Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance

[Brett Sheffield <bacs@librecast.net>]

On 2026-07-02 09:24, Simon Schippers wrote:
> On 7/1/26 22:56, Michael S. Tsirkin wrote:
> > On Wed, Jul 01, 2026 at 09:16:48PM +0200, Brett Sheffield wrote:
> >> TL;DR - Commit 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 causes
> >> significant performance regressions with TAP interfaces and multithreaded
> >> network code. Please revert.
> >>
> >>
> >> Librecast is an IPv6 multicast library. One of the tests (0055) fails under
> >> Linux 7.2-rc1. The test performs data synchronization over IPv6 multicast using a TAP
> >> interface. This test has run successfully on every stable, LTS and mainline RC
> >> released in the past year. Every kernel with my Tested-by has run this test.
> >>
> >> There have been a bunch of changes to MLDv2 so I started bisecting there, but
> >> the culprit is actually 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 "tun/tap &
> >> vhost-net: avoid ptr_ring tail-drop when a qdisc is present"
> >>
> >> Reverting this commit fixes the test.
> >>
> >> To eliminate my code and any multicast weirdness, I ran tests with iperf3
> >> comparing the same host running 7.2-rc1 both with and without 1d6e569b7d0
> >> reverted.
> 
> Thank you very much for your bisect!
> 
> As the author, I am sorry for that regression!

No worries. That's why we test :-)

> > - does it help to increase the tun queue size?
> 
> I agree, this would be great to know.
> 
> However, even then we must act. I am considering IFF_BACKPRESSURE
> as a feature flag, defaulting to off. It would just enable/disable
> the stopping logic in tun_net_xmit() and the waking logic
> in __tun_wake_queue(). If disabled, it would result in the same logic
> as before.
> 
> I could provide such a patch as [net] material.

I'm going to make myself a strong cup of tea and dig into it a bit more here and
will let you know if I find anything worth reporting.

If you need me to try re-testing with specific settings or test a patch I'm
happy to do so.

Cheers,


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