Re: [patch net-next v3] tc: introduce OpenFlow classifier

[David Miller <davem@davemloft.net>]

From: Jiri Pirko <jiri@resnulli.us>
Date: Sun, 12 Apr 2015 09:53:51 +0200

> Sat, Apr 11, 2015 at 06:12:25PM CEST, alexei.starovoitov@gmail.com wrote:
>>On Fri, Apr 10, 2015 at 02:45:17PM +0200, Jiri Pirko wrote:
>>> Okay. That was misunderstanding. I was thinking about using existing
>>> flow_dissect. There are couple things which I'm scared of:
>>> - there are eventually many fields to be added to dissection function and to
>>>   the structure as well. Not sure how acceptable that would be for
>>>   performance reasons when flow_dissect is used by different users...
>>
>>I share the same concern. I think flow_dissect is too performance
>>critical to reuse by expanding 'struct flow_keys'.
>>I think it would be better to generalize ovs's key_extract() into
>>common piece of code that TC classifier and ovs datapath can use.
>>It uses kernel internal 'struct sw_flow_key' which we can tweak to
>>accommodate more users. It's already gigantic at 392 bytes, so
>>split and a bit of diet would help too.
> 
> Yep, those are few next topics on my agenda.

This argument kinda ignores the fact that full flow dissection is run
on _every_ single RX packet on basically all Intel chipsets.

Therefore, I cannot take seriously someone saying that it is too much
overhead for a classifier.

And if it is that expensive, fix it, because that helps everyone not
just your special thing.