[PATCH v4 2/2] virtio_balloon: Use a workqueue instead of "vballoon" kthread

[Petr Mladek <pmladek@suse.com>]

From: Petr Mladek <pmladek@suse.cz>

This patch moves the deferred work from the "vballoon" kthread into a
system freezable workqueue.

We do not need to maintain and run a dedicated kthread. Also the event
driven workqueues API makes the logic much easier. Especially, we do
not longer need an own wait queue, wait function, and freeze point.

The conversion is pretty straightforward. One cycle of the main loop
is put into a work. The work is queued instead of waking the kthread.

fill_balloon() and leak_balloon() have a limit for the amount of modified
pages. The work re-queues itself when necessary.

My initial idea was to use a dedicated workqueue. Michael S. Tsirkin
suggested using a system one. Tejun Heo confirmed that the system
workqueue has a pretty high concurrency level (256) by default.
Therefore we need not be afraid of too long blocking.

Signed-off-by: Petr Mladek <pmladek@suse.cz>
---
 drivers/virtio/virtio_balloon.c | 82 +++++++++++++----------------------------
 1 file changed, 26 insertions(+), 56 deletions(-)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index d73a86db2490..960e54b1d0c1 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>
@@ -49,11 +48,8 @@ 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 balloon servicing is delegated to a freezable workqueue. */
+	struct work_struct wq_work;
 
 	/* Waiting for host to ack the pages we released. */
 	wait_queue_head_t acked;
@@ -255,14 +251,15 @@ 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 delegate the job to a freezable workqueue 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(system_freezable_wq, &vb->wq_work);
 }
 
 static void stats_handle_request(struct virtio_balloon *vb)
@@ -286,7 +283,7 @@ static void virtballoon_changed(struct virtio_device *vdev)
 {
 	struct virtio_balloon *vb = vdev->priv;
 
-	wake_up(&vb->config_change);
+	queue_work(system_freezable_wq, &vb->wq_work);
 }
 
 static inline s64 towards_target(struct virtio_balloon *vb)
@@ -349,43 +346,25 @@ static int virtballoon_oom_notify(struct notifier_block *self,
 	return NOTIFY_OK;
 }
 
-static int balloon(void *_vballoon)
+static void balloon(struct work_struct *work)
 {
-	struct virtio_balloon *vb = _vballoon;
-	DEFINE_WAIT_FUNC(wait, woken_wake_function);
-
-	set_freezable();
-	while (!kthread_should_stop()) {
-		s64 diff;
-
-		try_to_freeze();
-
-		add_wait_queue(&vb->config_change, &wait);
-		for (;;) {
-			if ((diff = towards_target(vb)) != 0 ||
-			    vb->need_stats_update ||
-			    kthread_should_stop() ||
-			    freezing(current))
-				break;
-			wait_woken(&wait, TASK_INTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);
-		}
-		remove_wait_queue(&vb->config_change, &wait);
+	struct virtio_balloon *vb;
+	s64 diff;
 
-		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);
+	vb = container_of(work, struct virtio_balloon, wq_work);
+	diff = towards_target(vb);
 
-		/*
-		 * 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;
+	if (vb->need_stats_update)
+		stats_handle_request(vb);
+
+	if (diff > 0)
+		diff -= fill_balloon(vb, diff);
+	else if (diff < 0)
+		diff += leak_balloon(vb, -diff);
+	update_balloon_size(vb);
+
+	if (diff)
+		queue_work(system_freezable_wq, work);
 }
 
 static int init_vqs(struct virtio_balloon *vb)
@@ -503,9 +482,9 @@ static int virtballoon_probe(struct virtio_device *vdev)
 		goto out;
 	}
 
+	INIT_WORK(&vb->wq_work, balloon);
 	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;
@@ -527,16 +506,8 @@ static int virtballoon_probe(struct virtio_device *vdev)
 
 	virtio_device_ready(vdev);
 
-	vb->thread = kthread_run(balloon, vb, "vballoon");
-	if (IS_ERR(vb->thread)) {
-		err = PTR_ERR(vb->thread);
-		goto out_del_vqs;
-	}
-
 	return 0;
 
-out_del_vqs:
-	unregister_oom_notifier(&vb->nb);
 out_oom_notify:
 	vdev->config->del_vqs(vdev);
 out_free_vb:
@@ -563,7 +534,7 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	struct virtio_balloon *vb = vdev->priv;
 
 	unregister_oom_notifier(&vb->nb);
-	kthread_stop(vb->thread);
+	cancel_work_sync(&vb->wq_work);
 	remove_common(vb);
 	kfree(vb);
 }
@@ -574,10 +545,9 @@ 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.
 	 */
-
 	remove_common(vb);
 	return 0;
 }
-- 
1.8.5.6