Subject: Re: [PATCH net-next v7 9/9] tun/tap & vhost-net: avoid ptr_ring tail-drop when qdisc is present

From: "Michael S. Tsirkin" <[email-masked]>
Date: Mon, 12 Jan 2026 06:18:14 -0500

On Mon, Jan 12, 2026 at 12:08:14PM +0100, Simon Schippers wrote:
> On 1/12/26 03:22, Jason Wang wrote:
> > On Fri, Jan 9, 2026 at 6:15 PM Simon Schippers
> > <[email-masked]> wrote:
> >>
> >> On 1/9/26 07:09, Jason Wang wrote:
> >>> On Thu, Jan 8, 2026 at 4:02 PM Simon Schippers
> >>> <[email-masked]> wrote:
> >>>>
> >>>> On 1/8/26 05:37, Jason Wang wrote:
> >>>>> On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
> >>>>> <[email-masked]> wrote:
> >>>>>>
> >>>>>> This commit prevents tail-drop when a qdisc is present and the ptr_ring
> >>>>>> becomes full. Once an entry is successfully produced and the ptr_ring
> >>>>>> reaches capacity, the netdev queue is stopped instead of dropping
> >>>>>> subsequent packets.
> >>>>>>
> >>>>>> If producing an entry fails anyways, the tun_net_xmit returns
> >>>>>> NETDEV_TX_BUSY, again avoiding a drop. Such failures are expected because
> >>>>>> LLTX is enabled and the transmit path operates without the usual locking.
> >>>>>> As a result, concurrent calls to tun_net_xmit() are not prevented.
> >>>>>>
> >>>>>> The existing __{tun,tap}_ring_consume functions free space in the
> >>>>>> ptr_ring and wake the netdev queue. Races between this wakeup and the
> >>>>>> queue-stop logic could leave the queue stopped indefinitely. To prevent
> >>>>>> this, a memory barrier is enforced (as discussed in a similar
> >>>>>> implementation in [1]), followed by a recheck that wakes the queue if
> >>>>>> space is already available.
> >>>>>>
> >>>>>> If no qdisc is present, the previous tail-drop behavior is preserved.
> >>>>>>
> >>>>>> +-------------------------+-----------+---------------+----------------+
> >>>>>> | 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              |
> >>>>>> +-----------+-------------+-----------+---------------+----------------+
> >>>>>>
> >>>>>> [1] Link: https://lore.kernel.org/all/[email-masked]/
> >>>>>>
> >>>>>> Co-developed-by: Tim Gebauer <[email-masked]>
> >>>>>> Signed-off-by: Tim Gebauer <[email-masked]>
> >>>>>> Signed-off-by: Simon Schippers <[email-masked]>
> >>>>>> ---
> >>>>>>  drivers/net/tun.c | 31 +++++++++++++++++++++++++++++--
> >>>>>>  1 file changed, 29 insertions(+), 2 deletions(-)
> >>>>>>
> >>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>>>>> index 71b6981d07d7..74d7fd09e9ba 100644
> >>>>>> --- a/drivers/net/tun.c
> >>>>>> +++ b/drivers/net/tun.c
> >>>>>> @@ -1008,6 +1008,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
> >>>>>>         struct netdev_queue *queue;
> >>>>>>         struct tun_file *tfile;
> >>>>>>         int len = skb->len;
> >>>>>> +       bool qdisc_present;
> >>>>>> +       int ret;
> >>>>>>
> >>>>>>         rcu_read_lock();
> >>>>>>         tfile = rcu_dereference(tun->tfiles[txq]);
> >>>>>> @@ -1060,13 +1062,38 @@ 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);
> >>>>>> +       qdisc_present = !qdisc_txq_has_no_queue(queue);
> >>>>>> +
> >>>>>> +       spin_lock(&tfile->tx_ring.producer_lock);
> >>>>>> +       ret = __ptr_ring_produce(&tfile->tx_ring, skb);
> >>>>>> +       if (__ptr_ring_produce_peek(&tfile->tx_ring) && qdisc_present) {
> >>>>>> +               netif_tx_stop_queue(queue);
> >>>>>> +               /* Avoid races with queue wake-up in
> >>>>>> +                * __{tun,tap}_ring_consume by waking if space is
> >>>>>> +                * available in a re-check.
> >>>>>> +                * The barrier makes sure that the stop is visible before
> >>>>>> +                * we re-check.
> >>>>>> +                */
> >>>>>> +               smp_mb__after_atomic();
> >>>>>> +               if (!__ptr_ring_produce_peek(&tfile->tx_ring))
> >>>>>> +                       netif_tx_wake_queue(queue);
> >>>>>
> >>>>> I'm not sure I will get here, but I think those should be moved to the
> >>>>> following if(ret) check. If __ptr_ring_produce() succeed, there's no
> >>>>> need to bother with those queue stop/wake logic?
> >>>>
> >>>> There is a need for that. If __ptr_ring_produce_peek() returns -ENOSPC,
> >>>> we stop the queue proactively.
> >>>
> >>> This seems to conflict with the following NETDEV_TX_BUSY. Or is
> >>> NETDEV_TX_BUSY prepared for the xdp_xmit?
> >>
> >> Am I not allowed to stop the queue and then return NETDEV_TX_BUSY?
> > 
> > No, I mean I don't understand why we still need to peek since we've
> > already used NETDEV_TX_BUSY.
> 
> Yes, if __ptr_ring_produce() returns -ENOSPC, there is no need to check
> __ptr_ring_produce_peek(). I agree with you on this point and will update
> the code accordingly. In all other cases, checking
> __ptr_ring_produce_peek() is still required in order to proactively stop
> the queue.
> 
> > 
> >> And I do not understand the connection with xdp_xmit.
> > 
> > Since there's we don't modify xdp_xmit path, so even if we peek next
> > ndo_start_xmit can still hit ring full.
> 
> Ah okay. Would you apply the same stop-and-recheck logic in
> tun_xdp_xmit when __ptr_ring_produce() fails to produce it, or is that
> not permitted there?
> 
> Apart from that, as noted in the commit message, since we are using LLTX,
> hitting a full ring is still possible anyway. I could see that especially
> at multiqueue tests with pktgen by looking at the qdisc requeues.
> 
> Thanks

