Re: [PATCH net-next v8 0/4] tun/tap & vhost-net: apply qdisc backpressure on full ptr_ring to reduce TX drops

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

On Fri, Mar 13, 2026 at 10:49:53AM +0100, Simon Schippers wrote:
> On 3/12/26 14:55, Michael S. Tsirkin wrote:
> > On Thu, Mar 12, 2026 at 02:06:35PM +0100, 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. 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.
> >> Pktgen benchmarks are provided per commit, with the final commit showing
> >> the overall performance.
> >>
> >> Thanks!
> > 
> > I posted a minor nit on patch 2.
> > 
> > Otherwise LGTM:
> > 
> > Acked-by: Michael S. Tsirkin <mst@redhat.com>
> > 
> > thanks for the work!
> 
> Thanks!
> 
> Should I do a new version for the minor nit?

It's easier if you do, yes.

> And how about the ptr_ring race:
> I see there is a seperate discussion for that now. [1]
> Should I wait for that?

If there's a conflict it will be trivial to resolve, so I'd say no.

> Before sending a potential new version I would of course wait for
> Jason's take.
> 
> [1] Link: https://lore.kernel.org/netdev/CANn89iJLC9p+N=Rqtuj7ZuPRdpSGCATCtZdz1Vi9mbzf3ATekQ@mail.gmail.com/
> 
> > 
> > 
> >> [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
> >> [3] Link: https://lore.kernel.org/r/174549940981.608169.4363875844729313831.stgit@firesoul
> >> [4] Link: https://lore.kernel.org/r/176295323282.307447.14790015927673763094.stgit@firesoul
> >>
> >> ---
> >> Changelog:
> >> V8:
> >> - 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 [3] (excluding the recently fixed 
> >> variant [4]), 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
> >>
> >> V6: https://lore.kernel.org/netdev/20251120152914.1127975-1-simon.schippers@tu-dortmund.de/
> >> General:
> >> - Major adjustments to the descriptions. Special thanks to Jon Kohler!
> >> - Fix git bisect by moving most logic into dedicated functions and only 
> >> start using them in patch 7.
> >> - Moved the main logic of the coupled producer and consumer into a single 
> >> patch to avoid a chicken-and-egg dependency between commits :-)
> >> - Rebased to 6.18-rc5 and ran benchmarks again that now also include lost 
> >> packets (previously I missed a 0, so all benchmark results were higher by 
> >> factor 10...).
> >> - Also include the benchmark in patch 7.
> >>
> >> Producer:
> >> - Move logic into the new helper tun_ring_produce()
> >> - Added a smp_rmb() paired with the consumer, ensuring freed space of the 
> >> consumer is visible
> >> - Assume that ptr_ring is not full when __ptr_ring_full_next() is called
> >>
> >> Consumer:
> >> - Use an unpaired smp_rmb() instead of barrier() to ensure that the 
> >> netdev_tx_queue_stopped() call completes before discarding
> >> - Also wake the netdev queue if it was stopped before discarding and then 
> >> becomes empty
> >> -> Fixes race with producer as identified by MST in V5
> >> -> Waking the netdev queues upon resize is not required anymore
> >> - Use __ptr_ring_consume_created_space() instead of messing with ptr_ring 
> >> internals
> >> -> Batched consume now just calls 
> >> __tun_ring_consume()/__tap_ring_consume() in a loop
> >> - Added an smp_wmb() before waking the netdev queue which is paired with 
> >> the smp_rmb() discussed above
> >>
> >> V5: https://lore.kernel.org/netdev/20250922221553.47802-1-simon.schippers@tu-dortmund.de/T/#u
> >> - Stop the netdev queue prior to producing the final fitting ptr_ring entry
> >> -> Ensures the consumer has the latest netdev queue state, making it safe 
> >> to wake the queue
> >> -> Resolves an issue in vhost-net where the netdev queue could remain 
> >> stopped despite being empty
> >> -> For TUN/TAP, the netdev queue no longer needs to be woken in the 
> >> blocking loop
> >> -> Introduces new helpers __ptr_ring_full_next and 
> >> __ptr_ring_will_invalidate for this purpose
> >> - vhost-net now uses wrappers of TUN/TAP for ptr_ring consumption rather 
> >> than maintaining its own rx_ring pointer
> >>
> >> V4: https://lore.kernel.org/netdev/20250902080957.47265-1-simon.schippers@tu-dortmund.de/T/#u
> >> - Target net-next instead of net
> >> - Changed to patch series instead of single patch
> >> - Changed to new title from old title
> >> "TUN/TAP: Improving throughput and latency by avoiding SKB drops"
> >> - Wake netdev queue with new helpers wake_netdev_queue when there is any 
> >> spare capacity in the ptr_ring instead of waiting for it to be empty
> >> - Use tun_file instead of tun_struct in tun_ring_recv as a more consistent 
> >> logic
> >> - Use smp_wmb() and smp_rmb() barrier pair, which avoids any packet drops 
> >> that happened rarely before
> >> - Use safer logic for vhost-net using RCU read locks to access TUN/TAP data
> >>
> >> V3: https://lore.kernel.org/netdev/20250825211832.84901-1-simon.schippers@tu-dortmund.de/T/#u
> >> - Added support for TAP and TAP+vhost-net.
> >>
> >> V2: https://lore.kernel.org/netdev/20250811220430.14063-1-simon.schippers@tu-dortmund.de/T/#u
> >> - Removed NETDEV_TX_BUSY return case in tun_net_xmit and removed 
> >> unnecessary netif_tx_wake_queue in tun_ring_recv.
> >>
> >> V1: https://lore.kernel.org/netdev/20250808153721.261334-1-simon.schippers@tu-dortmund.de/T/#u
> >> ---
> >>
> >> 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        | 91 +++++++++++++++++++++++++++++++++++++---
> >>  drivers/vhost/net.c      | 15 +++++--
> >>  include/linux/if_tun.h   |  3 ++
> >>  include/linux/ptr_ring.h | 14 ++++++-
> >>  4 files changed, 111 insertions(+), 12 deletions(-)
> >>
> >> -- 
> >> 2.43.0
> >