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

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Date:   Sun, 12 Dec 2021 17:40:51 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     John Fastabend <john.fastabend@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        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 4:11 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 12/10/21 8:54 PM, Tonghao Zhang wrote:
> > On Sat, Dec 11, 2021 at 1:46 AM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> >> On Sat, Dec 11, 2021 at 1:37 AM Tonghao Zhang <xiangxia.m.yue@...il.com> wrote:
> >>> On Sat, Dec 11, 2021 at 12:43 AM John Fastabend
> >>> <john.fastabend@...il.com> 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?
> >>> Hi John
> >>> Tx ethx -> __dev_queue_xmit -> sch_handle_egress
> >>> ->  execute BPF program on ethx with bpf_redirect(ifb0) ->
> >>> -> ifb_xmit -> ifb_ri_tasklet -> dev_queue_xmit -> __dev_queue_xmit
> >>> the packets loopbacks, that means bpf_redirect doesn't work with ifb
> >>> netdev, right ?
> >>> so in sch_handle_egress, I add the check skb_skip_tc_classify().
>
> But why would you do that? Usage like this is just broken by design..
> If you need to loop anything back to RX, just use bpf_redirect() with
> BPF_F_INGRESS? What is the concrete/actual rationale for ifb here?
Hi
note that: ifb_ri_tasklet can send out the packets or receive skb
ifb_ri_tasklet
                if (!skb->from_ingress) {
                        dev_queue_xmit(skb); // bpf_redirect to ifb
and ifb invoked the dev_queue_xmit in our case.
                } else {
                        skb_pull_rcsum(skb, skb->mac_len);
                        netif_receive_skb(skb);
                }
-- 
Best regards, Tonghao