Re: [REGRESSION][BISECTED] tun/tap & vhost-net: multi-threaded network performance
From: Simon Schippers <hidden>
Date: 2026-07-03 10:37:26
Also in:
lkml, regressions
On 7/3/26 00:44, Michael S. Tsirkin wrote:
Well, the issue was with host to guest right? Then testing what does bql do might be interesting. Might help. Something like this? Lightly tested.
Your approach calls netdev_tx_completed_queue() per individual packet which is wrong and will cause a constant BQL limit of 2 as we have seen in [1], causing a regression *100%*. Citing the documentation of netdev_tx_completed_queue() in netdevice.h: * Must be called at most once per TX completion round (and not per * individual packet), so that BQL can adjust its limits appropriately. [1] Link: https://lore.kernel.org/all/e8cdba04-aa9a-45c6-9807-8274b62920df@tu-dortmund.de/ (local)
quoted hunk ↗ jump to hunk
diff --git a/drivers/net/tun.c b/drivers/net/tun.c index bfa49fa9e3a1..abc46354c107 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c@@ -1076,6 +1076,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) queue = netdev_get_tx_queue(dev, txq); spin_lock(&tfile->tx_ring.producer_lock); + netdev_tx_sent_queue(queue, len); ret = __ptr_ring_produce(&tfile->tx_ring, skb); if (!qdisc_txq_has_no_queue(queue) && __ptr_ring_check_produce(&tfile->tx_ring) == -ENOSPC) {@@ -1088,6 +1089,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev) spin_unlock(&tfile->tx_ring.producer_lock); if (ret) { + netdev_tx_completed_queue(queue, 1, len); /* This should be a rare case if a qdisc is present, but * can happen due to lltx. * Since skb_tx_timestamp(), skb_orphan(),@@ -2148,15 +2150,19 @@ static ssize_t tun_put_user(struct tun_struct *tun, /* Callers must hold ring.consumer_lock */ static void __tun_wake_queue(struct tun_struct *tun, - struct tun_file *tfile, int consumed) + struct tun_file *tfile, + unsigned int pkts, unsigned int bytes) { struct netdev_queue *txq = netdev_get_tx_queue(tun->dev, tfile->queue_index); + if (bytes) + netdev_tx_completed_queue(txq, pkts, bytes); +
Right here.
quoted hunk ↗ jump to hunk
/* Paired with smp_mb__after_atomic() in tun_net_xmit() */ smp_mb(); if (netif_tx_queue_stopped(txq)) { - tfile->cons_cnt += consumed; + tfile->cons_cnt += pkts; if (tfile->cons_cnt >= tfile->tx_ring.size / 2 || __ptr_ring_empty(&tfile->tx_ring)) { netif_tx_wake_queue(txq);@@ -2167,12 +2173,16 @@ static void __tun_wake_queue(struct tun_struct *tun, static void *tun_ring_consume(struct tun_struct *tun, struct tun_file *tfile) { + unsigned int bytes = 0; void *ptr; spin_lock(&tfile->tx_ring.consumer_lock); ptr = __ptr_ring_consume(&tfile->tx_ring); - if (ptr) - __tun_wake_queue(tun, tfile, 1); + if (ptr) { + if (!tun_is_xdp_frame(ptr)) + bytes = ((struct sk_buff *)ptr)->len; + __tun_wake_queue(tun, tfile, 1, bytes); + } spin_unlock(&tfile->tx_ring.consumer_lock); return ptr;@@ -3805,7 +3815,7 @@ struct ptr_ring *tun_get_tx_ring(struct file *file) EXPORT_SYMBOL_GPL(tun_get_tx_ring); /* Callers must hold ring.consumer_lock */ -void tun_wake_queue(struct file *file, int consumed) +void tun_wake_queue(struct file *file, unsigned int pkts, unsigned int bytes) { struct tun_file *tfile; struct tun_struct *tun;@@ -3821,7 +3831,7 @@ void tun_wake_queue(struct file *file, int consumed) tun = rcu_dereference(tfile->tun); if (tun) - __tun_wake_queue(tun, tfile, consumed); + __tun_wake_queue(tun, tfile, pkts, bytes); rcu_read_unlock(); }diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index db341c922673..5267b323bd59 100644 --- a/drivers/vhost/net.c +++ b/drivers/vhost/net.c@@ -181,14 +181,23 @@ static int vhost_net_buf_produce(struct sock *sk, { struct file *file = sk->sk_socket->file; struct vhost_net_buf *rxq = &nvq->rxq; + unsigned int bytes = 0; + int i; rxq->head = 0; spin_lock(&nvq->rx_ring->consumer_lock); rxq->tail = __ptr_ring_consume_batched(nvq->rx_ring, rxq->queue, VHOST_NET_BATCH); - if (rxq->tail) - tun_wake_queue(file, rxq->tail); + if (rxq->tail) { + for (i = 0; i < rxq->tail; i++) { + void *ptr = rxq->queue[i]; + + if (!tun_is_xdp_frame(ptr)) + bytes += ((struct sk_buff *)ptr)->len; + } + tun_wake_queue(file, rxq->tail, bytes); + } spin_unlock(&nvq->rx_ring->consumer_lock); return rxq->tail;diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h index 5f3e206c7a73..49b85bf4f828 100644 --- a/include/linux/if_tun.h +++ b/include/linux/if_tun.h@@ -22,7 +22,7 @@ struct tun_msg_ctl { #if defined(CONFIG_TUN) || defined(CONFIG_TUN_MODULE) struct socket *tun_get_socket(struct file *); struct ptr_ring *tun_get_tx_ring(struct file *file); -void tun_wake_queue(struct file *file, int consumed); +void tun_wake_queue(struct file *file, unsigned int pkts, unsigned int bytes); static inline bool tun_is_xdp_frame(void *ptr) {@@ -56,7 +56,8 @@ static inline struct ptr_ring *tun_get_tx_ring(struct file *f) return ERR_PTR(-EINVAL); } -static inline void tun_wake_queue(struct file *f, int consumed) {} +static inline void tun_wake_queue(struct file *f, + unsigned int pkts, unsigned int bytes) {} static inline bool tun_is_xdp_frame(void *ptr) {