On 1/12/26 03:22, Jason Wang wrote:
> 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.
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
>
> 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
>>>
>>
>
