Re: [PATCH v2] virtio_balloon: Convert "vballon" kthread into a workqueue

[Michael S. Tsirkin]

On Wed, Nov 12, 2014 at 02:02:48PM +0100, Petr Mladek wrote:
> Workqueues have clean and rich API for all basic operations. The code is usually
> easier and better readable. It can be easily tuned for the given purpose.
> 
> In many cases, it allows to avoid an extra kernel thread. It helps to stop the
> growing number of them.  Also there will be less thread-specific hacks all over
> the kernel code.
> 
> It forces making the task selfcontained. There is no longer an unclear infinite
> loop. This helps to avoid problems with suspend. Also it will be very helpful
> for kGraft (kernel live patching).
> 
> The conversion is pretty straightforward. The main change is that there is not
> longer an "infinite" loop in the balloon() handler. Instead, another work item
> has to be scheduled from fill_balloon() and leak_balloon() when they do not
> do all requested changes in a single call.
> Changes in v2:
> 
> 	+ More elegant detection of the pending work in fill_balloon() and
> 	  leak_ballon(). It still needs to keep the original requested number
> 	  of pages but it does not add any extra boolean variable.
> 
> 	+ Remove WQ_MEM_RECLAIM workqueue parameter. If I get it correctly,
> 	  this is possible because the code manipulates memory but it is not
> 	  used in the memory reclaim path.
> 
> 	+ initialize the work item before allocation the workqueue
> 

changelog should come after ---

> Signed-off-by: Petr Mladek <pmladek@xxxxxxx>


I suggest a minor change below.
Otherwise looks ok

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>


> ---
>  drivers/virtio/virtio_balloon.c | 86 +++++++++++++++++++----------------------
>  1 file changed, 39 insertions(+), 47 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
> index c9703d4d6f67..5532d1222417 100644
> --- a/drivers/virtio/virtio_balloon.c
> +++ b/drivers/virtio/virtio_balloon.c
> @@ -22,8 +22,7 @@
>  #include <linux/virtio.h>
>  #include <linux/virtio_balloon.h>
>  #include <linux/swap.h>
> -#include <linux/kthread.h>
> -#include <linux/freezer.h>
> +#include <linux/workqueue.h>
>  #include <linux/delay.h>
>  #include <linux/slab.h>
>  #include <linux/module.h>
> @@ -42,11 +41,9 @@ struct virtio_balloon
>  	struct virtio_device *vdev;
>  	struct virtqueue *inflate_vq, *deflate_vq, *stats_vq;
>  
> -	/* Where the ballooning thread waits for config to change. */
> -	wait_queue_head_t config_change;
> -
> -	/* The thread servicing the balloon. */
> -	struct task_struct *thread;
> +	/* The workqueue servicing the balloon. */
> +	struct workqueue_struct *wq;
> +	struct work_struct wq_work;

We could use system_freezable_wq instead.
I do agree a dedicated wq is better since this can get blocked
for a long time while allocating memory.
However, please add a comment to this effect.

