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

[Jon Kohler <jon@nutanix.com>]

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