Subject: Re: [net-next V3 15/15] Documentation: networking: Add description for multi-pf netdev

From: Jiri Pirko <[email-masked]>
Date: Mon, 19 Feb 2024 19:04:23 +0100

Thu, Feb 15, 2024 at 04:08:14AM CET, [email-masked] wrote:
>From: Tariq Toukan <[email-masked]>
>
>Add documentation for the multi-pf netdev feature.
>Describe the mlx5 implementation and design decisions.
>
>Signed-off-by: Tariq Toukan <[email-masked]>
>Signed-off-by: Saeed Mahameed <[email-masked]>
>---
> Documentation/networking/index.rst           |   1 +
> Documentation/networking/multi-pf-netdev.rst | 157 +++++++++++++++++++
> 2 files changed, 158 insertions(+)
> create mode 100644 Documentation/networking/multi-pf-netdev.rst
>
>diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
>index 69f3d6dcd9fd..473d72c36d61 100644
>--- a/Documentation/networking/index.rst
>+++ b/Documentation/networking/index.rst
>@@ -74,6 +74,7 @@ Contents:
>    mpls-sysctl
>    mptcp-sysctl
>    multiqueue
>+   multi-pf-netdev
>    napi
>    net_cachelines/index
>    netconsole
>diff --git a/Documentation/networking/multi-pf-netdev.rst b/Documentation/networking/multi-pf-netdev.rst
>new file mode 100644
>index 000000000000..6ef2ac448d1e
>--- /dev/null
>+++ b/Documentation/networking/multi-pf-netdev.rst
>@@ -0,0 +1,157 @@
>+.. SPDX-License-Identifier: GPL-2.0
>+.. include:: <isonum.txt>
>+
>+===============
>+Multi-PF Netdev
>+===============
>+
>+Contents
>+========
>+
>+- `Background`_
>+- `Overview`_
>+- `mlx5 implementation`_
>+- `Channels distribution`_
>+- `Topology`_
>+- `Steering`_
>+- `Mutually exclusive features`_
>+
>+Background
>+==========
>+
>+The advanced Multi-PF NIC technology enables several CPUs within a multi-socket server to
>+connect directly to the network, each through its own dedicated PCIe interface. Through either a
>+connection harness that splits the PCIe lanes between two cards or by bifurcating a PCIe slot for a
>+single card. This results in eliminating the network traffic traversing over the internal bus
>+between the sockets, significantly reducing overhead and latency, in addition to reducing CPU
>+utilization and increasing network throughput.
>+
>+Overview
>+========
>+
>+This feature adds support for combining multiple devices (PFs) of the same port in a Multi-PF
>+environment under one netdev instance. Passing traffic through different devices belonging to
>+different NUMA sockets saves cross-numa traffic and allows apps running on the same netdev from
>+different numas to still feel a sense of proximity to the device and achieve improved performance.
>+
>+mlx5 implementation
>+===================
>+
>+Multi-PF or Socket-direct in mlx5 is achieved by grouping PFs together which belong to the same
>+NIC and has the socket-direct property enabled, once all PFS are probed, we create a single netdev

How do you enable this property?

>+to represent all of them, symmetrically, we destroy the netdev whenever any of the PFs is removed.
>+
>+The netdev network channels are distributed between all devices, a proper configuration would utilize
>+the correct close numa node when working on a certain app/cpu.
>+
>+We pick one PF to be a primary (leader), and it fills a special role. The other devices
>+(secondaries) are disconnected from the network at the chip level (set to silent mode). In silent
>+mode, no south <-> north traffic flowing directly through a secondary PF. It needs the assistance of
>+the leader PF (east <-> west traffic) to function. All RX/TX traffic is steered through the primary
>+to/from the secondaries.
>+
>+Currently, we limit the support to PFs only, and up to two PFs (sockets).

For the record, could you please describe why exactly you didn't use
drivers/base/component.c infrastructure for this? I know you told me,
but I don't recall. Better to have this written down, I believe.

