Message-ID: <20250902080957.47265-1-simon.schippers@tu-dortmund.de>
Date: Tue,  2 Sep 2025 10:09:53 +0200
From: Simon Schippers <simon.schippers@...dortmund.de>
To: willemdebruijn.kernel@...il.com, jasowang@...hat.com, mst@...hat.com,
        eperezma@...hat.com, stephen@...workplumber.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux.dev, kvm@...r.kernel.org
Cc: Simon Schippers <simon.schippers@...dortmund.de>
Subject: [PATCH net-next v4 0/4] TUN/TAP & vhost_net: netdev queue flow control to avoid ptr_ring tail drop

This patch series deals with TUN/TAP and 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 before this happens. 
This allows the connected qdisc to function correctly as reported by [1] 
and improves application-layer performance, see benchmarks.

This patch series includes TUN, TAP, and vhost_net because they share 
logic. Adjusting only one of them would break the others. Therefore, the 
patch series is structured as follows:
1. New ptr_ring_spare helper to check if the ptr_ring has spare capacity
2. Netdev queue flow control for TUN: Logic for stopping the queue upon 
full ptr_ring and waking the queue if ptr_ring has spare capacity
3. Additions for TAP: Similar logic for waking the queue
4. Additions for vhost_net: Calling TUN/TAP methods for waking the queue

Benchmarks ([2] & [3]):
- TUN: TCP throughput over real-world 120ms RTT OpenVPN connection 
improved by 36% (117Mbit/s vs 185 Mbit/s)
- TAP: TCP throughput to local qemu VM stays the same (2.2Gbit/s), an 
improvement by factor 2 at emulated 120ms RTT (98Mbit/s vs 198Mbit/s)
- TAP+vhost_net: TCP throughput to local qemu VM approx. the same 
(23.4Gbit/s vs 23.9Gbit/s), same performance at emulated 120ms RTT 
(200Mbit/s)
- TUN/TAP/TAP+vhost_net: Reduction of ptr_ring size to ~10 packets 
possible without losing performance

Possible future work:
- Introduction of Byte Queue Limits as suggested by Stephen Hemminger
- Adaption of the netdev queue flow control for ipvtap & macvtap

[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://github.com/tudo-cni/nodrop

Links to previous versions:
V3: 
https://lore.kernel.org/netdev/20250825211832.84901-1-simon.schippers@tu-dortmund.de/T/#u
V2: 
https://lore.kernel.org/netdev/20250811220430.14063-1-simon.schippers@tu-dortmund.de/T/#u
V1: 
https://lore.kernel.org/netdev/20250808153721.261334-1-simon.schippers@tu-dortmund.de/T/#u

Changelog:
V3 -> V4:
- 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

V2 -> V3: Added support for TAP and TAP+vhost_net.

V1 -> V2: Removed NETDEV_TX_BUSY return case in tun_net_xmit and removed 
unnecessary netif_tx_wake_queue in tun_ring_recv.



Simon Schippers (4):
  ptr_ring_spare: Helper to check if spare capacity of size cnt is
    available
  netdev queue flow control for TUN
  netdev queue flow control for TAP
  netdev queue flow control for vhost_net

 drivers/net/tap.c        | 28 ++++++++++++++++
 drivers/net/tun.c        | 39 ++++++++++++++++++++--
 drivers/vhost/net.c      | 34 +++++++++++++++----
 include/linux/if_tap.h   |  2 ++
 include/linux/if_tun.h   |  3 ++
 include/linux/ptr_ring.h | 71 ++++++++++++++++++++++++++++++++++++++++
 6 files changed, 168 insertions(+), 9 deletions(-)

-- 
2.43.0

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