Subject: [net-next RFC 0/1] mlx5: support per queue coalesce settings

From: "Nabil S. Alramli" <[email-masked]>
Date: Wed, 23 Aug 2023 18:31:20 -0400

Hello,

I am Submitting this as an RFC to get feedback and to find out if the folks
at Mellanox would accept this change.

Currently, only gobal coalescing configuration queries or changes are
supported in the `mlx5` driver. However, per-queue operations are not, and
result in `EOPNOTSUPP` errors when attempted with `ethtool`. This patch
adds support for per-queue coalesce operations, with a caveat described
below.

Here's an example use case:

- An mlx5 NIC is configured with 8 queues, each queue has its IRQ pinned
  to a unique CPU.
- Two custom RSS contexts are created: context 1 and context 2. Each
  context has a different set of queues where flows are distributed. For
  example, context 1 may distribute flows to queues 0-3, and context 2 may
  distribute flows to queues 4-7.
- A series of ntuple filters are installed which direct matching flows to
  RSS contexts. For example, perhaps port 80 is directed to context 1 and
  port 443 to context 2.
- Applications which receive network data associated with either context
  are pinned to the CPUs where the queues in the matching context have
  their IRQs pinned to maximize locality.

The apps themselves, however, may have different requirements on latency vs
CPU usage and so setting the per queue IRQ coalesce values would be very
helpful.

This patch would support this, with the caveat that DIM mode changes per
queue are not supported. DIM mode can only be changed NIC-wide. This is
because in the mlx5 driver, global operations that change the state of
adaptive-ex or adaptive-tx require a reset. So in the case of per-queue, we
reject such requests. This was done in the interest of simplicity for this
RFC as setting the DIM mode per queue appears to require significant
changes to mlx5 to be able to preserve the state of the indvidual channels
through a reset.

IMO, if a user is going to set per-queue coalesce settings it might be
reasonable to assume that they will disable adaptive rx/tx NIC wide first
and then go through and apply their desired per-queue settings.

Here's an example:

$ ethtool --per-queue eth0 queue_mask 0x4 --show-coalesce
Queue: 2
Adaptive RX: on  TX: on
stats-block-usecs: 0
sample-interval: 0
pkt-rate-low: 0
pkt-rate-high: 0

rx-usecs: 8
rx-frames: 128
...

Now, let's try to set adaptive-rx off rx-usecs 16 for queue 2:

$ sudo ethtool --per-queue eth0 queue_mask 0x4 --coalesce adaptive-rx off \
  rx-usecs 16
Cannot set device per queue parameters: Operation not supported

This is not supported; adaptive-rx must be disabled NIC wide first:

$ sudo ethtool -C eth0 adaptive-rx off

And now, queue_mask 0x4 can be applied to set rx-usecs:

$ sudo ethtool --per-queue eth0 queue_mask 0x4 --coalesce rx-usecs 16
$ ethtool --per-queue eth0 queue_mask 0x4 --show-coalesce
Queue: 2
Adaptive RX: off  TX: on
stats-block-usecs: 0
sample-interval: 0
pkt-rate-low: 0
pkt-rate-high: 0

rx-usecs: 16
rx-frames: 32
...

Previously a global `struct mlx5e_params` stored the options in
`struct mlx5e_priv.channels.params`. That was preserved, but a channel-
specific instance was added as well, in `struct mlx5e_channel.params`.

Note that setting global coalescing options will set the individual
channel settings to the same values as well.

Is Mellanox open to this change? What would be needed to get something like
this accepted?

Best Regards,

Nabil S. Alramli (1):
  mlx5: Add {get,set}_per_queue_coalesce()

 drivers/net/ethernet/mellanox/mlx5/core/en.h  |   3 +-
 .../ethernet/mellanox/mlx5/core/en_ethtool.c  | 212 ++++++++++++++----
 .../net/ethernet/mellanox/mlx5/core/en_main.c |   4 +
 3 files changed, 173 insertions(+), 46 deletions(-)

-- 
2.35.1