Re: [PATCH net 0/4] Revert "tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops"

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

On Tue, Jul 28, 2026 at 11:22:36AM +0200, 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 causes a
> significant throughput drop in an IPv6 multicast testcase. The regression
> can be pinpointed to multiple iperf3 TCP threads sending: for 8 threads
> the throughput dropped from 13.5 Gbit/s to 9.13 Gbit/s.
> 
> Therefore this series reverts the qdisc backpressure work.
> 
> Making the backpressure opt-in via a new IFF_BACKPRESSURE flag was
> proposed in [1], but a new IFF_* flag needs more review scrutiny than is
> available at the moment, so a revert was requested instead. The opt-in
> will be resubmitted for net-next later.


Acked-by: Michael S. Tsirkin <mst@redhat.com>


Thanks!

> [1] Link: https://lore.kernel.org/netdev/20260709095511.168235-1-simon.schippers@tu-dortmund.de/
> 
> Reported-by: Brett Sheffield <brett@librecast.net>
> Closes: https://lore.kernel.org/netdev/akVnoOYQOrt8k-Gu@karahi.librecast.net/
> 
> Simon Schippers (4):
>   Revert "tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is
>     present"
>   Revert "ptr_ring: move free-space check into separate helper"
>   Revert "vhost-net: wake queue of tun/tap after ptr_ring consume"
>   Revert "tun/tap: add ptr_ring consume helper with netdev queue wakeup"
> 
>  drivers/net/tun.c        | 109 +++------------------------------------
>  drivers/vhost/net.c      |  21 +++-----
>  include/linux/if_tun.h   |   3 --
>  include/linux/ptr_ring.h |  20 +------
>  4 files changed, 14 insertions(+), 139 deletions(-)
> 
> 
> base-commit: b14361aca6350ff7907b0e9903c7b94dc7d5d4a0
> -- 
> 2.43.0