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Message-ID: <CAPpAL=zqwLC5hgOokx-8MttoyrN2RXS6Q=eSJBJ_ZYSeZHpC0g@mail.gmail.com>
Date: Thu, 28 Aug 2025 09:56:03 +0800
From: Lei Yang <leiyang@...hat.com>
To: Simon Schippers <simon.schippers@...dortmund.de>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, jasowang@...hat.com,
netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
Tim Gebauer <tim.gebauer@...dortmund.de>
Subject: Re: [PATCH net v3] TUN/TAP: Improving throughput and latency by
avoiding SKB drops
Tested this patch with virtio-net regression tests, everything works fine.
Tested-by: Lei Yang <leiyang@...hat.com>
On Wed, Aug 27, 2025 at 5:35 PM Simon Schippers
<simon.schippers@...dortmund.de> wrote:
>
> Willem de Bruijn wrote:
> > Target net-next
> >
> > Also please have the subject line summarize the functional change
> > (only). Something like "tun: replace tail drop with queue pause
> > when full."
> >
>
> Thank you very much for your detailed reply!
> Yes, I will target net-next instead and change the title.
>
> > Simon Schippers wrote:
> >> This patch is a result of our paper [1] and 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. This behavior results in reduced
> >> TCP performance and packet loss for VPNs and VMs. In addition this patch
> >> also allows qdiscs to work properly (see [2]) and to reduce buffer bloat
> >> when reducing the TUN queue.
> >>
> >> TUN benchmarks:
> >> +-----------------------------------------------------------------+
> >> | Lab setup of our paper [1]: |
> >> | TCP throughput of VPN solutions at varying RTT (values in Mbps) |
> >> +-----------+---------------+---------------+----------+----------+
> >> | RTT [ms] | wireguard-go | wireguard-go | OpenVPN | OpenVPN |
> >> | | | patched | | patched |
> >> +-----------+---------------+---------------+----------+----------+
> >> | 10 | 787.3 | 679.0 | 402.4 | 416.9 |
> >> +-----------+---------------+---------------+----------+----------+
> >> | 20 | 765.1 | 718.8 | 401.6 | 393.18 |
> >> +-----------+---------------+---------------+----------+----------+
> >> | 40 | 441.5 | 529.4 | 96.9 | 411.8 |
> >> +-----------+---------------+---------------+----------+----------+
> >> | 80 | 218.7 | 265.7 | 57.9 | 262.7 |
> >> +-----------+---------------+---------------+----------+----------+
> >> | 120 | 145.4 | 181.7 | 52.8 | 178.0 |
> >> +-----------+---------------+---------------+----------+----------+
> >>
> >> +--------------------------------------------------------------------+
> >> | Real-world setup of our paper [1]: |
> >> | TCP throughput of VPN solutions without and with the patch |
> >> | at a RTT of ~120 ms (values in Mbps) |
> >> +------------------+--------------+--------------+---------+---------+
> >> | TUN queue | wireguard-go | wireguard-go | OpenVPN | OpenVPN |
> >> | length [packets] | | patched | | patched |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 5000 | 185.8 | 185.6 | 184.7 | 184.8 |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 1000 | 185.1 | 184.9 | 177.1 | 183.0 |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 500 (default) | 137.5 | 184.9 | 117.4 | 184.6 |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 100 | 99.8 | 185.3 | 66.4 | 183.5 |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 50 | 59.4 | 185.7 | 21.6 | 184.7 |
> >> +------------------+--------------+--------------+---------+---------+
> >> | 10 | 1.7 | 185.4 | 1.6 | 183.6 |
> >> +------------------+--------------+--------------+---------+---------+
> >>
> >> TAP benchmarks:
> >> +------------------------------------------------------------------+
> >> | Lab Setup [3]: |
> >> | TCP throughput from host to Debian VM using TAP (values in Mbps) |
> >> +----------------------------+------------------+------------------+
> >> | TUN queue | Default | Patched |
> >> | length [packets] | | |
> >> +----------------------------+------------------+------------------+
> >> | 1000 (default) | 2194.3 | 2185.0 |
> >> +----------------------------+------------------+------------------+
> >> | 100 | 1986.4 | 2268.5 |
> >> +----------------------------+------------------+------------------+
> >> | 10 | 625.0 | 1988.9 |
> >> +----------------------------+------------------+------------------+
> >> | 1 | 2.2 | 1112.7 |
> >> +----------------------------+------------------+------------------+
> >> | |
> >> +------------------------------------------------------------------+
