On Tue, Apr 28, 2026 at 02:38:59PM +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 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. > > If no qdisc is present, the previous tail-drop behavior is preserved. > > The existing __tun_wake_queue() function of the consumer races with the > producer for waking/stopping the netdev queue: the consumer may drain > the ring just as the producer stops the queue, leading to a permanent > stall. To avoid this, the producer re-checks the ring after stopping > and wakes the queue itself if space was just made. An > smp_mb__after_atomic() is required so the re-peek of the ring sees any > drain that the consumer performed. > smp_mb__after_atomic() pairs with the test_and_clear_bit() inside of > netif_wake_subqueue(): > > Consumer CPU Producer CPU > ======================== ========================= > __ptr_ring_consume() > netif_wake_subqueue() netif_tx_stop_queue() > /\ smp_mb__after_atomic() > || __ptr_ring_produce_peek() > contains RMW operation > test_and_clear_bit() > /\ > || > "Fully ordered RMW: > smp_mb() before + after" > - atomic_t.txt > > Benchmarks: > The benchmarks show a slight regression in raw transmission performance, > though no packets are lost anymore.
Could you include the packets received as well? To demonstrate the gains/lack of loss. > > 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 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 | Transmitted | 1.136 Mpps | 1.130 Mpps | -0.6% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 3.758 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 3.858 Mpps | 3.816 Mpps | -1.1% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 789.8 Kpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > > +--------------------------+--------------+----------------+----------+ > | 2 threads | Stock | Patched with | diff | > | sending | | fq_codel qdisc | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 1.117 Mpps | 1.087 Mpps | -2.7% | > | +-------------+--------------+----------------+----------+ > | | Lost/s | 8.476 Mpps | 0 pps | | > +------------+-------------+--------------+----------------+----------+ > | TAP | Transmitted | 3.679 Mpps | 3.464 Mpps | -5.8% | > | +-------------+--------------+----------------+----------+ > | +vhost-net | Lost/s | 5.306 Mpps | 0 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 | 30 ++++++++++++++++++++++++++++-- > 1 file changed, 28 insertions(+), 2 deletions(-) > > diff --git a/drivers/net/tun.c b/drivers/net/tun.c > index efe809597622..c2a1618cc9db 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]); > @@ -1065,13 +1067,37 @@ 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) { > + netif_tx_stop_queue(queue); > + /* Re-peek and wake if the consumer drained the ring > + * concurrently in a race. smp_mb__after_atomic() pairs > + * with the test_and_clear_bit() of netif_wake_subqueue() > + * 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); > + > + 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
