Re: [net-next V3 15/15] Documentation: networking: Add description for multi-pf netdev
From: Jiri Pirko <jiri@resnulli.us>
Date: 2024-02-19 18:04:28
Thu, Feb 15, 2024 at 04:08:14AM CET, saeed@kernel.org wrote:
quoted hunk ↗ jump to hunk
From: Tariq Toukan <tariqt@nvidia.com> Add documentation for the multi-pf netdev feature. Describe the mlx5 implementation and design decisions. Signed-off-by: Tariq Toukan <tariqt@nvidia.com> Signed-off-by: Saeed Mahameed <saeedm@nvidia.com> --- 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.rstdiff --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 netconsolediff --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