> >> | Measurement with 1000 packets queue and emulated delay |
> >> +----------------------------+------------------+------------------+
> >> | RTT [ms] | Default | Patched |
> >> +----------------------------+------------------+------------------+
> >> | 60 | 171.8 | 341.2 |
> >> +----------------------------+------------------+------------------+
> >> | 120 | 98.3 | 255.0 |
> >> +----------------------------+------------------+------------------+
> >>
> >> TAP+vhost_net benchmarks:
> >> +----------------------------------------------------------------------+
> >> | Lab Setup [3]: |
> >> | TCP throughput from host to Debian VM using TAP+vhost_net |
> >> | (values in Mbps) |
> >> +-----------------------------+--------------------+-------------------+
> >> | TUN queue | Default | Patched |
> >> | length [packets] | | |
> >> +-----------------------------+--------------------+-------------------+
> >> | 1000 (default) | 23403.9 | 23858.8 |
> >> +-----------------------------+--------------------+-------------------+
> >> | 100 | 23372.5 | 23889.9 |
> >> +-----------------------------+--------------------+-------------------+
> >> | 10 | 25837.5 | 23730.2 |
> >> +-----------------------------+--------------------+-------------------+
> >> | 1 | 0.7 | 19244.8 |
> >> +-----------------------------+--------------------+-------------------+
> >> | Note: Default suffers from many retransmits, while patched does not. |
> >> +----------------------------------------------------------------------+
> >> | |
> >> +----------------------------------------------------------------------+
> >> | Measurement with 1000 packets queue and emulated delay |
> >> +-----------------------------+--------------------+-------------------+
> >> | RTT [ms] | Default | Patched |
> >> +-----------------------------+--------------------+-------------------+
> >> | 60 | 397.1 | 397.8 |
> >> +-----------------------------+--------------------+-------------------+
> >> | 120 | 200.7 | 199.9 |
> >> +-----------------------------+--------------------+-------------------+
> >>
> >> Implementation details:
> >> - The netdev queue start/stop flow control is utilized.
> >> - Compatible with multi-queue by only stopping/waking the specific
> >> netdevice subqueue.
> >>
> >> 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.
> >> - When the tx_ring is configured to be small (for example to hold 1 SKB),
> >
> > That's an exaggerated case that hopefully we do not have to support.
> > Can this be configured? Maybe we should round_up user input to a sane
> > lower bound instead.
> >
>
> I do not think that this issue will disappear with a bigger tx_ring, it
> will just get more unlikely.
> Just waking the netdev queue in the blocking wait queue is fine in my
> opinion.
> And small tx_ring sizes like 1 might be used by a possible dynamic queue
> limits since my benchmarks showed that the performance can be okay with
> such small tx_ring sizes.
>
> >> 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.
> >>
> >> In the tap_do_read function:
> >> - Same behavior as in tun_ring_recv: Waking the subqueue when the tx_ring
> >> is empty & waking the subqueue in the blocking wait queue.
> >> - Here the netdev txq is obtained with a rcu read lock instead.
> >>
> >> In the vhost_net_buf_produce function:
> >> - Same behavior as in tun_ring_recv: Waking the subqueue when the tx_ring
> >> is empty.
> >> - Here the netdev_queue is saved in the vhost_net_virtqueue at init with
> >> new helpers.
> >>
> >> We are open to suggestions regarding the implementation :)
> >> Thank you for your work!
> >
> > Similarly, in the commit message, lead with the technical explanation.
> > Brief benchmark results are great, but this is not an academic paper.
> > Best concise and below the main take-away. Or in the cover letter if a
> > multi patch series. ..
> >
>
> Okay, I will shorten the benchmarks to a minimum and lead with the
> technical explanation.
>
> >>
> >> [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>
> >> ---
> >> V2 -> V3: Added support for TAP and TAP+vhost_net.
> >
> > .. please split into a series, with separate patches for TUN, TAP and
> > vhost-net.
> >
> > Or, start with one and once that is merged after revisions, repeat
> > for the others. That is likely less work.
> >
>
> I will split it into a series with separate changes.
> Merging one after another will not work since TUN, TAP and vhost-net share
> tun_net_xmit as a common method.
