linux-kernel - Re: [RFC 00/12] io

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Date:   Wed, 1 Dec 2021 13:51:57 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Pavel Begunkov <asml.silence@...il.com>
CC:     David Ahern <dsahern@...il.com>, <io-uring@...r.kernel.org>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Willem de Bruijn <willemb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: [RFC 00/12] io_uring zerocopy send

On Wed, Dec 01, 2021 at 08:15:28PM +0000, Pavel Begunkov wrote:
> On 12/1/21 19:20, David Ahern wrote:
> > On 12/1/21 12:11 PM, Pavel Begunkov wrote:
> > > btw, why a dummy device would ever go through loopback? It doesn't
> > > seem to make sense, though may be missing something.
> > 
> > You are sending to a local ip address, so the fib_lookup returns
> > RTN_LOCAL. The code makes dev_out the loopback:
> > 
> > https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/net/ipv4/route.c#n2773
> 
> I see, thanks. I still don't use the skb_orphan_frags_rx() hack
> and it doesn't go through the loopback (for my dummy tests), just
> dummy_xmit() and no mention of loopback in perf data, see the
> flamegraph. Don't know what is the catch.
> 
> I'm illiterate of the routing paths. Can it be related to
> the "ip route add"? How do you get an ipv4 address for the device?
I also bumped into the udp-connect() => ECONNREFUSED (111) error from send-zc.
because I assumed no server is needed by using dummy.  Then realized
the cover letter mentioned msg_zerocopy is used as the server.
Mentioning just in case someone hits it also.

To tx out dummy, I did:
#> ip a add 10.0.0.1/24 dev dummy0

#> ip -4 r
10.0.0.0/24 dev dummy0 proto kernel scope link src 10.0.0.1

#> ./send-zc -4 -D 10.0.0.(2) -t 10 udp
ip -s link show dev dummy0
2: dummy0: <BROADCAST,NOARP,UP,LOWER_UP> mtu 65535 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
   link/ether 82:0f:e0:dc:f7:e6 brd ff:ff:ff:ff:ff:ff
   RX:    bytes packets errors dropped  missed   mcast
              0       0      0       0       0       0
   TX:    bytes packets errors dropped carrier collsns
   140800890299 2150397      0       0       0       0