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 our paper [2]. Meanwhile the theoretical performance differs only slightly:
+------------------------+----------+----------+ | pktgen benchmarks | Stock | Patched | | i5 6300HQ, 20M packets | | | +------------------------+----------+----------+ | TAP | 2.10Mpps | 1.99Mpps | +------------------------+----------+----------+ | TAP+vhost_net | 6.05Mpps | 6.14Mpps | +------------------------+----------+----------+ | Note: Patched had no TX drops at all, | | while stock suffered numerous drops. | +----------------------------------------------+ 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+2: New ptr_ring helpers for 3 & 4 3: TUN & TAP: Stop netdev queue upon reaching a full ptr_ring 4: TUN & TAP: Wake netdev queue after consuming an entry 5+6+7: TUN & TAP: ptr_ring wrappers and other helpers to be called by vhost_net 8: vhost_net: Call the wrappers & helpers 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 Links to previous versions: V4: https://lore.kernel.org/netdev/20250902080957.47265-1-simon.schipp...@tu-dortmund.de/T/#u 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: V4 -> V5: - 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 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. Thanks, Simon :) Simon Schippers (8): __ptr_ring_full_next: Returns if ring will be full after next insertion Move the decision of invalidation out of __ptr_ring_discard_one TUN, TAP & vhost_net: Stop netdev queue before reaching a full ptr_ring TUN & TAP: Wake netdev queue after consuming an entry TUN & TAP: Provide ptr_ring_consume_batched wrappers for vhost_net TUN & TAP: Provide ptr_ring_unconsume wrappers for vhost_net TUN & TAP: Methods to determine whether file is TUN/TAP for vhost_net vhost_net: Replace rx_ring with calls of TUN/TAP wrappers drivers/net/tap.c | 115 +++++++++++++++++++++++++++++++-- drivers/net/tun.c | 136 +++++++++++++++++++++++++++++++++++---- drivers/vhost/net.c | 90 +++++++++++++++++--------- include/linux/if_tap.h | 15 +++++ include/linux/if_tun.h | 18 ++++++ include/linux/ptr_ring.h | 54 +++++++++++++--- 6 files changed, 367 insertions(+), 61 deletions(-) -- 2.43.0
