Re: [PATCH net v2] tun/tap & vhost-net: make qdisc backpressure opt-in via IFF_BACKPRESSURE

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 06, 2026 at 12:11:15PM +0200, Brett A C Sheffield wrote:
> On 2026-07-06 11:42, Simon Schippers wrote:
> > Commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop
> > when a qdisc is present") did not show a relevant performance regression
> > in my testing but on Brett Sheffield's librecast testbed it shows a
> > significant performance drop in a IPv6 multicast testcase. The regression
> > can be pinpointed when multiple iperf3 UDP threads are sending. For 8
> > threads the performance dropped from 13.5 Gbit/s to 9.13 Gbit/s. This is
> > the reason why this patch makes the qdisc backpressure behavior opt-in.
> 
> Your v1 commit message was correct.  The iperf3 tests were TCP, not UDP.
> 
> The original failing test that alerted me to the problem was IPv6 multicast
> (UDP), but the reproducer tests I provided stats for in the regression report
> were TCP "To eliminate my code and any multicast weirdness" and also to verify
> that this also affected TCP.
> 
> Sorry for the confusion. The command lines used are in the regression report.
> 
> I've tested the v2 patch (with IPv6 multicast), and verified the
> previously failing test passes.
> 
> Tested-by: Brett A C Sheffield <bacs@librecast.net>
> 
> Cheers,


Just to clarify, it's more of a work-around, not a fix.

It's not really great to have a flag that says "change something
opaque in the internals of the device, it affects performance
in some way, we can't predict how".

So maybe we really should revert for now, and work on something
more coherent for the next linux.


> 
> Brett
> -- 
> Brett Sheffield (he/him)
> Librecast - Decentralising the Internet with Multicast
> https://librecast.net/
> https://blog.brettsheffield.com/