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

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Date:   Sat, 11 Dec 2021 01:43:36 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "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>,
        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

On Sat, Dec 11, 2021 at 12:52 AM John Fastabend
<john.fastabend@...il.com> wrote:
>
> 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.
No, ifb netdev sets the skb->tc_skip_classify = 1, that means we
should not process the skb again, no matter on egress or ingress.
if the bpf programs don't set the skb->tc_skip_classify = 1, then we
can process them again.

-- 
Best regards, Tonghao