Understanding Linux Network Internals

softnet_data Structure

We will see in Chapter 10 that each CPU has its own queue for incoming frames . Because each CPU has its own data structure to manage ingress and egress traffic, there is no need for any locking among different CPUs. The data structure for this queue, softnet_data, is defined in include/linux/netdevice.h as follows:

struct softnet_data
{
    int            throttle;
    int            cng_level;
    int            avg_blog;
    struct sk_buff_head    input_pkt_queue;
    struct list_head       poll_list;
    struct net_device      *output_queue;
    struct sk_buff         *completion_queue;
    struct net_device      backlog_dev;
}

The structure includes both fields used for reception and fields used for transmission. In other words, both the NET_RX_SOFTIRQ and NET_TX_SOFTIRQ softirqs refer to the structure. Ingress frames are queued to input_pkt_queue, ^[*] and egress frames are placed into the specialized queues handled by Traffic Control (the QoS layer) instead of being handled by softirqs and the softnet_data structure, but softirqs are still used to clean up transmitted buffers afterward, to keep that task from slowing transmission.

Fields of softnet_data

The following is a brief field-by-field description of this data structure; details will be given in later chapters. Some drivers use the NAPI interface, whereas others have not yet been updated to NAPI; both types of driver use this structure, but some fields are reserved for the non-NAPI drivers.

throttle

avg_blog

cng_level

These three parameters are used by the congestion management algorithm ...