Re: [PATCH net-next v8 2/4] vhost-net: wake queue of tun/tap after ptr_ring consume
From: "Michael S. Tsirkin" <mst@redhat.com>
Date: 2026-03-12 13:54:58
Also in:
kvm, lkml, virtualization
On Thu, Mar 12, 2026 at 02:06:37PM +0100, Simon Schippers wrote:
Add tun_wake_queue() to tun.c and export it for use by vhost-net. The function validates that the file belongs to a tun/tap device, dereferences the tun_struct under RCU, and delegates to __tun_wake_queue(). vhost_net_buf_produce() now calls tun_wake_queue() after a successful batched consume of the ring to allow the netdev subqueue to be woken up.
A sentence missing here: the point is to allow queue to be stopped when it gets full, which is required for traffic shaping - implemented by the following "avoid ptr_ring tail-drop when a qdisc is present"
Without the corresponding queue stopping (introduced in a subsequent commit), this patch alone causes a slight throughput regression for a tap+vhost-net setup sending to a qemu VM: 3.948 Mpps to 3.888 Mpps (-1.5%). Details: AMD Ryzen 5 5600X at 4.3 GHz, 3200 MHz RAM, isolated QEMU threads, XDP drop program active in VM, pktgen sender; Avg over 20 runs @ 100,000,000 packets. SRSO and spectre v2 mitigations disabled.
quoted hunk ↗ jump to hunk
Co-developed-by: Tim Gebauer <redacted> Signed-off-by: Tim Gebauer <redacted> Signed-off-by: Simon Schippers <redacted> --- drivers/net/tun.c | 21 +++++++++++++++++++++ drivers/vhost/net.c | 15 +++++++++++---- include/linux/if_tun.h | 3 +++ 3 files changed, 35 insertions(+), 4 deletions(-)diff --git a/drivers/net/tun.c b/drivers/net/tun.c index a82d665dab5f..b86582cc6cb6 100644 --- a/drivers/net/tun.c +++ b/drivers/net/tun.c@@ -3760,6 +3760,27 @@ struct ptr_ring *tun_get_tx_ring(struct file *file) } EXPORT_SYMBOL_GPL(tun_get_tx_ring); +void tun_wake_queue(struct file *file) +{ + struct tun_file *tfile; + struct tun_struct *tun; + + if (file->f_op != &tun_fops) + return; + tfile = file->private_data; + if (!tfile) + return; + + rcu_read_lock(); + + tun = rcu_dereference(tfile->tun); + if (tun) + __tun_wake_queue(tun, tfile); + + rcu_read_unlock(); +} +EXPORT_SYMBOL_GPL(tun_wake_queue); + module_init(tun_init); module_exit(tun_cleanup); MODULE_DESCRIPTION(DRV_DESCRIPTION);diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c index 80965181920c..c8ef804ef28c 100644 --- a/drivers/vhost/net.c +++ b/drivers/vhost/net.c@@ -176,13 +176,19 @@ static void *vhost_net_buf_consume(struct vhost_net_buf *rxq) return ret; } -static int vhost_net_buf_produce(struct vhost_net_virtqueue *nvq) +static int vhost_net_buf_produce(struct sock *sk, + struct vhost_net_virtqueue *nvq) { + struct file *file = sk->sk_socket->file; struct vhost_net_buf *rxq = &nvq->rxq; rxq->head = 0; rxq->tail = ptr_ring_consume_batched(nvq->rx_ring, rxq->queue, VHOST_NET_BATCH); + + if (rxq->tail) + tun_wake_queue(file); + return rxq->tail; }@@ -209,14 +215,15 @@ static int vhost_net_buf_peek_len(void *ptr) return __skb_array_len_with_tag(ptr); } -static int vhost_net_buf_peek(struct vhost_net_virtqueue *nvq) +static int vhost_net_buf_peek(struct sock *sk, + struct vhost_net_virtqueue *nvq) { struct vhost_net_buf *rxq = &nvq->rxq; if (!vhost_net_buf_is_empty(rxq)) goto out; - if (!vhost_net_buf_produce(nvq)) + if (!vhost_net_buf_produce(sk, nvq)) return 0; out:@@ -995,7 +1002,7 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk) unsigned long flags; if (rvq->rx_ring) - return vhost_net_buf_peek(rvq); + return vhost_net_buf_peek(sk, rvq); spin_lock_irqsave(&sk->sk_receive_queue.lock, flags); head = skb_peek(&sk->sk_receive_queue);diff --git a/include/linux/if_tun.h b/include/linux/if_tun.h index 80166eb62f41..ab3b4ebca059 100644 --- a/include/linux/if_tun.h +++ b/include/linux/if_tun.h@@ -22,6 +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); static inline bool tun_is_xdp_frame(void *ptr) {@@ -55,6 +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) {} + static inline bool tun_is_xdp_frame(void *ptr) { return false;-- 2.43.0