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 -
but only when a qdisc is attached. If no qdisc is present the existing
behavior is preserved.
By applying proper backpressure, this change allows the connected qdisc to
operate correctly, as reported in [1], and significantly improves
performance in real-world scenarios, as demonstrated in our paper [2]. For
example, we observed a 36% TCP throughput improvement for an OpenVPN
connection between Germany and the USA.
At the same time, synthetic benchmarks (details below) show only minor
theoretical performance impact:
(1) With the noqueue qdisc, the patched behavior matches the stock
implementation, as expected. In both configurations, a significant
number of packets are dropped.
(2) pktgen benchmarks show a ~5-10% throughput reduction for TAP alone,
while no performance impact is observed for TAP + vhost-net. In both
cases, zero packet drops are observed.
(3) TCP benchmarks using iperf3 show no performance degradation for either
TAP or TAP combined with vhost-net.
This patch series touches tun/tap and vhost-net, as they share common
logic and must be updated together. Modifying only one of them would break
the others. The series is therefore structured as follows:
(1-2) ptr_ring: Introduce new helpers, which are used by patches (3)
and (9).
(3) tun/tap: add a ptr_ring consume helper with netdev queue wakeup.
(4-8) vhost-net: introduce and switch to the new tun/tap ptr_ring
wrappers with netdev queue wakeup.
(9) tun/tap: avoid ptr_ring tail-drop when a qdisc is present by
stopping the netdev queue.
+-------------------------+-----------+---------------+----------------+
| pktgen benchmarks to | Stock | Patched with | Patched with |
| Debian VM, i5 6300HQ, | | noqueue qdisc | fq_codel qdisc |
| 10M packets | | | |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 196 Kpps | 195 Kpps | 185 Kpps |
| +-------------+-----------+---------------+----------------+
| | Lost | 1618 Kpps | 1556 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 577 Kpps | 582 Kpps | 578 Kpps |
| + +-------------+-----------+---------------+----------------+
| vhost-net | Lost | 1170 Kpps | 1109 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
+-------------------------+-----------+---------------+----------------+
| pktgen benchmarks to | Stock | Patched with | Patched with |
| Debian VM, i5 6300HQ, | | noqueue qdisc | fq_codel qdisc |
| 10M packets, | | | |
| *4 threads* | | | |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 26 Kpps | 26 Kpps | 23 Kpps |
| +-------------+-----------+---------------+----------------+
| | Lost | 1535 Kpps | 1551 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
| TAP | Transmitted | 64 Kpps | 63 Kpps | 66 Kpps |
| + +-------------+-----------+---------------+----------------+
| vhost-net | Lost | 1550 Kpps | 1506 Kpps | 0 |
+-----------+-------------+-----------+---------------+----------------+
+-----------------------+-------------+---------------+----------------+
| iperf3 TCP benchmarks | Stock | Patched with | Patched with |
| to Debian VM | | noqueue qdisc | fq_codel qdisc |
| i5 6300HQ, 120s | | | |
+-----------------------+-------------+---------------+----------------+
| TAP | 1.71 Gbit/s | 1.71 Gbit/s | 1.71 Gbit/s |
+-----------------------+-------------+---------------+----------------+
| TAP + vhost-net | 22.1 Gbit/s | 22.0 Gbit/s | 22.0 Gbit/s |
+-----------------------+-------------+---------------+----------------+
[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
[3] Link:
https://lore.kernel.org/r/174549940981.608169.4363875844729313831.stgit@firesoul
[4] Link:
https://lore.kernel.org/r/176295323282.307447.14790015927673763094.stgit@firesoul
---
Changelog:
V7:
- Switch to an approach similar to veth [3] (excluding the recently fixed
variant [4]), as suggested by MST, with minor adjustments discussed in V6
- Rename the cover-letter title
- Add multithreaded pktgen and iperf3 benchmarks, as suggested by Jason
Wang
- Rework __ptr_ring_consume_created_space() so it can also be used after
batched consume
V6:
https://lore.kernel.org/netdev/[email protected]/
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/[email protected]/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/[email protected]/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/[email protected]/T/#u
- Added support for TAP and TAP+vhost-net.
V2:
https://lore.kernel.org/netdev/[email protected]/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/[email protected]/T/#u
---
Simon Schippers (9):
ptr_ring: move free-space check into separate helper
ptr_ring: add helper to detect newly freed space on consume
tun/tap: add ptr_ring consume helper with netdev queue wakeup
tun/tap: add batched ptr_ring consume functions with netdev queue
wakeup
tun/tap: add unconsume function for returning entries to ptr_ring
tun/tap: add helper functions to check file type
vhost-net: vhost-net: replace rx_ring with tun/tap ring wrappers
tun/tap: drop get ring exports
tun/tap & vhost-net: avoid ptr_ring tail-drop when qdisc is present
drivers/net/tap.c | 66 ++++++++++++++++++++++++---
drivers/net/tun.c | 99 ++++++++++++++++++++++++++++++++++++----
drivers/vhost/net.c | 92 ++++++++++++++++++++++++-------------
include/linux/if_tap.h | 16 +++++--
include/linux/if_tun.h | 18 ++++++--
include/linux/ptr_ring.h | 27 ++++++++++-
6 files changed, 263 insertions(+), 55 deletions(-)
--
2.43.0
