Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance
From: Simon Schippers <hidden>
Date: 2026-07-03 10:38:32
Also in:
lkml, regressions
On 7/3/26 00:55, Michael S. Tsirkin wrote:
Maybe it's the supposedly rare case? Does this change anything for you?
In the "rare case" we would have packet drops, visible by iperf3 TCP retransmissions. But these packet drops don't occur as I stated before. Also, we are still not allowed to return NETDEV_TX_BUSY here, because run_ebpf_filter() and pskb_trim() could have tinkered with the SKB.
quoted hunk ↗ jump to hunk
diff --git a/drivers/net/tun.c b/drivers/net/tun.c index bfa49fa9e3a1..bacd89460078 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c@@ -1018,7 +1018,6 @@ 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]);@@ -1064,19 +1063,24 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) goto drop; } - skb_tx_timestamp(skb); - - /* Orphan the skb - required as we might hang on to it - * for indefinite time. - */ - skb_orphan(skb); - - nf_reset_ct(skb); - queue = netdev_get_tx_queue(dev, txq); spin_lock(&tfile->tx_ring.producer_lock); - ret = __ptr_ring_produce(&tfile->tx_ring, skb); + if (__ptr_ring_check_produce(&tfile->tx_ring)) { + spin_unlock(&tfile->tx_ring.producer_lock); + netif_tx_stop_queue(queue); + smp_mb__after_atomic(); + if (!__ptr_ring_check_produce(&tfile->tx_ring)) + netif_tx_wake_queue(queue); + rcu_read_unlock(); + return NETDEV_TX_BUSY; + } + + skb_tx_timestamp(skb); + skb_orphan(skb); + nf_reset_ct(skb); + + __ptr_ring_produce(&tfile->tx_ring, skb); if (!qdisc_txq_has_no_queue(queue) && __ptr_ring_check_produce(&tfile->tx_ring) == -ENOSPC) { netif_tx_stop_queue(queue);@@ -1087,18 +1091,6 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) } spin_unlock(&tfile->tx_ring.producer_lock); - 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 */ txq_trans_cond_update(queue);