> Stopping the netdev queue there without waking it again will break stuff.
>
> >> 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/tap.c | 35 +++++++++++++++++++++++++++++++++++
> >> drivers/net/tun.c | 39 +++++++++++++++++++++++++++++++++++----
> >> drivers/vhost/net.c | 24 ++++++++++++++++++++++--
> >> include/linux/if_tap.h | 5 +++++
> >> include/linux/if_tun.h | 6 ++++++
> >> 5 files changed, 103 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/drivers/net/tap.c b/drivers/net/tap.c
> >> index 1197f245e873..df7e4063fb7c 100644
> >> --- a/drivers/net/tap.c
> >> +++ b/drivers/net/tap.c
> >> @@ -758,6 +758,8 @@ static ssize_t tap_do_read(struct tap_queue *q,
> >> int noblock, struct sk_buff *skb)
> >> {
> >> DEFINE_WAIT(wait);
> >> + struct netdev_queue *txq;
> >> + struct net_device *dev;
> >> ssize_t ret = 0;
> >>
> >> if (!iov_iter_count(to)) {
> >> @@ -785,12 +787,26 @@ static ssize_t tap_do_read(struct tap_queue *q,
> >> ret = -ERESTARTSYS;
> >> break;
> >> }
> >> + rcu_read_lock();
> >> + dev = rcu_dereference(q->tap)->dev;
> >> + txq = netdev_get_tx_queue(dev, q->queue_index);
> >> + netif_tx_wake_queue(txq);
> >> + rcu_read_unlock();
> >> +
> >
> > This wakes the queue only once entirely empty? That seems aggressive.
> >
>
> This waking of the netdev queue is only for the "exaggerated" case, see
> described above for TUN.
> However you are right that only waking the netdev queue when the tx_ring
> is empty (which is done with the condition below) is aggressive.
> In previous testing waking the queue when the tx_ring is not full anymore
> (instead of completely empty) showed crashes. But I will reevaluate this
> logic.
>
> > Where is the matching netif_tx_stop_queue. I had expected that
> > arund the ptr_ring_produce calls in tap_handle_frame.
> >
>
> TAP uses tun_net_xmit as .ndo_start_xmit. However, tap_handle_frame is
> used by ipvtap and macvtap. It could also be considered in the patch
> series, I guess?
>
> >> /* Nothing to read, let's sleep */
> >> schedule();
> >> }
> >> if (!noblock)
> >> finish_wait(sk_sleep(&q->sk), &wait);
> >>
> >> + if (ptr_ring_empty(&q->ring)) {
> >> + rcu_read_lock();
> >> + dev = rcu_dereference(q->tap)->dev;
> >> + txq = netdev_get_tx_queue(dev, q->queue_index);
> >> + netif_tx_wake_queue(txq);
> >> + rcu_read_unlock();
> >> + }
> >> +
> >
> > Why the second test for the same condition: ring empty?
> >
>
> See previous comment.
>
> >> put:
> >> if (skb) {
> >> ret = tap_put_user(q, skb, to);
> >> @@ -1176,6 +1192,25 @@ struct socket *tap_get_socket(struct file *file)
> >> }
> >> EXPORT_SYMBOL_GPL(tap_get_socket);
> >>
> >> +struct netdev_queue *tap_get_netdev_queue(struct file *file)
> >> +{
> >> + struct netdev_queue *txq;
> >> + struct net_device *dev;
> >> + struct tap_queue *q;
> >> +
> >> + if (file->f_op != &tap_fops)
> >> + return ERR_PTR(-EINVAL);
> >> + q = file->private_data;
> >> + if (!q)
> >> + return ERR_PTR(-EBADFD);
> >> + rcu_read_lock();
> >> + dev = rcu_dereference(q->tap)->dev;
> >> + txq = netdev_get_tx_queue(dev, q->queue_index);
> >> + rcu_read_unlock();
> >
> > If the dev is only safe to be accessed inside an RCU readside critical
> > section, is it safe to use txq outside of it?
> >
>
> You are right, this might be a bad idea as the queues might be messed with.
> However, I am not sure how to access the txq in another way?
