Re: [PATCH net-next v12 0/4] tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops

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

On Sun, May 10, 2026 at 05:15:25PM +0200, Simon Schippers wrote:
> This patch series deals with tun/tap & vhost-net which drop incoming
> SKBs whenever their internal ptr_ring buffer is full. Instead, with this 
> patch series, the associated netdev queue is stopped - but only when a
> qdisc is attached. If no qdisc is present the existing behavior is
> preserved. The XDP transmit path is not affected. This patch series
> touches tun/tap and vhost-net, as they share common logic and must be
> updated together. Modifying only one of them would break the other.
> 
> By applying proper backpressure, this change allows the connected qdisc to 
> operate correctly, as reported in [1], and significantly improves
> performance in real-world scenarios, as demonstrated in our paper [2]. For 
> example, we observed a 36% TCP throughput improvement for an OpenVPN 
> connection between Germany and the USA.
> 
> Synthetic pktgen benchmarks indicate a slight regression, and packet
> loss is reduced to near zero. Pktgen benchmarks are provided per commit,
> with the final commit showing the overall performance.

at v12, time to merge this.

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

> Thanks!
> 
> [1] Link: https://unix.stackexchange.com/questions/762935/traffic-shaping-ineffective-on-tun-device
> [2] Link: https://cni.etit.tu-dortmund.de/storages/cni-etit/r/Research/Publications/2025/Gebauer_2025_VTCFall/Gebauer_VTCFall2025_AuthorsVersion.pdf
> 
> ---
> Changelog:
> v12:
> Patch 1: 
> - Revert tun_queue_purge() to plain ptr_ring_consume() and instead
>   explicitly wake the queue in __tun_detach() for the ntfile taking
>   over the queue slot (if its ring is empty).
> - Inlined tun_reset_cons_cnt(), because only tun_attach() uses it.
> 
> - Patches 2-4 and cover letter unchanged.
> - Compiled and short pktgen test.
> 
> v11:
> - Renamed __ptr_ring_produce_peek() to __ptr_ring_check_produce()
>   (Sashiko)
> - Add return code -EINVAL to __ptr_ring_check_produce() which lets
>   tun_net_xmit() stop the queue only on -ENOSPC. (MST)
> - Resolve race on tfile->queue_index by locking tx_ring.consumer_lock
>   in __tun_detach(). (Sashiko)
> - Wake the queue in tun_queue_resize() to avoid possible stalls.
> - Other minor adjustments & reran the benchmarks.
> 
> v10: https://lore.kernel.org/netdev/20260506141033.180450-1-simon.schippers@tu-dortmund.de/
> - Changed the term "Transmitted" to "Received" in the benchmarks,
>   as correctly pointed out by MST, and reran the benchmarks.
> 
> Addressed the Sashiko AI review:
> - Avoid a data race on tfile->cons_cnt by always locking.
> - Correctly count the number of consumed packets for vhost-net.
> - Corrected a typo in the commit message of commit 3.
> - Added a missing barrier on the consumer side.
> --> The barriers now follow the "store buffering" principle.
> - No longer return NETDEV_TX_BUSY at all, because it is unsafe.
> --> Result: There are still a few drops with multiple senders, which
>             would be avoided by disabling LLTX.
> 
> V9: https://lore.kernel.org/netdev/20260428123859.19578-1-simon.schippers@tu-dortmund.de/
> - Addressed minor nit by MST in patches 1 and 2.
> - Rebased patch 3 because of commit d748047
>   ("ptr_ring: disable KCSAN warnings").
> - Documented the pair of the smp_mb__after_atomic() in tun_net_xmit()
>   with tun_ring_consume().
>   --> It simply pairs with the test_and_clear_bit() inside of
>       netif_wake_subqueue().
> - Use 1 ptr_ring consumer spinlock instead of 2.
> - Ran pktgen benchmarks with pg_set SHARED for 50 iterations on
>   latest kernel
>   --> No significant performance difference noticed
> 
> V8: https://lore.kernel.org/netdev/20260312130639.138988-1-simon.schippers@tu-dortmund.de/
> - Drop code changes in drivers/net/tap.c; The code there deals with
>   ipvtap/macvtap which are unrelated to the goal of this patch series
>   and I did not realize that before
> -> Greatly simplified logic, 4 instead of 9 commits
> -> No more duplicated logics and distinction in vhost required
> - Only wake after the queue stopped and half of the ring was consumed
>   as suggested by MST
> -> Performance improvements for TAP, but still slightly slower
> - Better benchmarking with pinned threads, XDP drop program for
>   tap+vhost-net and disabling CPU mitigations (and newer Ryzen 5 5600X
>   processor) as suggested by Jason Wang
> 
> V7: https://lore.kernel.org/netdev/20260107210448.37851-1-simon.schippers@tu-dortmund.de/
> - Switch to an approach similar to veth (excluding the recently fixed 
> variant), as suggested by MST, with minor adjustments discussed in V6
> - Rename the cover-letter title
> - Add multithreaded pktgen and iperf3 benchmarks, as suggested by Jason 
> Wang
> - Rework __ptr_ring_consume_created_space() so it can also be used after 
> batched consume
> 
> ...
> 
> ---
> 
> Simon Schippers (4):
>   tun/tap: add ptr_ring consume helper with netdev queue wakeup
>   vhost-net: wake queue of tun/tap after ptr_ring consume
>   ptr_ring: move free-space check into separate helper
>   tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present
> 
>  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, 139 insertions(+), 14 deletions(-)
> 
> -- 
> 2.43.0