Re: [PATCH net-next v8 4/4] tun/tap & vhost-net: avoid ptr_ring tail-drop when a qdisc is present

[Simon Schippers <simon.schippers@tu-dortmund.de>]

On 3/24/26 11:14, Simon Schippers wrote:
> On 3/24/26 02:47, Jason Wang wrote:
>> On Thu, Mar 12, 2026 at 9:07 PM Simon Schippers
>> <simon.schippers@tu-dortmund.de> 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 due to a race, tun_net_xmit returns
>>> NETDEV_TX_BUSY, again avoiding a drop. Such races are expected because
>>> LLTX is enabled and the transmit path operates without the usual locking.
>>>
>>> The existing __tun_wake_queue() function wakes 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.
>>
>> I wonder if we need a dedicated TUN flag to enable this. With this new
>> flag, we can even prevent TUN from using noqueue (not sure if it's
>> possible or not).
>>
> 
> Except of the slight regressions because of this patchset I do not see
> a reason for such a flag.
> 
> I have never seen that the driver prevents noqueue. For example you can
> set noqueue to your ethernet interface and under load you soon get 
> 
> net_crit_ratelimited("Virtual device %s asks to queue packet!\n",
> 					     dev->name);
> 
> followed by a -ENETDOWN. And this is not prevented even though it is
> clearly not something a user wants.
> 
>>>
>>> Benchmarks:
>>> The benchmarks show a slight regression in raw transmission performance,
>>> though no packets are lost anymore.
>>>
>>> The previously introduced threshold to only wake after the queue stopped
>>> and half of the ring was consumed showed to be a descent choice:
>>> Waking the queue whenever a consume made space in the ring strongly
>>> degrades performance for tap, while waking only when the ring is empty
>>> is too late and also hurts throughput for tap & tap+vhost-net.
>>> Other ratios (3/4, 7/8) showed similar results (not shown here), so
>>> 1/2 was chosen for the sake of simplicity for both tun/tap and
>>> tun/tap+vhost-net.
>>>
>>> Test setup:
>>> AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads;
>>> Average over 20 runs @ 100,000,000 packets. SRSO and spectre v2
>>> mitigations disabled.
>>>
>>> Note for tap+vhost-net:
>>> XDP drop program active in VM -> ~2.5x faster, slower for tap due to
>>> more syscalls (high utilization of entry_SYSRETQ_unsafe_stack in perf)
>>>
>>> +--------------------------+--------------+----------------+----------+
>>> | 1 thread                 | Stock        | Patched with   | diff     |
>>> | sending                  |              | fq_codel qdisc |          |
>>> +------------+-------------+--------------+----------------+----------+
>>> | TAP        | Transmitted | 1.151 Mpps   | 1.139 Mpps     | -1.1%    |
>>> |            +-------------+--------------+----------------+----------+
>>> |            | Lost/s      | 3.606 Mpps   | 0 pps          |          |
>>> +------------+-------------+--------------+----------------+----------+
>>> | TAP        | Transmitted | 3.948 Mpps   | 3.738 Mpps     | -5.3%    |
>>> |            +-------------+--------------+----------------+----------+
>>> | +vhost-net | Lost/s      | 496.5 Kpps   | 0 pps          |          |
>>> +------------+-------------+--------------+----------------+----------+
>>>
>>> +--------------------------+--------------+----------------+----------+
>>> | 2 threads                | Stock        | Patched with   | diff     |
>>> | sending                  |              | fq_codel qdisc |          |
>>> +------------+-------------+--------------+----------------+----------+
>>> | TAP        | Transmitted | 1.133 Mpps   | 1.109 Mpps     | -2.1%    |
>>> |            +-------------+--------------+----------------+----------+
>>> |            | Lost/s      | 8.269 Mpps   | 0 pps          |          |
>>> +------------+-------------+--------------+----------------+----------+
>>> | TAP        | Transmitted | 3.820 Mpps   | 3.513 Mpps     | -8.0%    |
>>> |            +-------------+--------------+----------------+----------+
>>> | +vhost-net | Lost/s      | 4.961 Mpps   | 0 pps          |          |
>>> +------------+-------------+--------------+----------------+----------+
>>>
>>> [1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/
>>>
>>> Co-developed-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
>>> Signed-off-by: Tim Gebauer <tim.gebauer@tu-dortmund.de>
>>> Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
>>> ---
>>>  drivers/net/tun.c | 30 ++++++++++++++++++++++++++++--
>>>  1 file changed, 28 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>> index b86582cc6cb6..9b7daec69acd 100644
>>> --- a/drivers/net/tun.c
>>> +++ b/drivers/net/tun.c
>>> @@ -1011,6 +1011,8 @@ 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]);
>>> @@ -1063,13 +1065,37 @@ 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) {
>>
>> So, it's possible that the administrator is switching between noqueue
>> and another qdisc. So ptr_ring_produce() can fail here, do we need to
>> check that or not?
>>
> 
> Do you mean that? My thoughts:
> 
> Switching from noqueue to some qdisc can cause a 
> 
> net_crit_ratelimited("Virtual device %s asks to queue packet!\n",
> 					     dev->name);
> 
> followed by a return of -ENETDOWN in __dev_queue_xmit().
> This is because tun_net_xmit detects some qdisc with 
> 
> qdisc_present = !qdisc_txq_has_no_queue(queue);
> 
> and returns NETDEV_TX_BUSY even though __dev_queue_xmit() did still
> detect noqueue.
> 
> I am not sure how to solve this/if this has to be solved.
> I do not see a proper way to avoid parallel execution of ndo_start_xmit
> and a qdisc change (dev_graft_qdisc only takes qdisc_skb_head lock).
> 
> And from my understanding the veth implementation faces the same issue.

How about rechecking if a qdisc is connected?
This would avoid -ENETDOWN.

diff --git a/net/core/dev.c b/net/core/dev.c
index f48dc299e4b2..2731a1a70732 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -4845,10 +4845,17 @@ int __dev_queue_xmit(struct sk_buff *skb, struct net_device *sb_dev)
                                if (is_list)
                                        rc = NETDEV_TX_OK;
                        }
+                       bool qdisc_present = !qdisc_txq_has_no_queue(txq);
                        HARD_TX_UNLOCK(dev, txq);
                        if (!skb) /* xmit completed */
                                goto out;
 
+                       /* Maybe a qdisc was connected in the meantime */
+                       if (qdisc_present) {
+                               kfree_skb(skb);
+                               goto out;
+                       }
+
                        net_crit_ratelimited("Virtual device %s asks to queue packet!\n",
                                             dev->name);
                        /* NETDEV_TX_BUSY or queue was stopped */


> 
> 
> Switching from some qdisc to noqueue is no problem I think.
> 
>>> +               netif_tx_stop_queue(queue);
>>> +               /* Avoid races with queue wake-ups in __tun_wake_queue 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();
>>
>> Let's document which barrier is paired with this.
>>
> 
> I am basically copying the (old) logic of veth [1] proposed by
> Jakub Kicinski. I must admit I am not 100% sure what it pairs with.
> 
> [1] Link: https://lore.kernel.org/all/20250424085358.75d817ae@kernel.org/
> 
>>> +               if (!__ptr_ring_produce_peek(&tfile->tx_ring))
>>> +                       netif_tx_wake_queue(queue);
>>> +       }
>>> +       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
>>