netdev - Re: Getting a handle on all these new NIC features

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Date:   Fri, 20 Jan 2017 09:06:47 -0800
From:   Tom Herbert <tom@...bertland.com>
To:     Martin Habets <mhabets@...arflare.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Getting a handle on all these new NIC features

On Fri, Jan 20, 2017 at 8:36 AM, Martin Habets <mhabets@...arflare.com> wrote:
> Hi Tom,
>
> On 17/01/17 22:05, Tom Herbert wrote:
>> There was some discussion about the problems of dealing with the
>> explosion of NIC features in the mlx directory restructuring proposal,
>> but I think the is a deeper issue here that should be discussed.
>>
>> It's hard not to notice that there has been quite a proliferation of
>> NIC features in several drivers. This trend had resulted in very
>> complex driver code that may or may not segment individual features.
>> One visible manifestation of this is number of ndo functions which is
>> somewhere around seventy-five now.
>>
>> I suspect the vast majority of these advances NIC features (e.g.
>> bridging, UDP offloads, tc offload, etc.) are only relevant to some of
>> the people some of the time. The problem we have, in this case those
>> of us that are attempting to deploy and maintain NICs at scale, is
>> when we have to deal with the ramifications of these features being
>> intertwined with core driver functionality that is relevant to
>> everyone. This becomes very obvious when we need to backport drivers
>> from later versions of kernel.
>>
>> I realize that backports of a driver is not a specific concern of the
>> Linux kernel, but nevertheless this is a real problem and a fact of
>> life for many users. Rebasing the full kernel is still a major effort
>> and it seems the best we could ever do is one rebase per year. In the
>> interim we need to occasionally backport drivers. Backporting drivers
>> is difficult precisely because of new features or API changes to
>> existing ones. These sort of changes tend to have a spiderweb of
>> dependencies in other parts of the stack so that the number of patches
>> we need to cherry-pick goes way beyond those that touch the driver we
>> are interested in.
>
> For the sfc driver (Solarflare Adapters) we currently do backports internally for:
>  - RedHat Enterprise Linux                        5.10,  5.11
>  - RedHat Enterprise Linux                        6.5, 6.6, 6.7, 6.8
>    - Redhat Messaging Realtime and Grid           2.5
>  - RedHat Enterprise Linux                        7.0, 7.1, 7.2
>    - RedHat Enterprise Linux for Realtime         7.1, 7.2
>  - SuSE Linux Enterprise Server 11                sp3, sp4
>    - SuSE Linux Enterprise RealTime Extension 11
>  - SuSE Linux Enterprise Server 12                base release, sp1
>  - Canonical Ubuntu Server LTS                    14.04, 16.04
>  - Canonical Ubuntu Server                        -
>  - Debian 7 "Wheezy"                              7.X
>  - Debian 8 "Jessie"                              8.X
>  - Linux                                          2.6.18 to 4.9-rc1
>
> We update this list as needed, and always try to support the latest kernel.
> I do not know if that would cover the kernel version you are using.
>
That really doesn't help us. We don't base which kernels we run in
datacenters on what distros are doing-- they don't seem to move as
fast in rebsing. Our general request is that vendors always do their
development upstream, if we need to do a backport in our kernel then
we take responsibility for that. As I mentioned, the churn and lack of
modularization seem to be making this process more and more difficult.

Tom

> Best regards,
> Martin
>
>> Currently we (FB) need to backport two NIC drivers. I've already gave
>> details of backporting mlx5 on the thread to restructure the driver
>> directories. The other driver being backporting seems to suffer from
>> the same type of feature complexity.
>>
>> In short, I would like to ask if driver maintainers to start to
>> modularize driver features. If something being added is obviously a
>> narrow feature that only a subset of users will need can we allow
>> config options to #ifdef those out somehow? Furthermore can the file
>> and directory structure of drivers reflect that; our lives would be
>> _so_ much simpler to maintain drivers in production if we have such
>> modularity and the ability to build drivers with the features of our
>> choosing.
>>
>> Thanks,
>> Tom