If it's an exceptional rare condition (i.e. a race), it's not a big deal. That's why
NETDEV_TX_BUSY is there in the 1st place. If it's the main modus
operadi - not good.

> > 
> > Thanks
> > 
> >>
> >>>
> >>>>
> >>>> I believe what you are aiming for is to always stop the queue if(ret),
> >>>> which I can agree with. In that case, I would simply change the condition
> >>>> to:
> >>>>
> >>>> if (qdisc_present && (ret || __ptr_ring_produce_peek(&tfile->tx_ring)))
> >>>>
> >>>>>
> >>>>>> +       }
> >>>>>> +       spin_unlock(&tfile->tx_ring.producer_lock);
> >>>>>> +
> >>>>>> +       if (ret) {
> >>>>>> +               /* If a qdisc is attached to our virtual device,
> >>>>>> +                * returning NETDEV_TX_BUSY is allowed.
> >>>>>> +                */
> >>>>>> +               if (qdisc_present) {
> >>>>>> +                       rcu_read_unlock();
> >>>>>> +                       return NETDEV_TX_BUSY;
> >>>>>> +               }
> >>>>>>                 drop_reason = SKB_DROP_REASON_FULL_RING;
> >>>>>>                 goto drop;
> >>>>>>         }
> >>>>>>
> >>>>>>         /* 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 */
> >>>>>> --
> >>>>>> 2.43.0
> >>>>>>
> >>>>>
> >>>>> Thanks
> >>>>>
> >>>>
> >>>
> >>> Thanks
> >>>
> >>
> >