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

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Date:   Wed, 13 Dec 2023 09:44:01 +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] virtio_balloon: stay awake while adjusting balloon

On 11.12.23 12:43, David Stevens wrote:
> From: David Stevens <stevensd@...omium.org>
> 

Hi David,

> Add a wakeup event for when the balloon is inflating or deflating.
> Userspace can enable this wakeup event to prevent the system from
> suspending while the balloon is being adjusted. This allows
> /sys/power/wakeup_count to be used without breaking virtio_balloon's
> cooperative memory management.


Can you add/share some more details

> 
> Signed-off-by: David Stevens <stevensd@...omium.org>
> ---
>   drivers/virtio/virtio_balloon.c | 59 +++++++++++++++++++++++++++------
>   1 file changed, 49 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index 1fe93e93f5bc..811d8937246a 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;
> +	u32 adjustment_seqno;

Using a simple flag that gets set when updating the balloon size and 
test-and-clear when testing for changes should be easier to get.

bool adjustment_balloon_size_changed;

or sth like that.

> +	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_seqno++;
> +	if (!vb->adjustment_in_progress) {
> +		vb->adjustment_in_progress = true;
> +		pm_stay_awake(&vb->vdev->dev);
> +	}
> +	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, u32 seqno)
> +{
> +	spin_lock(&vb->adjustment_lock);
> +	if (vb->adjustment_seqno == seqno && vb->adjustment_in_progress) {
> +		vb->adjustment_in_progress = false;
> +		pm_relax(&vb->vdev->dev);
> +	}
> +	spin_unlock(&vb->adjustment_lock);
> +}
> +
>   static void virtballoon_changed(struct virtio_device *vdev)
>   {
>   	struct virtio_balloon *vb = vdev->priv;
> @@ -444,8 +474,7 @@ static void virtballoon_changed(struct virtio_device *vdev)
>   
>   	spin_lock_irqsave(&vb->stop_update_lock, flags);
>   	if (!vb->stop_update) {
> -		queue_work(system_freezable_wq,
> -			   &vb->update_balloon_size_work);
> +		start_update_balloon_size(vb);
>   		virtio_balloon_queue_free_page_work(vb);
>   	}
>   	spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> @@ -473,22 +502,29 @@ static void update_balloon_size_func(struct work_struct *work)
>   {
>   	struct virtio_balloon *vb;
>   	s64 diff;
> +	u32 seqno;
>   
>   	vb = container_of(work, struct virtio_balloon,
>   			  update_balloon_size_work);
> -	diff = towards_target(vb);
>   
> -	if (!diff)
> -		return;
> +	spin_lock(&vb->adjustment_lock);
> +	seqno = vb->adjustment_seqno;
> +	spin_unlock(&vb->adjustment_lock);
>   
> -	if (diff > 0)
> -		diff -= fill_balloon(vb, diff);
> -	else
> -		diff += leak_balloon(vb, -diff);
> -	update_balloon_size(vb);
> +	diff = towards_target(vb);
> +
> +	if (diff) {
> +		if (diff > 0)
> +			diff -= fill_balloon(vb, diff);
> +		else
> +			diff += leak_balloon(vb, -diff);
> +		update_balloon_size(vb);
> +	}
>   
>   	if (diff)
>   		queue_work(system_freezable_wq, work);
> +	else
> +		end_update_balloon_size(vb, seqno);

What if we stop the workqueue and unload the driver -- see 
remove_common() -- won't you leave pm_stay_awake() wrongly set?

>   }
>   
>   static int init_vqs(struct virtio_balloon *vb)
> @@ -992,6 +1028,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
>   			goto out_unregister_oom;
>   	}
>   
> +	spin_lock_init(&vb->adjustment_lock);
> +	device_set_wakeup_capable(&vb->vdev->dev, true);


I'm a bit confused: Documentation/driver-api/pm/devices.rst documents

"
The :c:member:`power.can_wakeup` flag just records whether the device 
(and its driver) can physically support wakeup events.  The
:c:func:`device_set_wakeup_capable()` routine affects this flag.
"

...

"
Whether or not a device is capable of issuing wakeup events is a 
hardware matter, and the kernel is responsible for keeping track of it.
"

But how is the virtio-balloon device capable of waking up the machine? 
Your patch merely implies that the virtio-baloon device is capable to 
prohbit going to sleep.

What am I missing?

-- 
Cheers,

David / dhildenb