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? > 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
