netdev - Re: [PATCH net-next rfc 5/6] net-tc: convert tc_at to tc_at

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Date:   Wed, 4 Jan 2017 15:20:07 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Florian Westphal <fw@...len.de>, dborkman@...earbox.net,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next rfc 5/6] net-tc: convert tc_at to tc_at_ingress

On Wed, Jan 4, 2017 at 2:51 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
> On 01/04/2017 08:26 PM, Willem de Bruijn wrote:
>>>>
>>>> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
>>>> index ef53ede..be4e18d 100644
>>>> --- a/net/sched/sch_api.c
>>>> +++ b/net/sched/sch_api.c
>>>> @@ -1865,6 +1865,7 @@ int tc_classify(struct sk_buff *skb, const struct
>>>> tcf_proto *tp,
>>>>          const struct tcf_proto *old_tp = tp;
>>>>          int limit = 0;
>>>>
>>>> +       skb->tc_at_ingress = !!(tp && tp->q->flags & TCQ_F_INGRESS);
>>>
>>>
>>> I'd prefer if skb->tc_at_ingress is set directly to 0/1 in
>>> sch_handle_ingress()
>>> and __dev_queue_xmit() as we do right now, this would avoid above tests
>>> in
>>> fast
>>> path and it would also avoid to set the same thing in tc_classify()
>>> multiple
>>> times f.e. on egress path walking through multiple qdiscs. I don't see
>>> anything
>>> in layers above tc that would read it and expect an AT_STACK-like
>>> equivalent.
>>> skb_reset_tc() could thus still remain as you have above in fast-path
>>> like
>>> __netif_receive_skb_core().
>>
>>
>> I had been thinking about that. After submitting this I noticed that
>> Florian's
>> patchset had an elegant solution to avoid the branch: set tc_at_ingress in
>> handle_ing before tc_classify and clear it on the return path.
>>
>> Then we only set + clear it once on ingress regardless of the depth
>> of classifiers and do not touch it at all in other code.
>>
>> https://patchwork.ozlabs.org/patch/472698/
>>
>> What do you think of that approach?
>
>
> I think this approach might not work, it would certainly change semantics
> since right now *before* going into tc_classify() we always update
> skb->tc_verd's
> "at" location. After the patch we'd set TC_AT_INGRESS in ingress and could
> redirect within tc_classify() [and we'd skip that skb->tc_verd = 0 we have
> in
> __netif_receive_skb_core() for these] to xmit from there where next call
> into
> classifier from __dev_queue_xmit() call-site misses that we're not at
> ingress
> anymore but already at egress, so it would do wrong pull/push of headers in
> some cls, for example.

Oh, yes, of course. Okay, I will follow the existing code in v1. Thanks, Daniel.