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: <20251126035449-mutt-send-email-mst@kernel.org>
Date: Wed, 26 Nov 2025 03:56:49 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jon Kohler <jon@...anix.com>
Cc: Jason Wang <jasowang@...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 Tue, Nov 25, 2025 at 11:00:33AM -0700, Jon Kohler 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)) {


I also wonder why don't we assign len here.
I get the point about being fair to TX but it's not
indefinite poll, just a single peek ...

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