[PATCH 1/2] vhost: Reduce vhost_work_flush() wakeup latency

[Bart Van Assche <bvanassche@acm.org>]

If the TIF_NEED_RESCHED task flag is set, wake up any vhost_work_flush()
waiters before rescheduling instead of after rescheduling.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Asias He <asias@redhat.com>
---
 drivers/vhost/vhost.c |   42 +++++++++++++++++++-----------------------
 1 file changed, 19 insertions(+), 23 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index e58cf00..e7ffc10 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -201,47 +201,43 @@ static void vhost_vq_reset(struct vhost_dev *dev,
 static int vhost_worker(void *data)
 {
 	struct vhost_dev *dev = data;
-	struct vhost_work *work = NULL;
-	unsigned uninitialized_var(seq);
+	struct vhost_work *work;
+	unsigned seq;
 	mm_segment_t oldfs = get_fs();
 
 	set_fs(USER_DS);
 	use_mm(dev->mm);
 
-	for (;;) {
+	spin_lock_irq(&dev->work_lock);
+	while (!kthread_should_stop()) {
 		/* mb paired w/ kthread_stop */
 		set_current_state(TASK_INTERRUPTIBLE);
-
-		spin_lock_irq(&dev->work_lock);
-		if (work) {
-			work->done_seq = seq;
-			if (work->flushing)
-				wake_up_all(&work->done);
-		}
-
-		if (kthread_should_stop()) {
-			spin_unlock_irq(&dev->work_lock);
-			__set_current_state(TASK_RUNNING);
-			break;
-		}
 		if (!list_empty(&dev->work_list)) {
 			work = list_first_entry(&dev->work_list,
 						struct vhost_work, node);
 			list_del_init(&work->node);
 			seq = work->queue_seq;
-		} else
-			work = NULL;
-		spin_unlock_irq(&dev->work_lock);
+			spin_unlock_irq(&dev->work_lock);
 
-		if (work) {
 			__set_current_state(TASK_RUNNING);
 			work->fn(work);
-			if (need_resched())
-				schedule();
-		} else
+
+			spin_lock_irq(&dev->work_lock);
+			work->done_seq = seq;
+			if (work->flushing)
+				wake_up_all(&work->done);
+		}
+		if (list_empty(&dev->work_list) || need_resched()) {
+			spin_unlock_irq(&dev->work_lock);
+
 			schedule();
 
+			spin_lock_irq(&dev->work_lock);
+		}
 	}
+	spin_unlock_irq(&dev->work_lock);
+
+	__set_current_state(TASK_RUNNING);
 	unuse_mm(dev->mm);
 	set_fs(oldfs);
 	return 0;
-- 
1.7.10.4