On Tue, Apr 28, 2026 at 03:10:44PM +0200, Simon Schippers wrote: > On 4/28/26 14:50, Michael S. Tsirkin wrote: > > 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. > > > > Do you mean the number of packets received by the VM? > They should just be the same as the number sent (shown below), right?
Minus the loss? Which this is about, right? > I assume they would be visible as RX-DRP for TAP. > For TAP + vhost-net I would have to rewrite the XDP drop > program to count the number of dropped packets... > And I would have to automate it... > > >> > >> 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 > >
