netdev - Re: [net-next 10/16] net/mlx5: Support PCIe buffer congestion handling via Devlink

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Message-ID: <98071d31-bf99-d713-7fd1-4b1cc156377e@mellanox.com>
Date:   Wed, 25 Jul 2018 15:31:39 +0300
From:   Eran Ben Elisha <eranbe@...lanox.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Saeed Mahameed <saeedm@...lanox.com>,
        Jiri Pirko <jiri@...nulli.us>,
        "David S. Miller" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [net-next 10/16] net/mlx5: Support PCIe buffer congestion
 handling via Devlink



On 7/24/2018 10:51 PM, Jakub Kicinski wrote:
>>>
>>> The devlink params haven't been upstream even for a full cycle and
>>> already you guys are starting to use them to configure standard
>>> features like queuing.
>>
>> We developed the devlink params in order to support non-standard
>> configuration only. And for non-standard, there are generic and vendor
>> specific options.
> 
> I thought it was developed for performing non-standard and possibly
> vendor specific configuration.  Look at DEVLINK_PARAM_GENERIC_* for
> examples of well justified generic options for which we have no
> other API.  The vendor mlx4 options look fairly vendor specific if you
> ask me, too.
> 
> Configuring queuing has an API.  The question is it acceptable to enter
> into the risky territory of controlling offloads via devlink parameters
> or would we rather make vendors take the time and effort to model
> things to (a subset) of existing APIs.  The HW never fits the APIs
> perfectly.

I understand what you meant here, I would like to highlight that this 
mechanism was not meant to handle SRIOV, Representors, etc.
The vendor specific configuration suggested here is to handle a 
congestion state in Multi Host environment (which includes PF and 
multiple VFs per host), where one host is not aware to the other hosts, 
and each is running on its own pci/driver. It is a device working mode 
configuration.

This  couldn't fit into any existing API, thus creating this vendor 
specific unique API is needed.

> 
>> The queuing model is a standard. However here we are configuring the
>> outbound PCIe buffers on the receive path from NIC port toward the
>> host(s) in Single / MultiHost environment.
> 
> That's why we have PF representors.
> 
>> (You can see the driver processing based on this param as part of the RX
>> patch for the marked option here https://patchwork.ozlabs.org/patch/945998/)
>>
>>> I know your HW is not capable of doing full RED offload, it's a
>>> snowflake.
>>
>> The algorithm which is applied here for the drop option is not the core
>> of this feature.
>>
>>> You tell us you're doing custom DCB configuration hacks on
>>> one side (previous argument we had) and custom devlink parameter
>>> configuration hacks on PCIe.
>>>
>>> Perhaps the idea that we're trying to use the existing Linux APIs for
>>> HW configuration only applies to forwarding behaviour.
>>
>> Hopefully I explained above well why it is not related.
> 
> Sure ;)
>