On 12/1/21 19:20, David Ahern wrote:

On 12/1/21 12:11 PM, Pavel Begunkov wrote:

quoted

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?

(you are not using vrf so ignore the l3mdev reference). loopback device
has the logic to put the skb back in the stack for Rx processing:

https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/drivers/net/loopback.c#n68

-- 
Pavel Begunkov