On 23.09.25 16:55, Michael S. Tsirkin wrote: > On Tue, Sep 23, 2025 at 12:15:45AM +0200, Simon Schippers wrote: >> 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 > > > so what happens if you only apply patches 1-3? >
The netdev queue of vhost_net would be stopped by tun_net_xmit but will never be woken again. >> 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/[email protected]/T/#u >> V3: >> https://lore.kernel.org/netdev/[email protected]/T/#u >> V2: >> https://lore.kernel.org/netdev/[email protected]/T/#u >> V1: >> https://lore.kernel.org/netdev/[email protected]/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 >
