On Wed, May 06, 2026 at 06:28:06PM -0400, Michael S. Tsirkin wrote:
> On Wed, May 06, 2026 at 04:10:33PM +0200, Simon Schippers 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 no qdisc is present, the previous tail-drop
> > behavior is preserved.
> >
> > If producing an entry fails anyways due to a race, tun_net_xmit() drops
> > the packet. Such races are expected because LLTX is enabled and the
> > transmit path operates without the usual locking.
> >
> > The __tun_wake_queue() function of the consumer races with the producer
> > for waking/stopping the netdev queue, which could result in a stalled
> > queue. Therefore, an smp_mb__after_atomic() is introduced that pairs
> > with the smp_mb() of the consumer. It follows the principle of store
> > buffering described in tools/memory-model/Documentation/recipes.txt:
> >
> > - The producer in tun_net_xmit() first sets __QUEUE_STATE_DRV_XOFF,
> > followed by an smp_mb__after_atomic() (= smp_mb()), and then reads the
> > ring with __ptr_ring_produce_peek().
> >
> > - The consumer in __tun_wake_queue() first writes zero to the ring in
> > __ptr_ring_consume(), followed by an smp_mb(), and then reads the queue
> > status with netif_tx_queue_stopped().
> >
> > => Following the aforementioned principle, it is impossible for the
> > producer to see a full ring (and therefore not wake the queue on the
> > re-check) while the consumer simultaneously fails to see a stopped
> > queue (and therefore also does not wake it).
> >
> > Benchmarks:
> > The benchmarks show a slight regression in raw transmission performance
> > when using two sending threads. Packet loss also occurs only in the
> > two-thread sending case; no packet loss was observed with a single
> > sending thread.
> >
> > Test setup:
> > AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads;
> > Average over 50 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 | Received | 1.132 Mpps | 1.133 Mpps | +0.1% |
> > | +-------------+--------------+----------------+----------+
> > | | Lost/s | 3.765 Mpps | 0 pps | |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP | Received | 3.857 Mpps | 3.905 Mpps | +1.2% |
> > | +-------------+--------------+----------------+----------+
> > | +vhost-net | Lost/s | 0.802 Mpps | 0 pps | |
> > +------------+-------------+--------------+----------------+----------+
> >
> > +--------------------------+--------------+----------------+----------+
> > | 2 threads | Stock | Patched with | diff |
> > | sending | | fq_codel qdisc | |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP | Received | 1.115 Mpps | 1.092 Mpps | -2.1% |
> > | +-------------+--------------+----------------+----------+
> > | | Lost/s | 8.490 Mpps | 359 pps | |
> > +------------+-------------+--------------+----------------+----------+
> > | TAP | Received | 3.664 Mpps | 3.549 Mpps | -3.1% |
> > | +-------------+--------------+----------------+----------+
> > | +vhost-net | Lost/s | 5.330 Mpps | 832 pps | |
> > +------------+-------------+--------------+----------------+----------+
> >
> > 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 | 25 +++++++++++++++++++++++--
> > 1 file changed, 23 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> > index fc358c4c355b..d9ffbf88cfd8 100644
> > --- a/drivers/net/tun.c
> > +++ b/drivers/net/tun.c
> > @@ -1018,6 +1018,7 @@ 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;
> > + int ret;
> >
> > rcu_read_lock();
> > tfile = rcu_dereference(tun->tfiles[txq]);
> > @@ -1072,13 +1073,33 @@ 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);
> > +
> > + spin_lock(&tfile->tx_ring.producer_lock);
> > + ret = __ptr_ring_produce(&tfile->tx_ring, skb);
> > + if (!qdisc_txq_has_no_queue(queue) &&
> > + (__ptr_ring_produce_peek(&tfile->tx_ring) || ret)) {
> > + netif_tx_stop_queue(queue);
> > + /* Paired with smp_mb() in __tun_wake_queue() */
> > + smp_mb__after_atomic();
> > + if (!__ptr_ring_produce_peek(&tfile->tx_ring))
> > + netif_tx_wake_queue(queue);
> > + }
> > + spin_unlock(&tfile->tx_ring.producer_lock);
> > +
>
> There's a weird corner case here when tx_queue_len is 0
> but a qdisc has been configured - it looks like that
> currently it just drops all packets, with this change,
> the qdisc will get stuck permanently.
>
> I suspect just checking tx_ring.size should fix it.
> Or if you feel adventurous, change return code for __ptr_ring_produce
> to distinguish between "no ring" and "no space".
__ptr_ring_produce_peek really.
>
> > + if (ret) {
> > + /* This should be a rare case if a qdisc is present, but
> > + * can happen due to lltx.
> > + * Since skb_tx_timestamp(), skb_orphan(),
> > + * run_ebpf_filter() and pskb_trim() could have tinkered
> > + * with the SKB, returning NETDEV_TX_BUSY is unsafe and
> > + * we must drop instead.
> > + */
> > 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
