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Receive Packet Steering is a software implementation of RSS. Since it is implemented in software, this means it can be enabled for any NIC, even NICs which have only a single RX queue.

Mark Perlstein, Ceo, Datavail

However, since it is in software, this means that RPS can only enter into the flow after a packet has been harvested from the DMA memory region. Next, it’s time for netif_receive_skb to see how data is handed off to the protocol layers. Before this can be examined, we’ll need to take a look at Receive Packet Steering first. Generic Receive Offloading is a software implementation of a hardware optimization that is known as Large Receive Offloading .

The code path once the data hits __netif_receive_skb is the same as explained above for the RPS disabled case. Namely, __netif_receive_skb does some bookkeeping prior to calling __netif_receive_skb_core to pass network data up to the protocol layers.

Now it’s time to take two detours prior software downloads to proceeding up the network stack. First, let’s see how to monitor and tune the network subsystem’s softirqs. After that, the rest of the networking stack will make more sense as we enter napi_gro_receive. Otherwise, continue fetching additional buffers from the RX queue, adding them to the skb. This is necessary if a received data frame is larger than the buffer size.

The process_backlog function is a loop which runs until its weight has been consumed or no more data remains on the backlog. See the section above about monitoring /proc/net/softnet_stat. The dropped field is a counter that gets incremented each time data is dropped instead of queued to a CPU’s input_pkt_queue. RPS distributes packet processing load amongst multiple CPUs, but a single large flow can monopolize CPU processing time and starve smaller flows. Flow limits are a feature that can be used to limit the number of packets queued to the backlog for each flow to a certain amount.

For now, let’s see how to monitor the health of the net_rx_action processing loop and move on to the inner working of NAPI poll functions so we can progress up the network stack. If a driver’s poll function does NOT consume its entire weight, it must disable NAPI.

If the queue is longer than this value, the data is dropped. Similarly, the flow limit is checked and if it has been exceeded, the data is dropped. In both cases the drop count on the softnet_data structure is incremented. Note that this is the softnet_data structure of the CPU the data was going to be queued to.

This can help ensure that smaller flows are processed even though much larger flows are pushing packets in. The length of input_pkt_queue is first compared to netdev_max_backlog.

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Read the section above about /proc/net/softnet_stat to learn how to get the drop count for monitoring purposes. Configure your IRQ settings to ensure each RX queue is handled by one of your desired network processing CPUs. Consult your NIC’s data sheet to determine if this feature is supported. If your NIC’s driver exposes a function called ndo_rx_flow_steer, then the driver has support for accelerated RFS.

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