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
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()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;
}
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);
...
}
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 */
}