Date:   Tue, 26 Jun 2018 15:59:08 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Amritha Nambiar <amritha.nambiar@...el.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexander Duyck <alexander.h.duyck@...el.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Sridhar Samudrala <sridhar.samudrala@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [net-next PATCH v4 7/7] Documentation: Add explanation for XPS
 using Rx-queue(s) map

On Mon, Jun 25, 2018 at 11:04 AM, Amritha Nambiar
<amritha.nambiar@...el.com> wrote:
> Signed-off-by: Amritha Nambiar <amritha.nambiar@...el.com>

Acked-by: Tom Herbert <tom@...ntonium.net>

> ---
>  Documentation/ABI/testing/sysfs-class-net-queues |   11 ++++
>  Documentation/networking/scaling.txt             |   57 ++++++++++++++++++----
>  2 files changed, 58 insertions(+), 10 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-class-net-queues b/Documentation/ABI/testing/sysfs-class-net-queues
> index 0c0df91..978b763 100644
> --- a/Documentation/ABI/testing/sysfs-class-net-queues
> +++ b/Documentation/ABI/testing/sysfs-class-net-queues
> @@ -42,6 +42,17 @@ Description:
>                 network device transmit queue. Possible vaules depend on the
>                 number of available CPU(s) in the system.
>
> +What:          /sys/class/<iface>/queues/tx-<queue>/xps_rxqs
> +Date:          June 2018
> +KernelVersion: 4.18.0
> +Contact:       netdev@...r.kernel.org
> +Description:
> +               Mask of the receive queue(s) currently enabled to participate
> +               into the Transmit Packet Steering packet processing flow for this
> +               network device transmit queue. Possible values depend on the
> +               number of available receive queue(s) in the network device.
> +               Default is disabled.
> +
>  What:          /sys/class/<iface>/queues/tx-<queue>/byte_queue_limits/hold_time
>  Date:          November 2011
>  KernelVersion: 3.3
> diff --git a/Documentation/networking/scaling.txt b/Documentation/networking/scaling.txt
> index f55639d..8336116 100644
> --- a/Documentation/networking/scaling.txt
> +++ b/Documentation/networking/scaling.txt
> @@ -366,8 +366,13 @@ XPS: Transmit Packet Steering
>
>  Transmit Packet Steering is a mechanism for intelligently selecting
>  which transmit queue to use when transmitting a packet on a multi-queue
> -device. To accomplish this, a mapping from CPU to hardware queue(s) is
> -recorded. The goal of this mapping is usually to assign queues
> +device. This can be accomplished by recording two kinds of maps, either
> +a mapping of CPU to hardware queue(s) or a mapping of receive queue(s)
> +to hardware transmit queue(s).
> +
> +1. XPS using CPUs map
> +
> +The goal of this mapping is usually to assign queues
>  exclusively to a subset of CPUs, where the transmit completions for
>  these queues are processed on a CPU within this set. This choice
>  provides two benefits. First, contention on the device queue lock is
> @@ -377,12 +382,35 @@ transmit queue). Secondly, cache miss rate on transmit completion is
>  reduced, in particular for data cache lines that hold the sk_buff
>  structures.
>
> -XPS is configured per transmit queue by setting a bitmap of CPUs that
> -may use that queue to transmit. The reverse mapping, from CPUs to
> -transmit queues, is computed and maintained for each network device.
> -When transmitting the first packet in a flow, the function
> -get_xps_queue() is called to select a queue. This function uses the ID
> -of the running CPU as a key into the CPU-to-queue lookup table. If the
> +2. XPS using receive queues map
> +
> +This mapping is used to pick transmit queue based on the receive
> +queue(s) map configuration set by the administrator. A set of receive
> +queues can be mapped to a set of transmit queues (many:many), although
> +the common use case is a 1:1 mapping. This will enable sending packets
> +on the same queue associations for transmit and receive. This is useful for
> +busy polling multi-threaded workloads where there are challenges in
> +associating a given CPU to a given application thread. The application
> +threads are not pinned to CPUs and each thread handles packets
> +received on a single queue. The receive queue number is cached in the
> +socket for the connection. In this model, sending the packets on the same
> +transmit queue corresponding to the associated receive queue has benefits
> +in keeping the CPU overhead low. Transmit completion work is locked into
> +the same queue-association that a given application is polling on. This
> +avoids the overhead of triggering an interrupt on another CPU. When the
> +application cleans up the packets during the busy poll, transmit completion
> +may be processed along with it in the same thread context and so result in
> +reduced latency.
> +
> +XPS is configured per transmit queue by setting a bitmap of
> +CPUs/receive-queues that may use that queue to transmit. The reverse
> +mapping, from CPUs to transmit queues or from receive-queues to transmit
> +queues, is computed and maintained for each network device. When
> +transmitting the first packet in a flow, the function get_xps_queue() is
> +called to select a queue. This function uses the ID of the receive queue
> +for the socket connection for a match in the receive queue-to-transmit queue
> +lookup table. Alternatively, this function can also use the ID of the
> +running CPU as a key into the CPU-to-queue lookup table. If the
>  ID matches a single queue, that is used for transmission. If multiple
>  queues match, one is selected by using the flow hash to compute an index
>  into the set.
> @@ -404,11 +432,15 @@ acknowledged.
>
>  XPS is only available if the kconfig symbol CONFIG_XPS is enabled (on by
>  default for SMP). The functionality remains disabled until explicitly
> -configured. To enable XPS, the bitmap of CPUs that may use a transmit
> -queue is configured using the sysfs file entry:
> +configured. To enable XPS, the bitmap of CPUs/receive-queues that may
> +use a transmit queue is configured using the sysfs file entry:
>
> +For selection based on CPUs map:
>  /sys/class/net/<dev>/queues/tx-<n>/xps_cpus
>
> +For selection based on receive-queues map:
> +/sys/class/net/<dev>/queues/tx-<n>/xps_rxqs
> +
>  == Suggested Configuration
>
>  For a network device with a single transmission queue, XPS configuration
> @@ -421,6 +453,11 @@ best CPUs to share a given queue are probably those that share the cache
>  with the CPU that processes transmit completions for that queue
>  (transmit interrupts).
>
> +For transmit queue selection based on receive queue(s), XPS has to be
> +explicitly configured mapping receive-queue(s) to transmit queue(s). If the
> +user configuration for receive-queue map does not apply, then the transmit
> +queue is selected based on the CPUs map.
> +
>  Per TX Queue rate limitation:
>  =============================
>
>

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