On Wed, Nov 26, 2025 at 04:49:11PM +0000, Jon Kohler wrote:

quoted

On Nov 25, 2025, at 6:50 PM, Michael S. Tsirkin [off-list ref] wrote:

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On Tue, Nov 25, 2025 at 11:00:33AM -0700, Jon Kohler wrote:

quoted

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 <redacted>

Given this is supposed to be a performance improvement,
pls include info on the effect this has on performance. Thanks!

I had already mentioned we’re avoiding ~20k schedulers/IPIs in that
example, but I can add more detail. Let’s resolve the other parts of
the thread first and go from there?

the discussion seems to have died down.
I suggest reposting with perf data you have
(which test, how much improvement, what cpu usage)
collected in the commit log.

thanks!

quoted

quoted

---
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