a093bf006e
Add a brief howto to Documentation/networking for multiqueue. It explains how to use the multiqueue API in a driver to support multiqueue paths from the stack, as well as the qdiscs to use for feeding a multiqueue device. Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com> Signed-off-by: David S. Miller <davem@davemloft.net>
112 lines
4.9 KiB
Plaintext
112 lines
4.9 KiB
Plaintext
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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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Section 2: Qdisc support for multiqueue devices
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Section 3: Brief howto using PRIO or RR for multiqueue devices
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Intro: Kernel support for multiqueue devices
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---------------------------------------------------------
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Kernel support for multiqueue devices is only an API that is presented to the
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netdevice layer for base drivers to implement. This feature is part of the
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core networking stack, and all network devices will be running on the
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multiqueue-aware stack. If a base driver only has one queue, then these
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changes are transparent to that driver.
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Section 1: Base driver requirements for implementing multiqueue support
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-----------------------------------------------------------------------
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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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Finally, the base driver should indicate that it is a multiqueue device. The
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feature flag NETIF_F_MULTI_QUEUE should be added to the netdev->features
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bitmap on device initialization. Below is an example from e1000:
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#ifdef CONFIG_E1000_MQ
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if ( (adapter->hw.mac.type == e1000_82571) ||
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(adapter->hw.mac.type == e1000_82572) ||
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(adapter->hw.mac.type == e1000_80003es2lan))
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netdev->features |= NETIF_F_MULTI_QUEUE;
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#endif
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Section 2: Qdisc support for multiqueue devices
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-----------------------------------------------
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Currently two qdiscs support multiqueue devices. A new round-robin qdisc,
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sch_rr, and sch_prio. The qdisc is responsible for classifying the skb's to
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bands and queues, and will store the queue mapping into skb->queue_mapping.
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Use this field in the base driver to determine which queue to send the skb
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to.
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sch_rr has been added for hardware that doesn't want scheduling policies from
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software, so it's a straight round-robin qdisc. It uses the same syntax and
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classification priomap that sch_prio uses, so it should be intuitive to
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configure for people who've used sch_prio.
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The PRIO qdisc naturally plugs into a multiqueue device. If PRIO has been
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built with NET_SCH_PRIO_MQ, then upon load, it will make sure the number of
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bands requested is equal to the number of queues on the hardware. If they
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are equal, it sets a one-to-one mapping up between the queues and bands. If
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they're not equal, it will not load the qdisc. This is the same behavior
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for RR. Once the association is made, any skb that is classified will have
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skb->queue_mapping set, which will allow the driver to properly queue skb's
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to multiple queues.
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Section 3: Brief howto using PRIO and RR 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 PRIO qdisc to your network device, assuming the device is
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called eth0, run the following command:
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# tc qdisc add dev eth0 root handle 1: prio bands 4 multiqueue
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This will create 4 bands, 0 being highest priority, and associate those bands
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to the queues on your NIC. Assuming eth0 has 4 Tx queues, the band mapping
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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 if your TOS values are assigning
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traffic across the various bands. For example, ssh traffic will always try to
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go out band 0 based on TOS -> Linux priority conversion (realtime traffic),
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so it will be sent out queue 0. ICMP traffic (pings) fall into the "normal"
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traffic classification, which is band 1. Therefore pings will be send out
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queue 1 on the NIC.
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Note the use of the multiqueue keyword. This is only in versions of iproute2
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that support multiqueue networking devices; if this is omitted when loading
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a qdisc onto a multiqueue device, the qdisc will load and operate the same
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if it were loaded onto a single-queue device (i.e. - sends all traffic to
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queue 0).
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Another alternative to multiqueue band allocation can be done by using the
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multiqueue option and specify 0 bands. If this is the case, the qdisc will
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allocate the number of bands to equal the number of queues that the device
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reports, and bring the qdisc online.
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The behavior of tc filters remains the same, where it will override TOS priority
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classification.
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Author: Peter P. Waskiewicz Jr. <peter.p.waskiewicz.jr@intel.com>
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