linux-kernel - Re: [PATCH v2] virtio_balloon: stay awake while adjusting balloon

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Date: Mon, 18 Dec 2023 18:32:57 +0100
From: David Hildenbrand <david@...hat.com>
To: David Stevens <stevensd@...omium.org>,
 "Michael S . Tsirkin" <mst@...hat.com>
Cc: virtualization@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] virtio_balloon: stay awake while adjusting balloon

On 18.12.23 16:18, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 
> A virtio_balloon's parent device may be configured so that a
> configuration change interrupt is a wakeup event. Extend the processing
> of such a wakeup event until the balloon finishes inflating or deflating
> by calling pm_stay_awake/pm_relax in the virtio_balloon driver. Note
> that these calls are no-ops if the parent device doesn't support wakeup
> events or if the wakeup events are not enabled.
> 
> This change allows the guest to use system power states such as s2idle
> without running the risk the virtio_balloon's cooperative memory
> management becoming unresponsive to the host's requests.
> 
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
> v1 -> v2:
>   - Use adjustment_signal_pending flag instead of a sequence number
>   - Call pm_stay_awake/pm_relax on parent device instead of adding a wake
>     event to the virtio balloon device
> 
>   drivers/virtio/virtio_balloon.c | 57 +++++++++++++++++++++++++++------
>   1 file changed, 47 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 1fe93e93f5bc..a3c11159cbe0 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -119,6 +119,11 @@ struct virtio_balloon {
>   	/* Free page reporting device */
>   	struct virtqueue *reporting_vq;
>   	struct page_reporting_dev_info pr_dev_info;
> +
> +	/* State for keeping the wakeup_source active while adjusting the balloon */
> +	spinlock_t adjustment_lock;
> +	bool adjustment_signal_pending;
> +	bool adjustment_in_progress;
>   };
>   
>   static const struct virtio_device_id id_table[] = {
> @@ -437,6 +442,31 @@ static void virtio_balloon_queue_free_page_work(struct virtio_balloon *vb)
>   	queue_work(vb->balloon_wq, &vb->report_free_page_work);
>   }
>   
> +static void start_update_balloon_size(struct virtio_balloon *vb)
> +{
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&vb->adjustment_lock, flags);
> +	vb->adjustment_signal_pending = true;
> +	if (!vb->adjustment_in_progress) {
> +		vb->adjustment_in_progress = true;
> +		pm_stay_awake(vb->vdev->dev.parent);
> +	}
> +	spin_unlock_irqrestore(&vb->adjustment_lock, flags);
> +
> +	queue_work(system_freezable_wq, &vb->update_balloon_size_work);
> +}
> +
> +static void end_update_balloon_size(struct virtio_balloon *vb)
> +{
> +	spin_lock(&vb->adjustment_lock);
> +	if (!vb->adjustment_signal_pending && vb->adjustment_in_progress) {

How could vb->adjustment_in_progress ever not be set at this point?

> +		vb->adjustment_in_progress = false;
> +		pm_relax(vb->vdev->dev.parent);
> +	}
> +	spin_unlock(&vb->adjustment_lock);
> +}
> +

LGTM, although I wonder what happens when calling pm_stay_awake() etc. 
on a parent device that is not wakeup-even-capable?

-- 
Cheers,

David / dhildenb