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.
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
>
