linux-kernel - Re: [PATCH net-next] vhost/net: check peek_head_len after signal to guest to avoid delays

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Message-ID: <CACGkMEv22mEkVoEdN0iPdgeycOEn8TaXcg-y5PGHTjw9YvTKpw@mail.gmail.com>
Date: Wed, 26 Nov 2025 14:15:45 +0800
From: Jason Wang <jasowang@...hat.com>
To: Jon Kohler <jon@...anix.com>
Cc: "Michael S. Tsirkin" <mst@...hat.com>, Eugenio Pérez <eperezma@...hat.com>, 
	kvm@...r.kernel.org, virtualization@...ts.linux.dev, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] vhost/net: check peek_head_len after signal to
 guest to avoid delays

On Wed, Nov 26, 2025 at 1:18 AM Jon Kohler <jon@...anix.com> wrote:
>
> In non-busypoll handle_rx paths, if peek_head_len returns 0, the RX
> loop breaks, the RX wait queue is re-enabled, and vhost_net_signal_used
> is called to flush done_idx and notify the guest if needed.
>
> However, signaling the guest can take non-trivial time. During this
> window, additional RX payloads may arrive on rx_ring without further
> kicks. These new payloads will sit unprocessed until another kick
> arrives, increasing latency. In high-rate UDP RX workloads, this was
> observed to occur over 20k times per second.
>
> To minimize this window and improve opportunities to process packets
> promptly, immediately call peek_head_len after signaling. If new packets
> are found, treat it as a busy poll interrupt and requeue handle_rx,
> improving fairness to TX handlers and other pending CPU work. This also
> helps suppress unnecessary thread wakeups, reducing waker CPU demand.
>
> Signed-off-by: Jon Kohler <jon@...anix.com>
> ---
>  drivers/vhost/net.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 35ded4330431..04cb5f1dc6e4 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -1015,6 +1015,27 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net, struct sock *sk,
>         struct vhost_virtqueue *tvq = &tnvq->vq;
>         int len = peek_head_len(rnvq, sk);
>
> +       if (!len && rnvq->done_idx) {
> +               /* When idle, flush signal first, which can take some
> +                * time for ring management and guest notification.
> +                * Afterwards, check one last time for work, as the ring
> +                * may have received new work during the notification
> +                * window.
> +                */
> +               vhost_net_signal_used(rnvq, *count);
> +               *count = 0;
> +               if (peek_head_len(rnvq, sk)) {
> +                       /* More work came in during the notification
> +                        * window. To be fair to the TX handler and other
> +                        * potentially pending work items, pretend like
> +                        * this was a busy poll interruption so that
> +                        * the RX handler will be rescheduled and try
> +                        * again.
> +                        */
> +                       *busyloop_intr = true;
> +               }
> +       }

I'm not sure I will get here.

Once vhost_net_rx_peek_head_len() returns 0, we exit the loop to:

if (unlikely(busyloop_intr))
                vhost_poll_queue(&vq->poll);
        else if (!sock_len)
                vhost_net_enable_vq(net, vq);
out:
        vhost_net_signal_used(nvq, count);

Are you suggesting signalling before enabling vq actually?

Thanks

> +
>         if (!len && rvq->busyloop_timeout) {
>                 /* Flush batched heads first */
>                 vhost_net_signal_used(rnvq, *count);
> --
> 2.43.0
>