netdev - Re: [PATCH RFC net-next 00/11] udp gso

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Date:   Fri, 31 Aug 2018 11:09:44 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH RFC net-next 00/11] udp gso

Hi,

On Tue, 2018-04-17 at 17:07 -0400, Willem de Bruijn wrote:
> That said, for negotiated flows an inverse GRO feature could
> conceivably be implemented to reduce rx stack traversal, too.
> Though due to interleaving of packets on the wire, it aggregation
> would be best effort, similar to TCP TSO and GRO using the
> PSH bit as packetization signal.

Reviving this old thread, before I forgot again. I have some local
patches implementing UDP GRO in a dual way to current GSO_UDP_L4
implementation: several datagram with the same length are aggregated
into a single one, and the user space receive a single larger packet
instead of multiple ones. I hope quic can leverage such scenario, but I
really know nothing about the protocol.

I measure roughly a 50% performance improvement with udpgso_bench in
respect to UDP GSO, and ~100% using a pktgen sender, and a reduced CPU
usage on the receiver[1]. Some additional hacking to the general GRO
bits is required to avoid useless socket lookups for ingress UDP
packets when UDP_GSO is not enabled.

If there is interest on this topic, I can share some RFC patches
(hopefully somewhat next week).

Cheers,

Paolo

[1] With udpgso_bench_tx, the bottle-neck is again the sender, even
with GSO enabled. With a pktgen sender, the bottle-neck become the rx
softirqd, and I see a lot of time consumed due to retpolines in the GRO
code. In both scenarios skb_release_data() becomes the topmost perf
offender for the user space process.