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Date: Tue, 19 Dec 2023 16:39:54 +0100
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Subject: Re: [PATCH v2] virtio_balloon: stay awake while adjusting balloon
To: David Stevens <stevensd@chromium.org>
Cc: "Michael S . Tsirkin" <mst@redhat.com>,
 virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org
References: <20231218151842.2756673-1-stevensd@google.com>
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From: David Hildenbrand <david@redhat.com>
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On 19.12.23 15:37, David Stevens wrote:
> On Mon, Dec 18, 2023 at 12:33 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 18.12.23 16:18, David Stevens wrote:
>>> From: David Stevens <stevensd@chromium.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@chromium.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?
> 
> If the parent device is not wakeup capable or if wakeup isn't enabled,
> then the vb->vdev->dev.parent->power.wakeup pointer will be NULL, so
> the NULL checks in __pm_relax/__pm_stay_awake will immediately return.

Ah, I missed that NULL check, makes sense. Thanks!

-- 
Cheers,

David / dhildenb