netdev - Re: [PATCH net-next 4/4] act_mirred: use ACT

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Date:   Tue, 17 Jul 2018 11:15:59 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Jiri Pirko <jiri@...nulli.us>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Eyal Birger <eyal.birger@...il.com>
Subject: Re: [PATCH net-next 4/4] act_mirred: use ACT_REDIRECT when possible

Hi,

On Mon, 2018-07-16 at 16:39 -0700, Cong Wang wrote:
> On Fri, Jul 13, 2018 at 2:55 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > 
> > When mirred is invoked from the ingress path, and it wants to redirect
> > the processed packet, it can now use the ACT_REDIRECT action,
> > filling the tcf_result accordingly.
> > 
> > This avoids a skb_clone() in the TC S/W data path giving a ~10%
> > improvement in forwarding performances. Overall TC S/W performances
> > are now comparable to the kernel openswitch datapath.

Thank you for the feedback.

> Avoiding skb_clone() for redirection is cool, but why need to use
> skb_do_redirect() here?

Well, it's not needed. I tried to reduce code duplication, and I tried
to avoid adding another TC_ACT_* value.

> There is a subtle difference here:
> 
> skb_do_redirect() calls __bpf_rx_skb() which calls
> dev_forward_skb().
>
> while the current mirred action doesn't scrub packets when
> redirecting to ingress (from egress). Although I forget if it is
> intentionally.

Understood.

A possible option out of this issues would be adding another action
value - TC_ACT_MIRRED ? - and handle it in sch_handle_egress()[1] with
the appropriate semantic. That should address also Daniel and Eyal
concerns.

Would you consider the above acceptable?

Thanks,

Paolo