netdev - RE: [net v5 2/3] net: sched: add check tc_skip

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Date:   Fri, 10 Dec 2021 08:52:21 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     John Fastabend <john.fastabend@...il.com>,
        xiangxia.m.yue@...il.com, netdev@...r.kernel.org
Cc:     Tonghao Zhang <xiangxia.m.yue@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Antoine Tenart <atenart@...nel.org>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Wei Wang <weiwan@...gle.com>, Arnd Bergmann <arnd@...db.de>
Subject: RE: [net v5 2/3] net: sched: add check tc_skip_classify in sch egress

John Fastabend wrote:
> xiangxia.m.yue@ wrote:
> > From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> > 
> > Try to resolve the issues as below:
> > * We look up and then check tc_skip_classify flag in net
> >   sched layer, even though skb don't want to be classified.
> >   That case may consume a lot of cpu cycles. This patch
> >   is useful when there are a lot of filters with different
> >   prio. There is ~5 prio in in production, ~1% improvement.
> > 
> >   Rules as below:
> >   $ for id in $(seq 1 5); do
> >   $       tc filter add ... egress prio $id ... action mirred egress redirect dev ifb0
> >   $ done
> > 
> > * bpf_redirect may be invoked in egress path. If we don't
> >   check the flags and then return immediately, the packets
> >   will loopback.
> 
> This would be the naive case right? Meaning the BPF program is
> doing a redirect without any logic or is buggy?
> 
> Can you map out how this happens for me, I'm not fully sure I
> understand the exact concern. Is it possible for BPF programs
> that used to see packets no longer see the packet as expected?
> 
> Is this the path you are talking about?
> 
>  rx ethx  ->
>    execute BPF program on ethx with bpf_redirect(ifb0) ->
>      __skb_dequeue @ifb tc_skip_classify = 1 ->
>        dev_queue_xmit() -> 
>           sch_handle_egress() ->
>             execute BPF program again
> 
> I can't see why you want to skip that second tc BPF program,
> or for that matter any tc filter there. In general how do you
> know that is the correct/expected behavior? Before the above
> change it would have been called, what if its doing useful
> work.
> 
> Also its not clear how your ifb setup is built or used. That
> might help understand your use case. I would just remove the
> IFB altogether and the above discussion is mute.
> 
> Thanks,
> John

After a bit further thought (and coffee) I think this will
break some programs that exist today. Consider the case
where I pop a header off and resubmit to the same device
intentionally to reprocess the pkt without the header. I've
used this pattern in BPF a few times.