>
> >> + return txq;
> >> +}
> >> +EXPORT_SYMBOL_GPL(tap_get_netdev_queue);
> >> +
> >> struct ptr_ring *tap_get_ptr_ring(struct file *file)
> >> {
> >> struct tap_queue *q;
> >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >> index cc6c50180663..30ddcd20fcd3 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;
> >
> > again, stop the queue before dropping is needed. Which is what the
> > new ptr_ring_full code below does I guess. If so, when is this reached?
> >
>
> Yes, you are right this is what the ptr_ring_full code below does. It is
> reached when a SKB is successfully inserted into the tx_ring and with that
> the tx_ring becomes full. Then the queue is stopped which avoids packet
> drops.
>
> >> goto drop;
> >> }
> >> + if (ptr_ring_full(&tfile->tx_ring))
> >> + netif_tx_stop_queue(queue);
> >>
> >> /* dev->lltx requires to do our own update of trans_start */
> >> - queue = netdev_get_tx_queue(dev, txq);
> >> txq_trans_cond_update(queue);
> >>
> >> /* Notify and wake up reader process */
> >> @@ -2110,9 +2113,10 @@ static ssize_t tun_put_user(struct tun_struct *tun,
> >> return total;
> >> }
> >>
> >> -static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> >> +static void *tun_ring_recv(struct tun_struct *tun, struct tun_file *tfile, int noblock, int *err)
> >> {
> >> DECLARE_WAITQUEUE(wait, current);
> >> + struct netdev_queue *txq;
> >> void *ptr = NULL;
> >> int error = 0;
> >>
> >> @@ -2124,6 +2128,7 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> >> goto out;
> >> }
> >>
> >> + txq = netdev_get_tx_queue(tun->dev, tfile->queue_index);
> >> add_wait_queue(&tfile->socket.wq.wait, &wait);
> >>
> >> while (1) {
> >> @@ -2131,6 +2136,9 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> >> ptr = ptr_ring_consume(&tfile->tx_ring);
> >> if (ptr)
> >> break;
> >> +
> >> + netif_tx_wake_queue(txq);
> >> +
> >> if (signal_pending(current)) {
> >> error = -ERESTARTSYS;
> >> break;
> >> @@ -2147,6 +2155,10 @@ static void *tun_ring_recv(struct tun_file *tfile, int noblock, int *err)
> >> remove_wait_queue(&tfile->socket.wq.wait, &wait);
> >>
> >> out:
> >> + if (ptr_ring_empty(&tfile->tx_ring)) {
> >> + txq = netdev_get_tx_queue(tun->dev, tfile->queue_index);
> >> + netif_tx_wake_queue(txq);
> >> + }
> >> *err = error;
> >> return ptr;
> >> }
> >> @@ -2165,7 +2177,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> >>
> >> if (!ptr) {
> >> /* Read frames from ring */
> >> - ptr = tun_ring_recv(tfile, noblock, &err);
> >> + ptr = tun_ring_recv(tun, tfile, noblock, &err);
> >> if (!ptr)
> >> return err;
> >> }
> >> @@ -3712,6 +3724,25 @@ struct socket *tun_get_socket(struct file *file)
> >> }
> >> EXPORT_SYMBOL_GPL(tun_get_socket);
> >>
> >> +struct netdev_queue *tun_get_netdev_queue(struct file *file)
> >> +{
> >> + struct netdev_queue *txq;
> >> + struct net_device *dev;
> >> + struct tun_file *tfile;
> >> +
> >> + if (file->f_op != &tun_fops)
> >> + return ERR_PTR(-EINVAL);
> >> + tfile = file->private_data;
> >> + if (!tfile)
> >> + return ERR_PTR(-EBADFD);
> >> + rcu_read_lock();
> >> + dev = rcu_dereference(tfile->tun)->dev;
> >> + txq = netdev_get_tx_queue(dev, tfile->queue_index);
> >> + rcu_read_unlock();
> >> + return txq;
> >> +}
> >> +EXPORT_SYMBOL_GPL(tun_get_netdev_queue);
> >> +
> >> struct ptr_ring *tun_get_tx_ring(struct file *file)
> >> {
> >> struct tun_file *tfile;
