq_save(flags); /* 禁止本地中断 */
    rps_lock(sd);

    if (skb_queue_len(&sd->input_pkt_queue) <= netdev_max_backlog) {

        /* 如果接收队列不为空,则说明已经有软中断在处理数据包了,
         * 则不需要再次触发软中断,直接将数据包添加到接收队列尾部即可。
         */
        if (skb_queue_len(&sd->input_pkt_queue)) {
enqueue:
            __skb_queue_tail(&sd->input_pkt_queue, skb); /* 添加到接收队列尾部 */

            input_queue_tail_incr_save(sd, qtail);
            rps_unlock(sd);

            local_irq_restore(flags); /* 恢复本地中断 */
            return NET_RX_SUCCESS;
        }

        /* Schedule NAPI for backlog device.
         * 如果接收队列为空,说明当前没有软中断在处理数据包,
         * 把虚拟设备backlog添加到sd->poll_list中以便进行轮询,
         * 最后设置NET_RX_SOFTIRQ标志触发软中断。
         */
        if (! __test_and_set_bit(NAPT_STATE_SCHED, &sd->backlog.state)) {
            if (! rps_ipi_queued(sd))
                ____napi_schedule(sd, &sd->backlog);
        }
        goto enqueue;
    }

    sd->dropped++; /* 如果接收队列满了就直接丢弃 */
    rps_unlock(sd);
    local_irq_restore(flags); /* 恢复本地中断 */
    atomic_long_inc(&skb->dev->rx_dropped);
    kfree_skb(skb); /* 释放数据包 */
    return NET_RX_DROP;
}

int netdev_tstamp_prequeue = 1; /* 记录接收时间 */
int netdev_max_backlog = 1000; /* 接收队列的最大长度 */

 

napi_struct代表一个虚拟设备,用于兼容非NAPI的驱动。

/* Structure for NAPI scheduling similar to tasklet but with weighting */

struct napi_struct {
    /* The poll_list must only be managed by the entity which changes the
     * state of the NAPI_STATE_SCHED bit. This means whoever atomically
     * sets that bit can add this napi_struct to the per-cpu poll_list, and
     * whoever clears that bit can remove from the list right before clearing the bit.
     */
    struct list_head poll_list; /* 用于加入处于轮询状态的设备队列 */
    unsigned long state; /* 虚拟设备的状态 */
    int weight; /* 每次处理的最大数量,非NAPI为weight_p,默认为64 */
    int (*poll) (struct napi_struct *, int); /* 此设备的轮询方法,默认为process_backlog() */

#ifdef CONFIG_NETPOLL
    ...
#endif

    unsigned int gro_count;
    struct net_device *dev;
    struct list_head dev_list;
    struct sk_buff *gro_list;
    struct sk_buff *skb;
};
static inline void ____napi_schedule(struct softnet_data *sd, struct napi_struct *napi)
{
    /* 把napi_struct添加到softnet_data的poll_list中 */
    list_add_tail(&napi->poll_list, &sd->poll_list);
    __raise_softirq_irqoff(NET_RX_SOFTIRQ); /* 设置软中断标志位 */
}


 

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