Understanding Linux Network Internals

Processing the NET_RX_SOFTIRQ: net_rx_action

net_rx_action is the bottom-half function used to process incoming frames. Its execution is triggered whenever a driver notifies the kernel about the presence of input frames. Figure 10-5 shows the flow of control through the function.

Frames can wait in two places for net_rx_action to process them:

A shared CPU-specific queue

Non-NAPI devices' interrupt handlers, which call netif_rx, place frames into the softnet_data->input_pkt_queue of the CPU on which the interrupt handlers run.

Device memory

The poll method used by NAPI drivers extracts frames directly from the device (or the device driver receive rings).

The section "Old Versus New Driver Interfaces" showed how the kernel is notified about the need to run net_rx_action in both cases.

Figure 10-5. net_rx_action function

The job of net_rx_action is pretty simple: to browse the poll_list list of devices that have something in their ingress queue and invoke for each one the associated poll virtual function until one of the following conditions is met:

• There are no more devices in the list.

• net_rx_action has run for too long and therefore it is supposed to release the CPU so that it does not become a CPU hog.

• The number of frames already dequeued and processed has reached a given upper bound limit (budget). budget is initialized at the beginning of the function to netdev_max_backlog, which is ...