Re: [PATCH v1 1/2] virtio-balloon: tweak config_changed implementation

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

On Thu, Jan 03, 2019 at 01:31:01PM +0800, Wei Wang wrote:
> virtio-ccw has deadlock issues with reading config registers inside the
> interrupt context, so we tweak the virtballoon_changed implementation
> by moving the config read operations into the related workqueue contexts.
> 
> Signed-off-by: Wei Wang <wei.w.wang@intel.com>
> ---
>  drivers/virtio/virtio_balloon.c | 54 ++++++++++++++++++++---------------------
>  1 file changed, 26 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 728ecd1..9a82a11 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -394,33 +394,15 @@ static void virtballoon_changed(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  	unsigned long flags;
> -	s64 diff = towards_target(vb);
> -
> -	if (diff) {
> -		spin_lock_irqsave(&vb->stop_update_lock, flags);
> -		if (!vb->stop_update)
> -			queue_work(system_freezable_wq,
> -				   &vb->update_balloon_size_work);
> -		spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> -	}
>  
> -	if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> -		virtio_cread(vdev, struct virtio_balloon_config,
> -			     free_page_report_cmd_id, &vb->cmd_id_received);

There's one problem with this approach:

previously updating the cmd_id_received here would immediately
stop the report in send_free_pages.

With this approach we are waiting for the wq to schedule,
which might be blocked waiting for report to complete.
So host can no longer quickly stop the report in progress.

A simple work-around would be to set some kind of flag whenever there
is a change interrupt, then have send_free_pages test it
and re-read cmd_id_received.

Needs to be an atomic I guess ...


> -		if (vb->cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> -			/* Pass ULONG_MAX to give back all the free pages */
> -			return_free_pages_to_mm(vb, ULONG_MAX);
> -		} else if (vb->cmd_id_received != VIRTIO_BALLOON_CMD_ID_STOP &&
> -			   vb->cmd_id_received !=
> -			   virtio32_to_cpu(vdev, vb->cmd_id_active)) {
> -			spin_lock_irqsave(&vb->stop_update_lock, flags);
> -			if (!vb->stop_update) {
> -				queue_work(vb->balloon_wq,
> -					   &vb->report_free_page_work);
> -			}
> -			spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> -		}
> +	spin_lock_irqsave(&vb->stop_update_lock, flags);
> +	if (!vb->stop_update) {
> +		queue_work(system_freezable_wq,
> +			   &vb->update_balloon_size_work);
> +		if (virtio_has_feature(vdev, VIRTIO_BALLOON_F_FREE_PAGE_HINT))
> +			queue_work(vb->balloon_wq, &vb->report_free_page_work);
>  	}
> +	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
>  }
>  
>  static void update_balloon_size(struct virtio_balloon *vb)
> @@ -637,11 +619,9 @@ static int send_free_pages(struct virtio_balloon *vb)
>  	return 0;
>  }
>  
> -static void report_free_page_func(struct work_struct *work)
> +static void virtio_balloon_report_free_page(struct virtio_balloon *vb)
>  {
>  	int err;
> -	struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> -						 report_free_page_work);
>  	struct device *dev = &vb->vdev->dev;
>  
>  	/* Start by sending the received cmd id to host with an outbuf. */
> @@ -659,6 +639,24 @@ static void report_free_page_func(struct work_struct *work)
>  		dev_err(dev, "Failed to send a stop id, err = %d\n", err);
>  }
>  
> +static void report_free_page_func(struct work_struct *work)
> +{
> +	struct virtio_balloon *vb = container_of(work, struct virtio_balloon,
> +						 report_free_page_work);
> +
> +	virtio_cread(vb->vdev, struct virtio_balloon_config,
> +		     free_page_report_cmd_id, &vb->cmd_id_received);
> +
> +	if (vb->cmd_id_received == VIRTIO_BALLOON_CMD_ID_DONE) {
> +		/* Pass ULONG_MAX to give back all the free pages */
> +		return_free_pages_to_mm(vb, ULONG_MAX);
> +	} else if (vb->cmd_id_received != VIRTIO_BALLOON_CMD_ID_STOP &&
> +		   vb->cmd_id_received !=
> +		   virtio32_to_cpu(vb->vdev, vb->cmd_id_active)) {
> +		virtio_balloon_report_free_page(vb);
> +	}
> +}
> +
>  #ifdef CONFIG_BALLOON_COMPACTION
>  /*
>   * virtballoon_migratepage - perform the balloon page migration on behalf of
> -- 
> 2.7.4