[RFC PATCH 4/4] vhost: use workqueues for the works

[Bandan Das]

Use the common workqueue machanism for processing vhost work
by creating a unbound workqueue/vhost device. The backend workers
could still share work and are not visible to vhost.

Signed-off-by: Bandan Das <bsd@xxxxxxxxxx>
---
 drivers/vhost/vhost.c | 103 +++++++++++++++++++++++++++++++++++++++++++-------
 drivers/vhost/vhost.h |   2 +
 2 files changed, 92 insertions(+), 13 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index ad2146a..162e25e 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -30,6 +30,10 @@
 
 #include "vhost.h"
 
+static int cmwq_worker = 1;
+module_param(cmwq_worker, int, 0444);
+MODULE_PARM_DESC(cmwq_worker, "Use cmwq for worker threads - Experimental, 1 - Enable; 0 - Disable");
+
 static ushort max_mem_regions = 64;
 module_param(max_mem_regions, ushort, 0444);
 MODULE_PARM_DESC(max_mem_regions,
@@ -238,7 +242,10 @@ void vhost_work_queue(struct vhost_dev *dev, struct vhost_work *work)
 		list_add_tail(&work->node, &dev->work_list);
 		work->queue_seq++;
 		spin_unlock_irqrestore(&dev->work_lock, flags);
-		wake_up_process(dev->worker);
+		if (cmwq_worker)
+			queue_work(dev->qworker, &dev->qwork);
+		else
+			wake_up_process(dev->worker);
 	} else {
 		spin_unlock_irqrestore(&dev->work_lock, flags);
 	}
@@ -370,6 +377,52 @@ static void vhost_dev_free_iovecs(struct vhost_dev *dev)
 		vhost_vq_free_iovecs(dev->vqs[i]);
 }
 
+static void vhost_wq_worker(struct work_struct *qwork)
+{
+	struct vhost_dev *dev =
+		container_of(qwork, struct vhost_dev, qwork);
+	struct vhost_work *work = NULL;
+	unsigned uninitialized_var(seq);
+	struct mm_struct *prev_mm = NULL;
+	mm_segment_t oldfs = get_fs();
+
+	set_fs(USER_DS);
+
+	for (;;) {
+		spin_lock_irq(&dev->work_lock);
+		if (list_empty(&dev->work_list)) {
+			spin_unlock(&dev->work_lock);
+			break;
+		}
+
+		work = list_first_entry(&dev->work_list,
+					struct vhost_work, node);
+		list_del_init(&work->node);
+		seq = work->queue_seq;
+
+		if (prev_mm != dev->mm) {
+			if (prev_mm)
+				unuse_mm(prev_mm);
+			prev_mm = dev->mm;
+			use_mm(prev_mm);
+		}
+		spin_unlock(&dev->work_lock);
+
+		if (work) {
+			work->fn(work);
+			spin_lock_irq(&dev->work_lock);
+			work->done_seq = seq;
+			if (work->flushing)
+				wake_up_all(&work->done);
+			spin_unlock_irq(&dev->work_lock);
+		}
+	}
+
+	if (prev_mm)
+		unuse_mm(prev_mm);
+	set_fs(oldfs);
+}
+
 void vhost_dev_init(struct vhost_dev *dev,
 		    struct vhost_virtqueue **vqs, int nvqs)
 {
@@ -386,6 +439,10 @@ void vhost_dev_init(struct vhost_dev *dev,
 	spin_lock_init(&dev->work_lock);
 	INIT_LIST_HEAD(&dev->work_list);
 	dev->worker = NULL;
+	dev->qworker = NULL;
+
+	if (cmwq_worker)
+		INIT_WORK(&dev->qwork, vhost_wq_worker);
 
 	for (i = 0; i < dev->nvqs; ++i) {
 		vq = dev->vqs[i];
@@ -445,7 +502,8 @@ EXPORT_SYMBOL_GPL(vhost_dev_has_owner);
 /* Caller should have device mutex */
 long vhost_dev_set_owner(struct vhost_dev *dev)
 {
-	struct task_struct *worker;
+	struct task_struct *worker = NULL;
+	struct workqueue_struct *qworker;
 	int err;
 
 	/* Is there an owner already? */
@@ -456,18 +514,31 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
 
 	/* No owner, become one */
 	dev->mm = get_task_mm(current);
-	worker = kthread_create(vhost_worker, dev, "vhost-%d", current->pid);
-	if (IS_ERR(worker)) {
-		err = PTR_ERR(worker);
-		goto err_worker;
-	}
+	if (cmwq_worker) {
+		qworker = alloc_workqueue("vhost-wq-%d",
+					  WQ_UNBOUND|WQ_CGROUPS,
+					  0, current->pid);
+		if (!qworker) {
+			err = -ENOMEM;
+			goto err_worker;
+		}
+		dev->qworker = qworker;
+	} else {
+		worker = kthread_create(vhost_worker, dev,
+					"vhost-%d", current->pid);
+		if (IS_ERR(worker)) {
+			err = PTR_ERR(worker);
+			goto err_worker;
+		}
 
-	dev->worker = worker;
-	wake_up_process(worker);	/* avoid contributing to loadavg */
+		dev->worker = worker;
+		/* avoid contributing to loadavg */
+		wake_up_process(worker);
 
-	err = vhost_attach_cgroups(dev);
-	if (err)
-		goto err_cgroup;
+		err = vhost_attach_cgroups(dev);
+		if (err)
+			goto err_cgroup;
+	}
 
 	err = vhost_dev_alloc_iovecs(dev);
 	if (err)
@@ -475,7 +546,8 @@ long vhost_dev_set_owner(struct vhost_dev *dev)
 
 	return 0;
 err_cgroup:
-	kthread_stop(worker);
+	if (worker)
+		kthread_stop(worker);
 	dev->worker = NULL;
 err_worker:
 	if (dev->mm)
@@ -556,6 +628,11 @@ void vhost_dev_cleanup(struct vhost_dev *dev, bool locked)
 		kthread_stop(dev->worker);
 		dev->worker = NULL;
 	}
+	if (dev->qworker) {
+		/* destroy does flush */
+		destroy_workqueue(dev->qworker);
+		dev->qworker = NULL;
+	}
 	if (dev->mm)
 		mmput(dev->mm);
 	dev->mm = NULL;
diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
index d3f7674..e2ce0c3 100644
--- a/drivers/vhost/vhost.h
+++ b/drivers/vhost/vhost.h
@@ -127,6 +127,8 @@ struct vhost_dev {
 	spinlock_t work_lock;
 	struct list_head work_list;
 	struct task_struct *worker;
+	struct workqueue_struct *qworker;
+	struct work_struct qwork;
 };
 
 void vhost_dev_init(struct vhost_dev *, struct vhost_virtqueue **vqs, int nvqs);
-- 
2.5.0