linux-kernel - Re: [PATCH net 2/2] vhost-net: correctly flush batched packet before enabling notification

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Message-ID: <20250912044523-mutt-send-email-mst@kernel.org>
Date: Fri, 12 Sep 2025 04:50:35 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Jason Wang <jasowang@...hat.com>
Cc: eperezma@...hat.com, jonah.palmer@...cle.com, kuba@...nel.org,
	jon@...anix.com, kvm@...r.kernel.org,
	virtualization@...ts.linux.dev, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net 2/2] vhost-net: correctly flush batched packet before
 enabling notification

On Fri, Sep 12, 2025 at 04:26:58PM +0800, Jason Wang wrote:
> Commit 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after
> sendmsg") tries to defer the notification enabling by moving the logic
> out of the loop after the vhost_tx_batch() when nothing new is
> spotted. This will bring side effects as the new logic would be reused
> for several other error conditions.
> 
> One example is the IOTLB: when there's an IOTLB miss, get_tx_bufs()
> might return -EAGAIN and exit the loop and see there's still available
> buffers, so it will queue the tx work again until userspace feed the
> IOTLB entry correctly. This will slowdown the tx processing and may
> trigger the TX watchdog in the guest.

It's not that it might.
Pls clarify that it *has been reported* to do exactly that,
and add a link to the report.


> Fixing this by stick the notificaiton enabling logic inside the loop
> when nothing new is spotted and flush the batched before.
> 
> Reported-by: Jon Kohler <jon@...anix.com>
> Cc: stable@...r.kernel.org
> Fixes: 8c2e6b26ffe2 ("vhost/net: Defer TX queue re-enable until after sendmsg")
> Signed-off-by: Jason Wang <jasowang@...hat.com>

So this is mostly a revert, but with
                     vhost_tx_batch(net, nvq, sock, &msg);
added in to avoid regressing performance.

If you do not want to structure it like this (revert+optimization),
then pls make that clear in the message.


> ---
>  drivers/vhost/net.c | 33 +++++++++++++--------------------
>  1 file changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 16e39f3ab956..3611b7537932 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -765,11 +765,11 @@ 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;
>  	bool in_order = vhost_has_feature(vq, VIRTIO_F_IN_ORDER);
>  
>  	do {
> -		busyloop_intr = false;
> +		bool busyloop_intr = false;
> +
>  		if (nvq->done_idx == VHOST_NET_BATCH)
>  			vhost_tx_batch(net, nvq, sock, &msg);
>  
> @@ -780,10 +780,18 @@ 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) {
> -			/* Kicks are disabled at this point, break loop and
> -			 * process any remaining batched packets. Queue will
> -			 * be re-enabled afterwards.
> +			/* Flush batched packets before enabling
> +			 * virqtueue notification to reduce
> +			 * unnecssary virtqueue kicks.

typos: virtqueue, unnecessary

>  			 */
> +			vhost_tx_batch(net, nvq, sock, &msg);
> +			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;
> +			}
>  			break;
>  		}
>  
> @@ -839,22 +847,7 @@ 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)
> -- 
> 2.34.1