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

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Message-ID: <20171004191724.GJ1895@nanopsycho>
Date:   Wed, 4 Oct 2017 21:17:24 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     Simon Horman <simon.horman@...ronome.com>
Cc:     Tom Herbert <tom@...bertland.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

Wed, Oct 04, 2017 at 09:07:17PM CEST, simon.horman@...ronome.com wrote:
>On Wed, Oct 04, 2017 at 08:07:15PM +0200, Jiri Pirko wrote:
>> Wed, Oct 04, 2017 at 05:52:54PM CEST, tom@...bertland.com wrote:
>> >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.
>> 
>> Yeah. I just wanted the flow dissector to stay compact. But if needed,
>> could be split. I just fear that it will become a mess that's all.
>> 
>> 
>> >
>> >> 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.
>> 
>> Yeah, you are right.
>
>Hi Tom, Hi Jiri,
>
>I'm happy to make a patch to move the call to
>__skb_flow_dissect_tunnel_info() from __skb_flow_dissect() to
>fl_classify(). It seems that approach has been agreed on above.

If the consensus is that the right way is to cut-out flow dissector,
so be it. But first, I believe it is reasonable to request to see some
numbers that would indicate that it actually resolves anything.