Understanding Linux Network Internals

Chapter 12. General and Reference Material About Interrupts

This chapter contains several general types of information that apply to the material presented in the previous three chapters on interrupts and frame handling.

Statistics

Statistics about frame reception are kept in the per-CPU array netdev_rx_stat, whose elements are of type netif_rx_stats (see include/linux/netdevice.h):

struct netif_rx_stats netdev_rx_stat[NR_CPUS];
 
struct netif_rx_stats
{
        unsigned total;
        unsigned dropped;
        unsigned time_squeeze;
        unsigned throttled;
        unsigned fastroute_hit;
        unsigned fastroute_success;
        unsigned fastroute_defer;
        unsigned fastroute_deferred_out;
        unsigned fastroute_latency_reduction;
        unsigned cpu_collision;
} __ _  _cacheline_aligned;
 

The elements of netif_rx_stats are:

total

Total number of ingress frames processed, including any that might be discarded. This value is updated both in netif_rx and in netif_receive_skb, which means that (by mistake) the same frame is accounted for twice when the driver does not use NAPI (i.e., it uses the netif_rx interface; see Figure 10-2 in Chapter 10).

dropped

Number of frames that were dropped because they were received when the CPU was in the throttle state.

time_squeeze

Number of times net_rx_action had to return while frames were still in the CPU ingress queue, so as not to become a CPU hog. See the section "Processing the NET_RX_SOFTIRQ: net_rx_action" in Chapter 10.

throttled

Number of times the CPU went into the throttle state. This value is incremented ...