netdev - Re: [PATCH v2] net: sched: check tc_skip

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Date:   Thu, 4 Nov 2021 09:23:25 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH v2] net: sched: check tc_skip_classify as far as possible

On Wed, Nov 3, 2021 at 10:59 PM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 11/3/21 3:47 PM, Willem de Bruijn wrote:
> > On Wed, Nov 3, 2021 at 10:32 AM <xiangxia.m.yue@...il.com> wrote:
> >>
> >> From: Tonghao Zhang <xiangxia.m.yue@...il.com>
> >>
> >> 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.
> >>
> >> Install the rules as below:
> >> $ for id in $(seq 1 100); do
> >> $       tc filter add ... egress prio $id ... action mirred egress redirect dev ifb0
> >> $ done
> >>
> >> netperf:
> >> $ taskset -c 1 netperf -t TCP_RR -H ip -- -r 32,32
> >> $ taskset -c 1 netperf -t TCP_STREAM -H ip -- -m 32
> >>
> >> Before: 10662.33 tps, 108.95 Mbit/s
> >> After:  12434.48 tps, 145.89 Mbit/s
> >>
> >> For TCP_RR, there are 16.6% improvement, TCP_STREAM 33.9%.
> >>
> >> Cc: Willem de Bruijn <willemb@...gle.com>
> >> Cc: Cong Wang <xiyou.wangcong@...il.com>
> >> Cc: Jakub Kicinski <kuba@...nel.org>
> >> Cc: Daniel Borkmann <daniel@...earbox.net>
> >> Signed-off-by: Tonghao Zhang <xiangxia.m.yue@...il.com>
> >> ---
> >> v2: don't delete skb_skip_tc_classify in act_api
> >> ---
> >>   net/core/dev.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >>
> >> diff --git a/net/core/dev.c b/net/core/dev.c
> >> index edeb811c454e..fc29a429e9ad 100644
> >> --- a/net/core/dev.c
> >> +++ b/net/core/dev.c
> >> @@ -3940,6 +3940,9 @@ sch_handle_egress(struct sk_buff *skb, int *ret, struct net_device *dev)
> >>          if (!miniq)
> >>                  return skb;
> >>
> >> +       if (skb_skip_tc_classify(skb))
> >> +               return skb;
> >> +
> >
> > This bit and test exist to make sure that packets redirected through
> > the ifb device are redirected only once.
> >
> > I was afraid that on second loop, this will also short-circuit other
> > unrelated tc classifiers and actions that should take place.
> >
> > The name and the variable comment make clear that the intention is
> > indeed to bypass all classification.
> >
> > However, the current implementation acts at tcf_action_exec. So it
> > does not stop processing by fused classifier-action objects, notably tc_bpf.
> >
> > So I agree both that this seems to follow the original intent, but also
> > has the chance to break existing production configurations.
>
> Agree, I share your concern.
I got it, thanks Daniel, Willem.
One question, If users use the bpf_redirect, instead of tc-mirred, to
redirect the packets to ifb in egress.
If there is not a test with *skb_skip_tc_classify, the packets will
loop and be dropped, right ? but we hope the packets sent out the
linux box.


-- 
Best regards, Tonghao