>  
>  	/* Waiting for host to ack the pages we released. */
>  	wait_queue_head_t acked;
> @@ -128,12 +125,13 @@ static void set_page_pfns(u32 pfns[], struct page *page)
>  static void fill_balloon(struct virtio_balloon *vb, size_t num)
>  {
>  	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
> +	size_t limit;
>  
>  	/* We can only do one array worth at a time. */
> -	num = min(num, ARRAY_SIZE(vb->pfns));
> +	limit = min(num, ARRAY_SIZE(vb->pfns));
>  
>  	mutex_lock(&vb->balloon_lock);
> -	for (vb->num_pfns = 0; vb->num_pfns < num;
> +	for (vb->num_pfns = 0; vb->num_pfns < limit;
>  	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
>  		struct page *page = balloon_page_enqueue(vb_dev_info);
>  
> @@ -153,6 +151,9 @@ static void fill_balloon(struct virtio_balloon *vb, size_t num)
>  	/* Did we get any? */
>  	if (vb->num_pfns != 0)
>  		tell_host(vb, vb->inflate_vq);
> +	/* Do we need to get more? */
> +	if (vb->num_pfns < num)
> +		queue_work(vb->wq, &vb->wq_work);
>  	mutex_unlock(&vb->balloon_lock);
>  }
>  
> @@ -172,12 +173,13 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
>  {
>  	struct page *page;
>  	struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
> +	size_t limit;
>  
>  	/* We can only do one array worth at a time. */
> -	num = min(num, ARRAY_SIZE(vb->pfns));
> +	limit = min(num, ARRAY_SIZE(vb->pfns));
>  
>  	mutex_lock(&vb->balloon_lock);
> -	for (vb->num_pfns = 0; vb->num_pfns < num;
> +	for (vb->num_pfns = 0; vb->num_pfns < limit;
>  	     vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
>  		page = balloon_page_dequeue(vb_dev_info);
>  		if (!page)
> @@ -193,6 +195,9 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
>  	 */
>  	if (vb->num_pfns != 0)
>  		tell_host(vb, vb->deflate_vq);
> +	/* Schedule another call if a bigger change is requested */
> +	if (vb->num_pfns < num)
> +		queue_work(vb->wq, &vb->wq_work);
>  	mutex_unlock(&vb->balloon_lock);
>  	release_pages_by_pfn(vb->pfns, vb->num_pfns);
>  }
> @@ -234,14 +239,14 @@ static void update_balloon_stats(struct virtio_balloon *vb)
>   * with a single buffer.  From that point forward, all conversations consist of
>   * a hypervisor request (a call to this function) which directs us to refill
>   * the virtqueue with a fresh stats buffer.  Since stats collection can sleep,
> - * we notify our kthread which does the actual work via stats_handle_request().
> + * we queue a work that will do the actual work via stats_handle_request().
>   */
>  static void stats_request(struct virtqueue *vq)
>  {
>  	struct virtio_balloon *vb = vq->vdev->priv;
>  
>  	vb->need_stats_update = 1;
> -	wake_up(&vb->config_change);
> +	queue_work(vb->wq, &vb->wq_work);
>  }
>  
>  static void stats_handle_request(struct virtio_balloon *vb)
> @@ -265,7 +270,7 @@ static void virtballoon_changed(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> -	wake_up(&vb->config_change);
> +	queue_work(vb->wq, &vb->wq_work);
>  }
>  
>  static inline s64 towards_target(struct virtio_balloon *vb)
> @@ -287,35 +292,22 @@ static void update_balloon_size(struct virtio_balloon *vb)
>  		      &actual);
>  }
>  
> -static int balloon(void *_vballoon)
> +static void balloon(struct work_struct *work)
>  {
> -	struct virtio_balloon *vb = _vballoon;
> -
> -	set_freezable();
> -	while (!kthread_should_stop()) {
> -		s64 diff;
> -
> -		try_to_freeze();
> -		wait_event_interruptible(vb->config_change,
> -					 (diff = towards_target(vb)) != 0
> -					 || vb->need_stats_update
> -					 || kthread_should_stop()
> -					 || freezing(current));
> -		if (vb->need_stats_update)
> -			stats_handle_request(vb);
> -		if (diff > 0)
> -			fill_balloon(vb, diff);
> -		else if (diff < 0)
> -			leak_balloon(vb, -diff);
> -		update_balloon_size(vb);
> +	struct virtio_balloon *vb;
> +	s64 diff;
>  
> -		/*
> -		 * For large balloon changes, we could spend a lot of time
> -		 * and always have work to do.  Be nice if preempt disabled.
> -		 */
> -		cond_resched();
> -	}
> -	return 0;
> +	vb = container_of(work, struct virtio_balloon, wq_work);
> +	diff = towards_target(vb);
> +
> +	if (vb->need_stats_update)
> +		stats_handle_request(vb);
> +
> +	if (diff > 0)
> +		fill_balloon(vb, diff);
> +	else if (diff < 0)
> +		leak_balloon(vb, -diff);
> +	update_balloon_size(vb);
>  }
>  
>  static int init_vqs(struct virtio_balloon *vb)
> @@ -429,7 +421,6 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  
>  	vb->num_pages = 0;
>  	mutex_init(&vb->balloon_lock);
> -	init_waitqueue_head(&vb->config_change);
>  	init_waitqueue_head(&vb->acked);
>  	vb->vdev = vdev;
>  	vb->need_stats_update = 0;
> @@ -443,9 +434,10 @@ static int virtballoon_probe(struct virtio_device *vdev)
>  	if (err)
>  		goto out_free_vb;
>  
> -	vb->thread = kthread_run(balloon, vb, "vballoon");
> -	if (IS_ERR(vb->thread)) {
> -		err = PTR_ERR(vb->thread);
> +	INIT_WORK(&vb->wq_work, balloon);
> +	vb->wq = alloc_workqueue("vballoon_wq", WQ_FREEZABLE, 0);
> +	if (!vb->wq) {
> +		err = -ENOMEM;
>  		goto out_del_vqs;
>  	}
>  
> @@ -476,7 +468,7 @@ static void virtballoon_remove(struct virtio_device *vdev)
>  {
>  	struct virtio_balloon *vb = vdev->priv;
>  
> -	kthread_stop(vb->thread);
> +	destroy_workqueue(vb->wq);
>  	remove_common(vb);
>  	kfree(vb);
>  }
> @@ -487,7 +479,7 @@ static int virtballoon_freeze(struct virtio_device *vdev)
>  	struct virtio_balloon *vb = vdev->priv;
>  
>  	/*
> -	 * The kthread is already frozen by the PM core before this
> +	 * The workqueue is already frozen by the PM core before this
>  	 * function is called.
>  	 */
>  
> -- 
> 1.8.5.2
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