netdev - Re: [PATCHv8 bpf-next] samples/bpf: add xdp program on egress for xdp_redirect

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Date:   Wed, 20 Jan 2021 12:16:28 +0800
From:   Hangbin Liu <liuhangbin@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Yonghong Song <yhs@...com>,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCHv8 bpf-next] samples/bpf: add xdp program on egress for
 xdp_redirect_map

On Tue, Jan 19, 2021 at 03:51:27PM +0100, Jesper Dangaard Brouer wrote:
> > @@ -73,13 +90,63 @@ int xdp_redirect_map_prog(struct xdp_md *ctx)
> >  
> >  	/* count packet in global counter */
> >  	value = bpf_map_lookup_elem(&rxcnt, &key);
> > -	if (value)
> > +	if (value) {
> >  		*value += 1;
> > +		if (*value % 2 == 1)
> > +			vport = 1;
> 
> This will also change the base behavior of the program, e.g when we are
> not testing the 2nd xdp-prog.  It will become hard to compare the
> performance between xdp_redirect and xdp_redirect_map.

I just did a test with/without this patch on 5.10 using pktgen

By ./xdp_redirect_map -N/-S eno1 eno1

Without this patch
- S 1.8M pps
- N 7.4M pps

With this patch
- S 1.9M pps
- N 7.4M pps

So I don't see much difference.

> 
> It looks like you are populating vport=0 and vport=1 with the same ifindex.
> Thus, this code is basically doing packet reordering, due to the per
> CPU bulking layer (of 16 packets) in devmap.
> Is this the intended behavior?

I didn't expect this could cause reordering. If we want only do it when
attached the 2nd prog, we need add a check like

key = 1;
value = bpf_map_lookup_elem(&tx_port_native , &key);
if (value)
	do_2nd_prog_test
else
	do_nothing_and_redirect_with_port_0

But an extra bpf_map_lookup_elem() for every packet may cause more performance
drop. So WDYT?

Thanks
Hangbin