Re: [PATCH net-next v8 00/11] net: tunnel: introduce noref xmit flows for tunnels

[Paolo Abeni <pabeni@redhat.com>]

On 3/12/26 4:56 PM, Marek Mietus wrote:
> Currently, tunnel xmit flows always take a reference on the dst_entry
> for each xmitted packet. These atomic operations are redundant in some
> flows.
> 
> This patchset introduces the infrastructure required for converting
> the tunnel xmit flows to noref, and converts them where possible.
> 
> These changes improve tunnel performance, since less atomic operations
> are used.
> 
> There are already noref optimizations in both ipv4 and ip6.
> (See __ip_queue_xmit, inet6_csk_xmit)
> This patchset implements similar optimizations in ip and udp tunnels.
> 
> Benchmarks:
> I used a vxlan tunnel over a pair of veth peers and measured the average
> throughput over multiple samples.
> 
> I ran 100 samples on a clean build, and another 100 on a patched
> build. Each sample ran for 120 seconds. These were my results:
> 
> clean:      72.52 gb/sec, stddev = 1.39
> patched:    75.39 gb/sec, stddev = 0.94
> 
> TL;DR - This patchset results in a 4% improvement in throughput for
> vxlan. It's safe to assume that we might see similar results when testing
> other tunnels.

Sabrina noted I wrongly replied on an old revision. Reporting my
statements here for completeness.

IMHO this performance delta is not enough to justify this amount of changes.

Additionally, the measured impact of removing the dst_hold/dst_release
does not fit with my direct experience on the same matter: it should be
below noise level in practice, as dst are per-cpu and and no
contention/false sharing is expected in a good setup.

I think you are observing larger impact because in the veth test
dst_release can happen on a remote CPU. Note that this setup (vxlan over
veth) is not very relevant in practice.

I'm sorry I'm not applying this series.

Side note: if you are interested into improving (UDP) tunnel
performances have a look to big TCP support work from Alice Mikityanska:

https://lore.kernel.org/netdev/20260226201600.222044-1-alice.kernel@fastmail.im/

/P