netdev - Re: [PATCH net-next] net: use indirect call wrappers for skb_copy_datagram

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Date:   Wed, 25 Mar 2020 16:58:24 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] net: use indirect call wrappers for skb_copy_datagram_iter()

On Wed, Mar 25, 2020 at 12:00 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On Wed, 2020-03-25 at 10:55 -0400, Willem de Bruijn wrote:
> > On the UDP front this reminded me of another indirect function call
> > without indirect call wrapper: getfrag in __ip_append_data.
> >
> > That is called for each datagram once per linear + once per page. That
> > said, the noise in my quick RR test was too great to measure any
> > benefit from the following.
>
> Why an RR test ?
>
> I think you should be able to measure some raw tput improvement with
> large UDP GSO write towards a blackhole dst/or dropping ingress pkts
> with XDP (just to be sure the bottle-neck is on the sender side).

Thanks for the suggestion. I ran a send-only udpgso_bench_tx test
to a dummy device with NETIF_F_GSO_UDP_L4.

    ip link add dummy0 type dummy
    ip link set dev dummy0 mtu 1500
    ip link set dev dummy0 up
    ip addr add 10.0.0.1/24 dev dummy0
    perf stat -- ./udpgso_bench_tx -C 1 -4 -D 10.0.0.2 -l 5 -S 0

By default, this generates only 3 getfrag calls per sendmsg, due to
sk_page_frag_refill generating 32KB compound pages.

When disabling compound pages by setting sysctl
net.core.high_order_alloc_disable , this increased to 17 getfrag calls
per sendmsg.

Even then any benefit appears to be in the noise. Both reported
10900-11700 MB/s.

The effect of that sysctl, and thus compound pages, was much larger
than I expected. With that disabled, I observed 16500-18100 MBps.

In summary, this particular indirect call does not appear worthwhile.