Re: [PATCH net-next v3] vhost/net: Defer TX queue re-enable until after sendmsg

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Apr 30, 2025 at 07:04:28PM -0700, Jon Kohler wrote:
> In handle_tx_copy, TX batching processes packets below ~PAGE_SIZE and
> batches up to 64 messages before calling sock->sendmsg.
> 
> Currently, when there are no more messages on the ring to dequeue,
> handle_tx_copy re-enables kicks on the ring *before* firing off the
> batch sendmsg. However, sock->sendmsg incurs a non-zero delay,
> especially if it needs to wake up a thread (e.g., another vhost worker).
> 
> If the guest submits additional messages immediately after the last ring
> check and disablement, it triggers an EPT_MISCONFIG vmexit to attempt to
> kick the vhost worker. This may happen while the worker is still
> processing the sendmsg, leading to wasteful exit(s).
> 
> This is particularly problematic for single-threaded guest submission
> threads, as they must exit, wait for the exit to be processed
> (potentially involving a TTWU), and then resume.
> 
> In scenarios like a constant stream of UDP messages, this results in a
> sawtooth pattern where the submitter frequently vmexits, and the
> vhost-net worker alternates between sleeping and waking.
> 
> A common solution is to configure vhost-net busy polling via userspace
> (e.g., qemu poll-us). However, treating the sendmsg as the "busy"
> period by keeping kicks disabled during the final sendmsg and
> performing one additional ring check afterward provides a significant
> performance improvement without any excess busy poll cycles.
> 
> If messages are found in the ring after the final sendmsg, requeue the
> TX handler. This ensures fairness for the RX handler and allows
> vhost_run_work_list to cond_resched() as needed.
> 
> Test Case
>     TX VM: taskset -c 2 iperf3  -c rx-ip-here -t 60 -p 5200 -b 0 -u -i 5
>     RX VM: taskset -c 2 iperf3 -s -p 5200 -D
>     6.12.0, each worker backed by tun interface with IFF_NAPI setup.
>     Note: TCP side is largely unchanged as that was copy bound
> 
> 6.12.0 unpatched
>     EPT_MISCONFIG/second: 5411
>     Datagrams/second: ~382k
>     Interval         Transfer     Bitrate         Lost/Total Datagrams
>     0.00-30.00  sec  15.5 GBytes  4.43 Gbits/sec  0/11481630 (0%)  sender
> 
> 6.12.0 patched
>     EPT_MISCONFIG/second: 58 (~93x reduction)
>     Datagrams/second: ~650k  (~1.7x increase)
>     Interval         Transfer     Bitrate         Lost/Total Datagrams
>     0.00-30.00  sec  26.4 GBytes  7.55 Gbits/sec  0/19554720 (0%)  sender
> 
> Acked-by: Jason Wang <jasowang@redhat.com>
> Signed-off-by: Jon Kohler <jon@nutanix.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
> v2->v3: Address MST's comments regarding busyloop_intr
> 	https://patchwork.kernel.org/project/netdevbpf/patch/20250420010518.2842335-1-jon@nutanix.com/
> v1->v2: Move from net to net-next (no changes)
> 	https://patchwork.kernel.org/project/netdevbpf/patch/20250401043230.790419-1-jon@nutanix.com/
> ---
>  drivers/vhost/net.c | 30 +++++++++++++++++++++---------
>  1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index b9b9e9d40951..7cbfc7d718b3 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -755,10 +755,10 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  	int err;
>  	int sent_pkts = 0;
>  	bool sock_can_batch = (sock->sk->sk_sndbuf == INT_MAX);
> +	bool busyloop_intr;
>  
>  	do {
> -		bool busyloop_intr = false;
> -
> +		busyloop_intr = false;
>  		if (nvq->done_idx == VHOST_NET_BATCH)
>  			vhost_tx_batch(net, nvq, sock, &msg);
>  
> @@ -769,13 +769,10 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  			break;
>  		/* Nothing new?  Wait for eventfd to tell us they refilled. */
>  		if (head == vq->num) {
> -			if (unlikely(busyloop_intr)) {
> -				vhost_poll_queue(&vq->poll);
> -			} else if (unlikely(vhost_enable_notify(&net->dev,
> -								vq))) {
> -				vhost_disable_notify(&net->dev, vq);
> -				continue;
> -			}
> +			/* Kicks are disabled at this point, break loop and
> +			 * process any remaining batched packets. Queue will
> +			 * be re-enabled afterwards.
> +			 */
>  			break;
>  		}
>  
> @@ -825,7 +822,22 @@ static void handle_tx_copy(struct vhost_net *net, struct socket *sock)
>  		++nvq->done_idx;
>  	} while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
>  
> +	/* Kicks are still disabled, dispatch any remaining batched msgs. */
>  	vhost_tx_batch(net, nvq, sock, &msg);
> +
> +	if (unlikely(busyloop_intr))
> +		/* If interrupted while doing busy polling, requeue the
> +		 * handler to be fair handle_rx as well as other tasks
> +		 * waiting on cpu.
> +		 */
> +		vhost_poll_queue(&vq->poll);
> +	else
> +		/* All of our work has been completed; however, before
> +		 * leaving the TX handler, do one last check for work,
> +		 * and requeue handler if necessary. If there is no work,
> +		 * queue will be reenabled.
> +		 */
> +		vhost_net_busy_poll_try_queue(net, vq);
>  }
>  
>  static void handle_tx_zerocopy(struct vhost_net *net, struct socket *sock)
> -- 
> 2.43.0