netdev - Re: [net-next 07/10] net/mlx5: Enable PCIe buffer congestion handling workaround via devlink

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Date:   Wed, 1 Aug 2018 15:18:21 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     Saeed Mahameed <saeedm@...lanox.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>, Jiri Pirko <jiri@...lanox.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Eran Ben Elisha <eranbe@...lanox.com>
Subject: Re: [net-next 07/10] net/mlx5: Enable PCIe buffer congestion handling
 workaround via devlink

On Wed, Aug 1, 2018 at 2:52 PM, Saeed Mahameed <saeedm@...lanox.com> wrote:
> From: Eran Ben Elisha <eranbe@...lanox.com>
>
> Add support for two driver parameters via devlink params interface:
> - Congestion action
>         HW mechanism in the PCIe buffer which monitors the amount of
>         consumed PCIe buffer per host. This mechanism supports the
>         following actions in case of threshold overflow:
>         - disabled - NOP (Default)
>         - drop
>         - mark - Mark CE bit in the CQE of received packet

Any chance you could clarify the differences between "disabled" and
"drop"? I am assuming the "drop" is a head-of-line drop versus the
"disabled" being a incoming packet drop?

Also I still don't see this as necessarily being all that unique of a
feature/issue. Basically being PCIe bus limited is not all that
uncommon of a thing and has existed since the early days of PCI. In
the case of the Intel NICs we just throw a warning and end up dropping
the incoming packets instead of providing the two other options you
have listed.

> - Congestion mode
>         - aggressive - Aggressive static trigger threshold (Default)
>         - dynamic - Dynamically change the trigger threshold
>
> These driver-specific params enable the NIC HW workaround to handle
> buffer congestion on the current NIC generation.

Is there any documentation anywhere for any of these features? In the
patch set I see you adding interfaces, but I don't see them documented
anywhere.

> Signed-off-by: Eran Ben Elisha <eranbe@...lanox.com>
> Reviewed-by: Moshe Shemesh <moshe@...lanox.com>
> Reviewed-by: Jiri Pirko <jiri@...lanox.com>
> Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
> ---
>  .../net/ethernet/mellanox/mlx5/core/devlink.c | 204 +++++++++++++++++-
>  1 file changed, 203 insertions(+), 1 deletion(-)

Ideally there should be some documentation going into the kernel when
you extend the devlink interface at least so that I know how to use
your new interfaces when you define them. Just updating devlink.c
seems like a messy way to do things.

- Alex