Message-ID: <20251120152914.1127975-1-simon.schippers@tu-dortmund.de>
Date: Thu, 20 Nov 2025 16:29:05 +0100
From: Simon Schippers <simon.schippers@...dortmund.de>
To: willemdebruijn.kernel@...il.com, jasowang@...hat.com,
        andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com,
        kuba@...nel.org, pabeni@...hat.com, mst@...hat.com,
        eperezma@...hat.com, jon@...anix.com, tim.gebauer@...dortmund.de,
        simon.schippers@...dortmund.de, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        virtualization@...ts.linux.dev
Subject: [PATCH net-next v6 0/8] tun/tap & vhost-net: netdev queue flow control to avoid ptr_ring tail drop

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 to Debian VM | Stock     | Patched  |
| i5 6300HQ, 20M packets         |           |          |
+-----------------+--------------+-----------+----------+
| TAP             | Transmitted  | 195 Kpps  | 183 Kpps |
|                 +--------------+-----------+----------+
|                 | Lost         | 1615 Kpps | 0 pps    |
+-----------------+--------------+-----------+----------+
| TAP+vhost_net   | Transmitted  | 589 Kpps  | 588 Kpps |
|                 +--------------+-----------+----------+
|                 | Lost         | 1164 Kpps | 0 pps    |
+-----------------+--------------+-----------+----------+

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
3: tun/tap: tun/tap: add synchronized ring produce/consume with queue 
management
4+5+6: tun/tap: ptr_ring wrappers and other helpers to be called by 
vhost-net
7: tun/tap & vhost-net: only now use the previous implemented functions to 
not break git bisect
8: tun/tap: drop get ring exports (not used anymore)

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

Changelog:
V6:
General:
- Major adjustments to the descriptions. Special thanks to Jon Kohler!
- Fix git bisect by moving most logic into dedicated functions and only 
start using them in patch 7.
- Moved the main logic of the coupled producer and consumer into a single 
patch to avoid a chicken-and-egg dependency between commits :-)
- Rebased to 6.18-rc5 and ran benchmarks again that now also include lost 
packets (previously I missed a 0, so all benchmark results were higher by 
factor 10...).
- Also include the benchmark in patch 7.

Producer:
- Move logic into the new helper tun_ring_produce()
- Added a smp_rmb() paired with the consumer, ensuring freed space of the 
consumer is visible
- Assume that ptr_ring is not full when __ptr_ring_full_next() is called

Consumer:
- Use an unpaired smp_rmb() instead of barrier() to ensure that the 
netdev_tx_queue_stopped() call completes before discarding
- Also wake the netdev queue if it was stopped before discarding and then 
becomes empty
-> Fixes race with producer as identified by MST in V5
-> Waking the netdev queues upon resize is not required anymore
- Use __ptr_ring_consume_created_space() instead of messing with ptr_ring 
internals
-> Batched consume now just calls 
__tun_ring_consume()/__tap_ring_consume() in a loop
- Added an smp_wmb() before waking the netdev queue which is paired with 
the smp_rmb() discussed above

V5: https://lore.kernel.org/netdev/20250922221553.47802-1-simon.schippers@tu-dortmund.de/T/#u
- 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

V4: https://lore.kernel.org/netdev/20250902080957.47265-1-simon.schippers@tu-dortmund.de/T/#u
- 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

V3: https://lore.kernel.org/netdev/20250825211832.84901-1-simon.schippers@tu-dortmund.de/T/#u
- Added support for TAP and TAP+vhost-net.

V2: https://lore.kernel.org/netdev/20250811220430.14063-1-simon.schippers@tu-dortmund.de/T/#u
- Removed NETDEV_TX_BUSY return case in tun_net_xmit and removed 
unnecessary netif_tx_wake_queue in tun_ring_recv.

V1: https://lore.kernel.org/netdev/20250808153721.261334-1-simon.schippers@tu-dortmund.de/T/#u

Thanks,
Simon :)

Simon Schippers (8):
  ptr_ring: add __ptr_ring_full_next() to predict imminent fullness
  ptr_ring: add helper to check if consume created space
  tun/tap: add synchronized ring produce/consume with queue management
  tun/tap: add batched ring consume function
  tun/tap: add uncomsume function for returning entries to ring
  tun/tap: add helper functions to check file type
  tun/tap & vhost-net: use {tun|tap}_ring_{consume|produce} to avoid
    tail drops
  tun/tap: drop get ring exports

 drivers/net/tap.c        | 106 +++++++++++++++++++++++++--
 drivers/net/tun.c        | 154 +++++++++++++++++++++++++++++++++++----
 drivers/vhost/net.c      |  92 +++++++++++++++--------
 include/linux/if_tap.h   |  16 +++-
 include/linux/if_tun.h   |  18 ++++-
 include/linux/ptr_ring.h |  42 +++++++++++
 6 files changed, 372 insertions(+), 56 deletions(-)

-- 
2.43.0

Powered by blists - more mailing lists