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

[Simon Schippers <simon.schippers@tu-dortmund.de>]

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?

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

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
>