1.. SPDX-License-Identifier: GPL-2.0 2 3=========================================== 4HOWTO for multiqueue network device support 5=========================================== 6 7Section 1: Base driver requirements for implementing multiqueue support 8======================================================================= 9 10Intro: Kernel support for multiqueue devices 11--------------------------------------------------------- 12 13Kernel support for multiqueue devices is always present. 14 15Base drivers are required to use the new alloc_etherdev_mq() or 16alloc_netdev_mq() functions to allocate the subqueues for the device. The 17underlying kernel API will take care of the allocation and deallocation of 18the subqueue memory, as well as netdev configuration of where the queues 19exist in memory. 20 21The base driver will also need to manage the queues as it does the global 22netdev->queue_lock today. Therefore base drivers should use the 23netif_{start|stop|wake}_subqueue() functions to manage each queue while the 24device is still operational. netdev->queue_lock is still used when the device 25comes online or when it's completely shut down (unregister_netdev(), etc.). 26 27 28Section 2: Qdisc support for multiqueue devices 29=============================================== 30 31Currently two qdiscs are optimized for multiqueue devices. The first is the 32default pfifo_fast qdisc. This qdisc supports one qdisc per hardware queue. 33A new round-robin qdisc, sch_multiq also supports multiple hardware queues. The 34qdisc is responsible for classifying the skb's and then directing the skb's to 35bands and queues based on the value in skb->queue_mapping. Use this field in 36the base driver to determine which queue to send the skb to. 37 38sch_multiq has been added for hardware that wishes to avoid head-of-line 39blocking. It will cycle though the bands and verify that the hardware queue 40associated with the band is not stopped prior to dequeuing a packet. 41 42On qdisc load, the number of bands is based on the number of queues on the 43hardware. Once the association is made, any skb with skb->queue_mapping set, 44will be queued to the band associated with the hardware queue. 45 46 47Section 3: Brief howto using MULTIQ for multiqueue devices 48========================================================== 49 50The userspace command 'tc,' part of the iproute2 package, is used to configure 51qdiscs. To add the MULTIQ qdisc to your network device, assuming the device 52is called eth0, run the following command:: 53 54 # tc qdisc add dev eth0 root handle 1: multiq 55 56The qdisc will allocate the number of bands to equal the number of queues that 57the device reports, and bring the qdisc online. Assuming eth0 has 4 Tx 58queues, the band mapping would look like:: 59 60 band 0 => queue 0 61 band 1 => queue 1 62 band 2 => queue 2 63 band 3 => queue 3 64 65Traffic will begin flowing through each queue based on either the simple_tx_hash 66function or based on netdev->select_queue() if you have it defined. 67 68The behavior of tc filters remains the same. However a new tc action, 69skbedit, has been added. Assuming you wanted to route all traffic to a 70specific host, for example 192.168.0.3, through a specific queue you could use 71this action and establish a filter such as:: 72 73 tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \ 74 match ip dst 192.168.0.3 \ 75 action skbedit queue_mapping 3 76 77:Author: Alexander Duyck <alexander.h.duyck@intel.com> 78:Original Author: Peter P. Waskiewicz Jr. <peter.p.waskiewicz.jr@intel.com> 79