Virtualization - May 2023 - [PATCH V2] virtio-fs: Improved request latencies when Virtio queue is full

When the Virtio queue is full, a work item is scheduled
to execute in 1ms that retries adding the request to the queue.
This is a large amount of time on the scale on which a
virtio-fs device can operate. When using a DPU this is around
40us baseline without going to a remote server (4k, QD=1).
This patch queues requests when the Virtio queue is full,
and when a completed request is taken off, immediately fills
it back up with queued requests.

This reduces the 99.9th percentile latencies in our tests by
60x and slightly increases the overall throughput, when using a
queue depth 2x the size of the Virtio queue size, with a
DPU-powered virtio-fs device.

Signed-off-by: Peter-Jan Gootzen <peter-jan at gootzen.net>
---
V1 -> V2: Not scheduling dispatch work anymore when not needed
and changed delayed_work structs to work_struct structs

 fs/fuse/virtio_fs.c | 32 +++++++++++++++++---------------
 1 file changed, 17 insertions(+), 15 deletions(-)

diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
index 4d8d4f16c727..a676297db09b 100644
--- a/fs/fuse/virtio_fs.c
+++ b/fs/fuse/virtio_fs.c
@@ -45,7 +45,7 @@ struct virtio_fs_vq {
 	struct work_struct done_work;
 	struct list_head queued_reqs;
 	struct list_head end_reqs;	/* End these requests */
-	struct delayed_work dispatch_work;
+	struct work_struct dispatch_work;
 	struct fuse_dev *fud;
 	bool connected;
 	long in_flight;
@@ -202,7 +202,7 @@ static void virtio_fs_drain_queue(struct virtio_fs_vq *fsvq)
 	}
 
 	flush_work(&fsvq->done_work);
-	flush_delayed_work(&fsvq->dispatch_work);
+	flush_work(&fsvq->dispatch_work);
 }
 
 static void virtio_fs_drain_all_queues_locked(struct virtio_fs *fs)
@@ -346,6 +346,9 @@ static void virtio_fs_hiprio_done_work(struct work_struct
*work)
 			dec_in_flight_req(fsvq);
 		}
 	} while (!virtqueue_enable_cb(vq) &&
likely(!virtqueue_is_broken(vq)));
+
+	if (!list_empty(&fsvq->queued_reqs))
+		schedule_work(&fsvq->dispatch_work);
 	spin_unlock(&fsvq->lock);
 }
 
@@ -353,7 +356,7 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
 {
 	struct fuse_req *req;
 	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
-						 dispatch_work.work);
+						 dispatch_work);
 	int ret;
 
 	pr_debug("virtio-fs: worker %s called.\n", __func__);
@@ -388,8 +391,6 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
 			if (ret == -ENOMEM || ret == -ENOSPC) {
 				spin_lock(&fsvq->lock);
 				list_add_tail(&req->list, &fsvq->queued_reqs);
-				schedule_delayed_work(&fsvq->dispatch_work,
-						      msecs_to_jiffies(1));
 				spin_unlock(&fsvq->lock);
 				return;
 			}
@@ -436,8 +437,6 @@ static int send_forget_request(struct virtio_fs_vq *fsvq,
 			pr_debug("virtio-fs: Could not queue FORGET: err=%d. Will try
later\n",
 				 ret);
 			list_add_tail(&forget->list, &fsvq->queued_reqs);
-			schedule_delayed_work(&fsvq->dispatch_work,
-					      msecs_to_jiffies(1));
 			if (!in_flight)
 				inc_in_flight_req(fsvq);
 			/* Queue is full */
@@ -469,7 +468,7 @@ static void virtio_fs_hiprio_dispatch_work(struct
work_struct *work)
 {
 	struct virtio_fs_forget *forget;
 	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
-						 dispatch_work.work);
+						 dispatch_work);
 	pr_debug("virtio-fs: worker %s called.\n", __func__);
 	while (1) {
 		spin_lock(&fsvq->lock);
@@ -647,6 +646,11 @@ static void virtio_fs_requests_done_work(struct work_struct
*work)
 			virtio_fs_request_complete(req, fsvq);
 		}
 	}
+
+	spin_lock(&fsvq->lock);
+	if (!list_empty(&fsvq->queued_reqs))
+		schedule_work(&fsvq->dispatch_work);
+	spin_unlock(&fsvq->lock);
 }
 
