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

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Date:   Fri, 10 Dec 2021 21:11:03 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Tonghao Zhang <xiangxia.m.yue@...il.com>,
        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>,
        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 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?