>+
>+Channels distribution
>+=====================
>+
>+We distribute the channels between the different PFs to achieve local NUMA node performance
>+on multiple NUMA nodes.
>+
>+Each combined channel works against one specific PF, creating all its datapath queues against it. We distribute
>+channels to PFs in a round-robin policy.
>+
>+::
>+
>+        Example for 2 PFs and 6 channels:
>+        +--------+--------+
>+        | ch idx | PF idx |
>+        +--------+--------+
>+        |    0   |    0   |
>+        |    1   |    1   |
>+        |    2   |    0   |
>+        |    3   |    1   |
>+        |    4   |    0   |
>+        |    5   |    1   |
>+        +--------+--------+
>+
>+
>+We prefer this round-robin distribution policy over another suggested intuitive distribution, in
>+which we first distribute one half of the channels to PF0 and then the second half to PF1.
>+
>+The reason we prefer round-robin is, it is less influenced by changes in the number of channels. The
>+mapping between a channel index and a PF is fixed, no matter how many channels the user configures.
>+As the channel stats are persistent across channel's closure, changing the mapping every single time
>+would turn the accumulative stats less representing of the channel's history.
>+
>+This is achieved by using the correct core device instance (mdev) in each channel, instead of them
>+all using the same instance under "priv->mdev".
>+
>+Topology
>+========
>+Currently the sysfs is kept untouched, letting the netdev sysfs point to its primary PF.
>+Enhancing sysfs to reflect the actual topology is to be discussed and contributed separately.
>+For now, debugfs is being used to reflect the topology:
>+
>+.. code-block:: bash
>+
>+        $ grep -H . /sys/kernel/debug/mlx5/0000\:08\:00.0/sd/*
>+        /sys/kernel/debug/mlx5/0000:08:00.0/sd/group_id:0x00000101
>+        /sys/kernel/debug/mlx5/0000:08:00.0/sd/primary:0000:08:00.0 vhca 0x0
>+        /sys/kernel/debug/mlx5/0000:08:00.0/sd/secondary_0:0000:09:00.0 vhca 0x2

Ugh :/

SD is something that is likely going to stay with us for some time.
Can't we have some proper UAPI instead of this? IDK.

>+
>+Steering
>+========
>+Secondary PFs are set to "silent" mode, meaning they are disconnected from the network.
>+
>+In RX, the steering tables belong to the primary PF only, and it is its role to distribute incoming
>+traffic to other PFs, via cross-vhca steering capabilities. Nothing special about the RSS table
>+content, except that it needs a capable device to point to the receive queues of a different PF.
>+
>+In TX, the primary PF creates a new TX flow table, which is aliased by the secondaries, so they can
>+go out to the network through it.
>+
>+In addition, we set default XPS configuration that, based on the cpu, selects an SQ belonging to the
>+PF on the same node as the cpu.
>+
>+XPS default config example:
>+
>+NUMA node(s):          2
>+NUMA node0 CPU(s):     0-11
>+NUMA node1 CPU(s):     12-23

How can user know which queue is bound to which cpu?

>+
>+PF0 on node0, PF1 on node1.
>+
>+- /sys/class/net/eth2/queues/tx-0/xps_cpus:000001
>+- /sys/class/net/eth2/queues/tx-1/xps_cpus:001000
>+- /sys/class/net/eth2/queues/tx-2/xps_cpus:000002
>+- /sys/class/net/eth2/queues/tx-3/xps_cpus:002000
>+- /sys/class/net/eth2/queues/tx-4/xps_cpus:000004
>+- /sys/class/net/eth2/queues/tx-5/xps_cpus:004000
>+- /sys/class/net/eth2/queues/tx-6/xps_cpus:000008
>+- /sys/class/net/eth2/queues/tx-7/xps_cpus:008000
>+- /sys/class/net/eth2/queues/tx-8/xps_cpus:000010
>+- /sys/class/net/eth2/queues/tx-9/xps_cpus:010000
>+- /sys/class/net/eth2/queues/tx-10/xps_cpus:000020
>+- /sys/class/net/eth2/queues/tx-11/xps_cpus:020000
>+- /sys/class/net/eth2/queues/tx-12/xps_cpus:000040
>+- /sys/class/net/eth2/queues/tx-13/xps_cpus:040000
>+- /sys/class/net/eth2/queues/tx-14/xps_cpus:000080
>+- /sys/class/net/eth2/queues/tx-15/xps_cpus:080000
>+- /sys/class/net/eth2/queues/tx-16/xps_cpus:000100
>+- /sys/class/net/eth2/queues/tx-17/xps_cpus:100000
>+- /sys/class/net/eth2/queues/tx-18/xps_cpus:000200
>+- /sys/class/net/eth2/queues/tx-19/xps_cpus:200000
>+- /sys/class/net/eth2/queues/tx-20/xps_cpus:000400
>+- /sys/class/net/eth2/queues/tx-21/xps_cpus:400000
>+- /sys/class/net/eth2/queues/tx-22/xps_cpus:000800
>+- /sys/class/net/eth2/queues/tx-23/xps_cpus:800000
>+
>+Mutually exclusive features
>+===========================
>+
>+The nature of Multi-PF, where different channels work with different PFs, conflicts with
>+stateful features where the state is maintained in one of the PFs.
>+For example, in the TLS device-offload feature, special context objects are created per connection
>+and maintained in the PF.  Transitioning between different RQs/SQs would break the feature. Hence,
>+we disable this combination for now.
>-- 
>2.43.0
>
>