| HOWTO for multiqueue network device support | |
| =========================================== | |
| Section 1: Base driver requirements for implementing multiqueue support | |
| Intro: Kernel support for multiqueue devices | |
| --------------------------------------------------------- | |
| Kernel support for multiqueue devices is always present. | |
| Section 1: Base driver requirements for implementing multiqueue support | |
| ----------------------------------------------------------------------- | |
| Base drivers are required to use the new alloc_etherdev_mq() or | |
| alloc_netdev_mq() functions to allocate the subqueues for the device. The | |
| underlying kernel API will take care of the allocation and deallocation of | |
| the subqueue memory, as well as netdev configuration of where the queues | |
| exist in memory. | |
| The base driver will also need to manage the queues as it does the global | |
| netdev->queue_lock today. Therefore base drivers should use the | |
| netif_{start|stop|wake}_subqueue() functions to manage each queue while the | |
| device is still operational. netdev->queue_lock is still used when the device | |
| comes online or when it's completely shut down (unregister_netdev(), etc.). | |
| Section 2: Qdisc support for multiqueue devices | |
| ----------------------------------------------- | |
| Currently two qdiscs are optimized for multiqueue devices. The first is the | |
| default pfifo_fast qdisc. This qdisc supports one qdisc per hardware queue. | |
| A new round-robin qdisc, sch_multiq also supports multiple hardware queues. The | |
| qdisc is responsible for classifying the skb's and then directing the skb's to | |
| bands and queues based on the value in skb->queue_mapping. Use this field in | |
| the base driver to determine which queue to send the skb to. | |
| sch_multiq has been added for hardware that wishes to avoid head-of-line | |
| blocking. It will cycle though the bands and verify that the hardware queue | |
| associated with the band is not stopped prior to dequeuing a packet. | |
| On qdisc load, the number of bands is based on the number of queues on the | |
| hardware. Once the association is made, any skb with skb->queue_mapping set, | |
| will be queued to the band associated with the hardware queue. | |
| Section 3: Brief howto using MULTIQ for multiqueue devices | |
| --------------------------------------------------------------- | |
| The userspace command 'tc,' part of the iproute2 package, is used to configure | |
| qdiscs. To add the MULTIQ qdisc to your network device, assuming the device | |
| is called eth0, run the following command: | |
| # tc qdisc add dev eth0 root handle 1: multiq | |
| The qdisc will allocate the number of bands to equal the number of queues that | |
| the device reports, and bring the qdisc online. Assuming eth0 has 4 Tx | |
| queues, the band mapping would look like: | |
| band 0 => queue 0 | |
| band 1 => queue 1 | |
| band 2 => queue 2 | |
| band 3 => queue 3 | |
| Traffic will begin flowing through each queue based on either the simple_tx_hash | |
| function or based on netdev->select_queue() if you have it defined. | |
| The behavior of tc filters remains the same. However a new tc action, | |
| skbedit, has been added. Assuming you wanted to route all traffic to a | |
| specific host, for example 192.168.0.3, through a specific queue you could use | |
| this action and establish a filter such as: | |
| tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \ | |
| match ip dst 192.168.0.3 \ | |
| action skbedit queue_mapping 3 | |
| Author: Alexander Duyck <alexander.h.duyck@intel.com> | |
| Original Author: Peter P. Waskiewicz Jr. <peter.p.waskiewicz.jr@intel.com> |