On Fri, Jan 9, 2026 at 6:15 PM Simon Schippers
<[email protected]> wrote:
>
> On 1/9/26 07:09, Jason Wang wrote:
> > 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?
>
> 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.
> 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.
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
> >
>
