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? 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 >
