79 lines
3.4 KiB
ReStructuredText
79 lines
3.4 KiB
ReStructuredText
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.. SPDX-License-Identifier: GPL-2.0
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===========================================
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HOWTO for multiqueue network device support
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===========================================
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Section 1: Base driver requirements for implementing multiqueue support
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=======================================================================
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Intro: Kernel support for multiqueue devices
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---------------------------------------------------------
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Kernel support for multiqueue devices is always present.
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Base drivers are required to use the new alloc_etherdev_mq() or
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alloc_netdev_mq() functions to allocate the subqueues for the device. The
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underlying kernel API will take care of the allocation and deallocation of
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the subqueue memory, as well as netdev configuration of where the queues
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exist in memory.
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The base driver will also need to manage the queues as it does the global
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netdev->queue_lock today. Therefore base drivers should use the
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netif_{start|stop|wake}_subqueue() functions to manage each queue while the
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device is still operational. netdev->queue_lock is still used when the device
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comes online or when it's completely shut down (unregister_netdev(), etc.).
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Section 2: Qdisc support for multiqueue devices
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===============================================
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Currently two qdiscs are optimized for multiqueue devices. The first is the
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default pfifo_fast qdisc. This qdisc supports one qdisc per hardware queue.
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A new round-robin qdisc, sch_multiq also supports multiple hardware queues. The
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qdisc is responsible for classifying the skb's and then directing the skb's to
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bands and queues based on the value in skb->queue_mapping. Use this field in
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the base driver to determine which queue to send the skb to.
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sch_multiq has been added for hardware that wishes to avoid head-of-line
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blocking. It will cycle though the bands and verify that the hardware queue
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associated with the band is not stopped prior to dequeuing a packet.
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On qdisc load, the number of bands is based on the number of queues on the
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hardware. Once the association is made, any skb with skb->queue_mapping set,
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will be queued to the band associated with the hardware queue.
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Section 3: Brief howto using MULTIQ for multiqueue devices
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==========================================================
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The userspace command 'tc,' part of the iproute2 package, is used to configure
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qdiscs. To add the MULTIQ qdisc to your network device, assuming the device
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is called eth0, run the following command::
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# tc qdisc add dev eth0 root handle 1: multiq
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The qdisc will allocate the number of bands to equal the number of queues that
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the device reports, and bring the qdisc online. Assuming eth0 has 4 Tx
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queues, the band mapping would look like::
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band 0 => queue 0
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band 1 => queue 1
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band 2 => queue 2
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band 3 => queue 3
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Traffic will begin flowing through each queue based on either the simple_tx_hash
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function or based on netdev->select_queue() if you have it defined.
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The behavior of tc filters remains the same. However a new tc action,
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skbedit, has been added. Assuming you wanted to route all traffic to a
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specific host, for example 192.168.0.3, through a specific queue you could use
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this action and establish a filter such as::
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tc filter add dev eth0 parent 1: protocol ip prio 1 u32 \
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match ip dst 192.168.0.3 \
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action skbedit queue_mapping 3
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:Author: Alexander Duyck <alexander.h.duyck@intel.com>
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:Original Author: Peter P. Waskiewicz Jr. <peter.p.waskiewicz.jr@intel.com>
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