netdev - Re: [PATCH net-next 2/2] flow_dissector: dissect tunnel info

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Message-ID: <CALx6S36c=BUXb7-53Ym_SZYuWqmPfEqdYUwwxyM78SfBnEPL4Q@mail.gmail.com>
Date:   Wed, 4 Oct 2017 08:52:54 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     Simon Horman <simon.horman@...ronome.com>,
        David Miller <davem@...emloft.net>,
        Jiri Pirko <jiri@...lanox.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com
Subject: Re: [PATCH net-next 2/2] flow_dissector: dissect tunnel info

On Wed, Oct 4, 2017 at 1:15 AM, Jiri Pirko <jiri@...nulli.us> wrote:
> Wed, Oct 04, 2017 at 10:08:57AM CEST, simon.horman@...ronome.com wrote:
>>On Tue, Oct 03, 2017 at 11:17:46AM -0700, Tom Herbert wrote:
>>> On Tue, Oct 3, 2017 at 2:40 AM, Simon Horman <simon.horman@...ronome.com> wrote:
>>> > On Mon, Oct 02, 2017 at 01:37:55PM -0700, Tom Herbert wrote:
>>> >> On Mon, Oct 2, 2017 at 1:41 AM, Simon Horman <simon.horman@...ronome.com> wrote:
>>> >> > Move dissection of tunnel info from the flower classifier to the flow
>>> >> > dissector where all other dissection occurs.  This should not have any
>>> >> > behavioural affect on other users of the flow dissector.
>>> >
>>> > ...
>>>
>>> > I feel that we are circling back the perennial issue of flower using the
>>> > flow dissector in a somewhat broader/different way than many/all other
>>> > users of the flow dissector.
>>> >
>>> Simon,
>>>
>>> It's more like __skb_flow_dissect is already an incredibly complex
>>> function and because of that it's difficult to maintain. We need to
>>> measure changes against that fact. For this patch, there is precisely
>>> one user (cls_flower.c) and it's not at all clear to me if there will
>>> be ever any more (e.g. for hashing we don't need tunnel info). IMO, it
>>> should be just as easy and less convolution for everyone to have
>>> flower call __skb_flow_dissect_tunnel_info directly and not call if
>>> from __skb_flow_dissect.
>>
>>Hi Tom,
>>
>>my original suggestion was just that, but Jiri indicated a strong preference
>>for the approach taken by this patch. I think we need to widen the
>>participants in this discussion.
>
> I like the __skb_flow_dissect to be the function to call and it will do
> the job according to the configuration. I don't like to split in
> multiple calls.

Those are not technical arguments. As I already mentioned, I don't
like it when we add stuff for the benefit of a 1% use case that
negatively impacts the rest of the 99% cases which is what I believe
is happening here.

> Does not make sense in the most of the cases as the
> dissection state would have to be carried in between calls.

Please elaborate. This code is being moved into __skb_flow_dissect, so
the functionality was already there. I don't see any description in
this discussion that things were broken and that this patch is a
necessary fix.

Thanks,
Tom