netdev - Re: [patch net-next v3 00/15] introduce programable flow dissector and cls

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Date:	Wed, 13 May 2015 15:27:59 -0400
From:	Tom Herbert <tom@...bertland.com>
To:	David Miller <davem@...emloft.net>
Cc:	Jiří Pírko <jiri@...nulli.us>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Thomas Graf <tgraf@...g.ch>, jesse@...ira.com,
	Patrick McHardy <kaber@...sh.net>,
	Eric Dumazet <edumazet@...gle.com>,
	alexander.h.duyck@...hat.com,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Alexei Starovoitov <ast@...mgrid.com>, daniel@...earbox.net,
	Herbert Xu <herbert@...dor.apana.org.au>,
	cwang@...pensource.com, john fastabend <john.fastabend@...il.com>
Subject: Re: [patch net-next v3 00/15] introduce programable flow dissector
 and cls_flower

On Wed, May 13, 2015 at 3:21 PM, David Miller <davem@...emloft.net> wrote:
> From: Jiri Pirko <jiri@...nulli.us>
> Date: Tue, 12 May 2015 14:56:06 +0200
>
>> Per Davem's request, I prepared this patchset which introduces programmable
>> flow dissector. For current users of flow_keys, there is a wrapper
>> skb_flow_dissect_flow_keys which maintains the previous behaviour.
>> For purposes of cls_flower, couple of new dissection keys were introduced.
>>
>> Note that this dissector can be also eventually used by openvswitch code.
>>
>> Also, as a next step, I plan to get rid of *skb_flow_get_ports(export)
>> and *__skb_get_poff as their functionality can be now implemented by
>> skb_flow_dissect as well.
>>
>> v2->v3:
>> - remove TCA_FLOWER_POLICE attr suggested by Jamal
>>
>> v1->v2:
>> - move __skb_tx_hash rather to dev.c as suggested by Alex
>
> Ok, assuming this passes all of my build tests, I'll push this into
> net-next.
>
> I'm sure there will be some performance improvements possible, and I
> hope you will look into making sure this new programmable classifier
> is as light weight as possible.
>
> Anyways, thanks a lot.

David,

I still have concerns about making flow_dissector more complex like
this. This still seems like it should this programmable logic be done
in a separate function. We call flow_dissector at least once per
packet via skb_get_hash, it is in the critical path, and adding
several conditionals can only slow it down and provides no new value
to skb_get_hash. At the very least can we at least get some
performance numbers to show impact of this?

Thanks,
Tom
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