Re: [PATCH net-next v5 0/8] TUN/TAP & vhost_net: netdev queue flow control to avoid ptr_ring tail drop

["Michael S. Tsirkin" <mst@redhat.com>]

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/20250902080957.47265-1-simon.schippers@tu-dortmund.de/T/#u
> V3: 
> https://lore.kernel.org/netdev/20250825211832.84901-1-simon.schippers@tu-dortmund.de/T/#u
> V2: 
> https://lore.kernel.org/netdev/20250811220430.14063-1-simon.schippers@tu-dortmund.de/T/#u
> V1: 
> https://lore.kernel.org/netdev/20250808153721.261334-1-simon.schippers@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