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.schipp...@tu-dortmund.de/T/#u V2: https://lore.kernel.org/netdev/20250811220430.14063-1-simon.schipp...@tu-dortmund.de/T/#u V1: https://lore.kernel.org/netdev/20250808153721.261334-1-simon.schipp...@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
