Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 02, 2026 at 01:07:47PM +0200, Brett Sheffield wrote:
> On 2026-07-02 09:24, Simon Schippers wrote:
> > On 7/1/26 22:56, Michael S. Tsirkin wrote:
> > > On Wed, Jul 01, 2026 at 09:16:48PM +0200, Brett Sheffield wrote:
> > >> TL;DR - Commit 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 causes
> > >> significant performance regressions with TAP interfaces and multithreaded
> > >> network code. Please revert.
> > >>
> > >>
> > >> Librecast is an IPv6 multicast library. One of the tests (0055) fails under
> > >> Linux 7.2-rc1. The test performs data synchronization over IPv6 multicast using a TAP
> > >> interface. This test has run successfully on every stable, LTS and mainline RC
> > >> released in the past year. Every kernel with my Tested-by has run this test.
> > >>
> > >> There have been a bunch of changes to MLDv2 so I started bisecting there, but
> > >> the culprit is actually 1d6e569b7d0c0b2736636749e4be0a27f3cefcb3 "tun/tap &
> > >> vhost-net: avoid ptr_ring tail-drop when a qdisc is present"
> > >>
> > >> Reverting this commit fixes the test.
> > >>
> > >> To eliminate my code and any multicast weirdness, I ran tests with iperf3
> > >> comparing the same host running 7.2-rc1 both with and without 1d6e569b7d0
> > >> reverted.
> > 
> > Thank you very much for your bisect!
> > 
> > As the author, I am sorry for that regression!
> 
> No worries. That's why we test :-)
> 
> > > - does it help to increase the tun queue size?
> > 
> > I agree, this would be great to know.
> > 
> > However, even then we must act. I am considering IFF_BACKPRESSURE
> > as a feature flag, defaulting to off. It would just enable/disable
> > the stopping logic in tun_net_xmit() and the waking logic
> > in __tun_wake_queue(). If disabled, it would result in the same logic
> > as before.
> > 
> > I could provide such a patch as [net] material.
> 
> I'm going to make myself a strong cup of tea and dig into it a bit more here and
> will let you know if I find anything worth reporting.
> 
> If you need me to try re-testing with specific settings or test a patch I'm
> happy to do so.
> 
> Cheers,
> 
> 
> Brett
> -- 
> Brett Sheffield (he/him)
> Librecast - Decentralising the Internet with Multicast
> https://librecast.net/
> https://blog.brettsheffield.com/

Maybe it's the supposedly rare case? Does this change anything
for you?


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);