Message-ID: <8982048b-f5dc-4fec-bc53-f7ad88fbe199@tu-dortmund.de>
Date: Wed, 13 Aug 2025 20:27:10 +0200
From: Simon Schippers <simon.schippers@...dortmund.de>
To: Jason Wang <jasowang@...hat.com>
CC: <willemdebruijn.kernel@...il.com>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: [PATCH net v2] TUN/TAP: Improving throughput and latency by avoiding
 SKB drops

Jason Wang wrote:
> On Tue, Aug 12, 2025 at 6:04 AM Simon Schippers
> <simon.schippers@...dortmund.de> wrote:
>>
>> This patch is the result of our paper with the title "The NODROP Patch:
>> Hardening Secure Networking for Real-time Teleoperation by Preventing
>> Packet Drops in the Linux TUN Driver" [1].
>> It deals with the tun_net_xmit function which drops SKB's with the reason
>> SKB_DROP_REASON_FULL_RING whenever the tx_ring (TUN queue) is full,
>> resulting in reduced TCP performance and packet loss for bursty video
>> streams when used over VPN's.
>>
>> The abstract reads as follows:
>> "Throughput-critical teleoperation requires robust and low-latency
>> communication to ensure safety and performance. Often, these kinds of
>> applications are implemented in Linux-based operating systems and transmit
>> over virtual private networks, which ensure encryption and ease of use by
>> providing a dedicated tunneling interface (TUN) to user space
>> applications. In this work, we identified a specific behavior in the Linux
>> TUN driver, which results in significant performance degradation due to
>> the sender stack silently dropping packets. This design issue drastically
>> impacts real-time video streaming, inducing up to 29 % packet loss with
>> noticeable video artifacts when the internal queue of the TUN driver is
>> reduced to 25 packets to minimize latency. Furthermore, a small queue
>> length also drastically reduces the throughput of TCP traffic due to many
>> retransmissions. Instead, with our open-source NODROP Patch, we propose
>> generating backpressure in case of burst traffic or network congestion.
>> The patch effectively addresses the packet-dropping behavior, hardening
>> real-time video streaming and improving TCP throughput by 36 % in high
>> latency scenarios."
>>
>> In addition to the mentioned performance and latency improvements for VPN
>> applications, this patch also allows the proper usage of qdisc's. For
>> example a fq_codel can not control the queuing delay when packets are
>> already dropped in the TUN driver. This issue is also described in [2].
>>
>> The performance evaluation of the paper (see Fig. 4) showed a 4%
>> performance hit for a single queue TUN with the default TUN queue size of
>> 500 packets. However it is important to notice that with the proposed
>> patch no packet drop ever occurred even with a TUN queue size of 1 packet.
>> The utilized validation pipeline is available under [3].
>>
>> As the reduction of the TUN queue to a size of down to 5 packets showed no
>> further performance hit in the paper, a reduction of the default TUN queue
>> size might be desirable accompanying this patch. A reduction would
>> obviously reduce buffer bloat and memory requirements.
>>
>> Implementation details:
>> - The netdev queue start/stop flow control is utilized.
>> - Compatible with multi-queue by only stopping/waking the specific
>> netdevice subqueue.
>> - No additional locking is used.
>>
>> In the tun_net_xmit function:
>> - Stopping the subqueue is done when the tx_ring gets full after inserting
>> the SKB into the tx_ring.
>> - In the unlikely case when the insertion with ptr_ring_produce fails, the
>> old dropping behavior is used for this SKB.
>>
>> In the tun_ring_recv function:
>> - Waking the subqueue is done after consuming a SKB from the tx_ring when
>> the tx_ring is empty. Waking the subqueue when the tx_ring has any
>> available space, so when it is not full, showed crashes in our testing. We
>> are open to suggestions.
>> - When the tx_ring is configured to be small (for example to hold 1 SKB),
>> queuing might be stopped in the tun_net_xmit function while at the same
>> time, ptr_ring_consume is not able to grab a SKB. This prevents
>> tun_net_xmit from being called again and causes tun_ring_recv to wait
>> indefinitely for a SKB in the blocking wait queue. Therefore, the netdev
>> queue is woken in the wait queue if it has stopped.
>> - Because the tun_struct is required to get the tx_queue into the new txq
>> pointer, the tun_struct is passed in tun_do_read aswell. This is likely
>> faster then trying to get it via the tun_file tfile because it utilizes a
>> rcu lock.
>>
>> We are open to suggestions regarding the implementation :)
>> Thank you for your work!
>>
>
> I would like to see some benchmark results. Not only VPN but also a
> classical VM setup that is using vhost-net + TAP.
>

I completely overlooked that in tap.c there is also a tap_do_read function.
I would like to apologize for that and also implement the same behavior
from the tun_ring_recv function there.
The implementation is already done and it proved to be working but I will
test it a bit more before submitting a v3.

Regarding your proposed vhost-net + TAP setup: I need more time to
implement such a setup. However I am wondering what kind of tests you
would like to see exactly? TCP connections to a remote host like in our
paper?

>> [1] Link:
>> https://cni.etit.tu-dortmund.de/storages/cni-etit/r/Research/Publications/2025/Gebauer_2025_VTCFall/Gebauer_VTCFall2025_AuthorsVersion.pdf
>> [2] Link:
>> https://unix.stackexchange.com/questions/762935/traffic-shaping-ineffective-on-tun-device
>> [3] Link: https://github.com/tudo-cni/nodrop
>>
>> Co-developed-by: Tim Gebauer <tim.gebauer@...dortmund.de>
>> Signed-off-by: Tim Gebauer <tim.gebauer@...dortmund.de>
>> Signed-off-by: Simon Schippers <simon.schippers@...dortmund.de>
>> ---
>> V1 -> V2: Removed NETDEV_TX_BUSY return case in tun_net_xmit and removed
>> unnecessary netif_tx_wake_queue in tun_ring_recv.
>>
>>  drivers/net/tun.c | 21 +++++++++++++++++----
>>  1 file changed, 17 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index cc6c50180663..81abdd3f9aca 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1060,13 +1060,16 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>>
>>         nf_reset_ct(skb);
>>
>> -       if (ptr_ring_produce(&tfile->tx_ring, skb)) {
>> +       queue = netdev_get_tx_queue(dev, txq);
>> +       if (unlikely(ptr_ring_produce(&tfile->tx_ring, skb))) {
>> +               netif_tx_stop_queue(queue);
>>                 drop_reason = SKB_DROP_REASON_FULL_RING;
>
> This would still drop the packet. Should we detect if the ring is
> about to be full and stop then like a virtio-net?
>
> Thanks
>

I am a bit confused. You omitted the important part of the code which
comes right after that. There I stop the netdev queue when the tx_ring
gets full. Therefore tun_net_xmit is not (very very unlikely) called when
there is no space for another SKB and no SKB's are dropped. It is only
called again after the netdev queue is activated again in the
tun_ring_recv function.

In virtio-net whole SKB's they basically do the same in the tx_may_stop
function. The function is called after inserting a SKB and see if there is
enough space for another max size SKB, which is the same statement as if
their send_queue is full.
Correct me if I am wrong!
Thank you!

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