> >> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> >> index 6edac0c1ba9b..045fc31c59ff 100644
> >> --- a/drivers/vhost/net.c
> >> +++ b/drivers/vhost/net.c
> >> @@ -130,6 +130,7 @@ struct vhost_net_virtqueue {
> >> struct vhost_net_buf rxq;
> >> /* Batched XDP buffs */
> >> struct xdp_buff *xdp;
> >> + struct netdev_queue *netdev_queue;
> >> };
> >>
> >> struct vhost_net {
> >> @@ -182,6 +183,8 @@ static int vhost_net_buf_produce(struct vhost_net_virtqueue *nvq)
> >> rxq->head = 0;
> >> rxq->tail = ptr_ring_consume_batched(nvq->rx_ring, rxq->queue,
> >> VHOST_NET_BATCH);
> >> + if (ptr_ring_empty(nvq->rx_ring))
> >> + netif_tx_wake_queue(nvq->netdev_queue);
> >> return rxq->tail;
> >> }
> >>
> >> @@ -1469,6 +1472,21 @@ static struct socket *get_raw_socket(int fd)
> >> return ERR_PTR(r);
> >> }
> >>
> >> +static struct netdev_queue *get_tap_netdev_queue(struct file *file)
> >> +{
> >> + struct netdev_queue *q;
> >> +
> >> + q = tun_get_netdev_queue(file);
> >> + if (!IS_ERR(q))
> >> + goto out;
> >> + q = tap_get_netdev_queue(file);
> >> + if (!IS_ERR(q))
> >> + goto out;
> >> + q = NULL;
> >> +out:
> >> + return q;
> >> +}
> >> +
> >> static struct ptr_ring *get_tap_ptr_ring(struct file *file)
> >> {
> >> struct ptr_ring *ring;
> >> @@ -1570,10 +1588,12 @@ static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
> >> if (r)
> >> goto err_used;
> >> if (index == VHOST_NET_VQ_RX) {
> >> - if (sock)
> >> + if (sock) {
> >> nvq->rx_ring = get_tap_ptr_ring(sock->file);
> >> - else
> >> + nvq->netdev_queue = get_tap_netdev_queue(sock->file);
> >> + } else {
> >> nvq->rx_ring = NULL;
> >> + }
> >> }
> >>
> >> oldubufs = nvq->ubufs;
> >> diff --git a/include/linux/if_tap.h b/include/linux/if_tap.h
> >> index 553552fa635c..b15c40c86819 100644
> >> --- a/include/linux/if_tap.h
> >> +++ b/include/linux/if_tap.h
> >> @@ -10,6 +10,7 @@ struct socket;
> >>
> >> #if IS_ENABLED(CONFIG_TAP)
> >> struct socket *tap_get_socket(struct file *);
> >> +struct netdev_queue *tap_get_netdev_queue(struct file *file);
> >> struct ptr_ring *tap_get_ptr_ring(struct file *file);
> >> #else
> >> #include <linux/err.h>
> >> @@ -18,6 +19,10 @@ static inline struct socket *tap_get_socket(struct file *f)
> >> {
> >> return ERR_PTR(-EINVAL);
> >> }
> >> +static inline struct netdev_queue *tap_get_netdev_queue(struct file *f)
> >> +{
> >> + return ERR_PTR(-EINVAL);
> >> +}
> >> static inline struct ptr_ring *tap_get_ptr_ring(struct file *f)
> >> {
> >> return ERR_PTR(-EINVAL);
> >> diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h
> >> index 80166eb62f41..552eb35f0299 100644
> >> --- a/include/linux/if_tun.h
> >> +++ b/include/linux/if_tun.h
> >> @@ -21,6 +21,7 @@ struct tun_msg_ctl {
> >>
> >> #if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE)
> >> struct socket *tun_get_socket(struct file *);
> >> +struct netdev_queue *tun_get_netdev_queue(struct file *file);
> >> struct ptr_ring *tun_get_tx_ring(struct file *file);
> >>
> >> static inline bool tun_is_xdp_frame(void *ptr)
> >> @@ -50,6 +51,11 @@ static inline struct socket *tun_get_socket(struct file *f)
> >> return ERR_PTR(-EINVAL);
> >> }
> >>
> >> +static inline struct netdev_queue *tun_get_netdev_queue(struct file *f)
> >> +{
> >> + return ERR_PTR(-EINVAL);
> >> +}
> >> +
> >> static inline struct ptr_ring *tun_get_tx_ring(struct file *f)
> >> {
> >> return ERR_PTR(-EINVAL);
> >> --
> >> 2.43.0
> >>
> >
> >
>
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