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?
And I do not understand the connection with 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
>
