On Thu, Jan 8, 2026 at 4:02 PM Simon Schippers
<[email protected]> wrote:
>
> On 1/8/26 05:37, Jason Wang wrote:
> > On Thu, Jan 8, 2026 at 5:06 AM Simon Schippers
> > <[email protected]> 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 protected]/
> >>
> >> Co-developed-by: Tim Gebauer <[email protected]>
> >> Signed-off-by: Tim Gebauer <[email protected]>
> >> Signed-off-by: Simon Schippers <[email protected]>
> >> ---
> >> 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?
>
> 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
