netdev - Re: [PATCH net-next] net, sched: add clsact qdisc

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Date:	Thu, 07 Jan 2016 16:16:55 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:	davem@...emloft.net, jhs@...atatu.com, john.fastabend@...il.com,
	eric.dumazet@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net, sched: add clsact qdisc

On 01/07/2016 03:42 PM, Hannes Frederic Sowa wrote:
> On 07.01.2016 12:58, Daniel Borkmann wrote:
>> On 01/07/2016 11:09 AM, Hannes Frederic Sowa wrote:
>>> Hi Daniel and Alexei,
>>>
>>> On 07.01.2016 04:53, Alexei Starovoitov wrote:
>>>> On Wed, Jan 06, 2016 at 02:00:56AM +0100, Daniel Borkmann wrote:
>>>>>
>>>>> I decided to extend the sch_ingress module with clsact functionality so
>>>>> that commonly used code can be reused, the module is being aliased with
>>>>> sch_clsact so that it can be auto-loaded properly. Alternative would
>>>>> have been
>>>>> to add a flag when initializing ingress to alter its behaviour plus
>>>>> aliasing
>>>>> to a different name (as it's more than just ingress). However, the
>>>>> first would
>>>>> end up, based on the flag, choosing the new/old behaviour by calling
>>>>> different
>>>>> function implementations to handle each anyway, the latter would
>>>>> require to
>>>>> register ingress qdisc once again under different alias. So, this
>>>>> really begs
>>>>> to provide a minimal, cleaner approach to have Qdisc_ops and
>>>>> Qdisc_class_ops
>>>>> by its own that share callbacks used by both.
>>>> ...
>>>>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>>>>
>>>> we've been going back and forth on the design and this final approach
>>>> presented seems to be the best, since pros outweigh the cons.
>>>>
>>>> Acked-by: Alexei Starovoitov <ast@...nel.org>
>>>
>>> One question:
>>>
>>> With the advance in lockless qdiscs by John Fastabend, is it possible
>>> to push the handle_egress hook further down into sched layer?
>>
>> Idea was that this is done before we pick txq as stated. F.e., could also
>> be that we end up not having enqueue handler, thus moving this further down
>> (not sure if there's a good place?), might make it all more scattered resp.
>> complex to cover all parts.
>
> My understanding is that using redirects and something like ifb/imq-alike could already solve some of those problems while not adding yet another handler to the stack?

Hmm, imq at least is not even upstream [1], quick glance over the code, I'm
also not sure it's a clean or minimal solution. Certainly you need to go
through another dev_queue_xmit() pass, need an additional netdev with extra
classful qdisc just for exec of tc_classify() for the lower dev_queue_xmit().

Thanks,
Daniel

   [1] https://github.com/imq/linuximq/tree/master/kernel/v4.x
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