On Wed, Sep 24, 2025 at 07:59:46AM +0200, Simon Schippers wrote: > 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.
So this breaks bisect. Don't split patches like this please. > >> 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 > >