 /* Virtqueue interrupt handler */
@@ -670,12 +674,12 @@ static void virtio_fs_init_vq(struct virtio_fs_vq *fsvq,
char *name,
 
 	if (vq_type == VQ_REQUEST) {
 		INIT_WORK(&fsvq->done_work, virtio_fs_requests_done_work);
-		INIT_DELAYED_WORK(&fsvq->dispatch_work,
-				  virtio_fs_request_dispatch_work);
+		INIT_WORK(&fsvq->dispatch_work,
+			  virtio_fs_request_dispatch_work);
 	} else {
 		INIT_WORK(&fsvq->done_work, virtio_fs_hiprio_done_work);
-		INIT_DELAYED_WORK(&fsvq->dispatch_work,
-				  virtio_fs_hiprio_dispatch_work);
+		INIT_WORK(&fsvq->dispatch_work,
+			  virtio_fs_hiprio_dispatch_work);
 	}
 }
 
@@ -1254,8 +1258,6 @@ __releases(fiq->lock)
 			spin_lock(&fsvq->lock);
 			list_add_tail(&req->list, &fsvq->queued_reqs);
 			inc_in_flight_req(fsvq);
-			schedule_delayed_work(&fsvq->dispatch_work,
-						msecs_to_jiffies(1));
 			spin_unlock(&fsvq->lock);
 			return;
 		}
@@ -1265,7 +1267,7 @@ __releases(fiq->lock)
 		/* Can't end request in submission context. Use a worker */
 		spin_lock(&fsvq->lock);
 		list_add_tail(&req->list, &fsvq->end_reqs);
-		schedule_delayed_work(&fsvq->dispatch_work, 0);
+		schedule_work(&fsvq->dispatch_work);
 		spin_unlock(&fsvq->lock);
 		return;
 	}
-- 
2.34.1

On Wed, May 31, 2023 at 07:10:32PM +0200, Peter-Jan Gootzen
wrote:
> When the Virtio queue is full, a work item is scheduled
> to execute in 1ms that retries adding the request to the queue.
> This is a large amount of time on the scale on which a
> virtio-fs device can operate. When using a DPU this is around
> 40us baseline without going to a remote server (4k, QD=1).
> This patch queues requests when the Virtio queue is full,
> and when a completed request is taken off, immediately fills
> it back up with queued requests.
> 
> This reduces the 99.9th percentile latencies in our tests by
> 60x and slightly increases the overall throughput, when using a
> queue depth 2x the size of the Virtio queue size, with a
> DPU-powered virtio-fs device.
> 
> Signed-off-by: Peter-Jan Gootzen <peter-jan at gootzen.net>
> ---
> V1 -> V2: Not scheduling dispatch work anymore when not needed
> and changed delayed_work structs to work_struct structs
> 
>  fs/fuse/virtio_fs.c | 32 +++++++++++++++++---------------
>  1 file changed, 17 insertions(+), 15 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 4d8d4f16c727..a676297db09b 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -45,7 +45,7 @@ struct virtio_fs_vq {
>  	struct work_struct done_work;
>  	struct list_head queued_reqs;
>  	struct list_head end_reqs;	/* End these requests */
> -	struct delayed_work dispatch_work;
> +	struct work_struct dispatch_work;
>  	struct fuse_dev *fud;
>  	bool connected;
>  	long in_flight;
> @@ -202,7 +202,7 @@ static void virtio_fs_drain_queue(struct virtio_fs_vq
*fsvq)
>  	}
>  
>  	flush_work(&fsvq->done_work);
> -	flush_delayed_work(&fsvq->dispatch_work);
> +	flush_work(&fsvq->dispatch_work);
>  }
>  
>  static void virtio_fs_drain_all_queues_locked(struct virtio_fs *fs)
> @@ -346,6 +346,9 @@ static void virtio_fs_hiprio_done_work(struct
work_struct *work)
>  			dec_in_flight_req(fsvq);
>  		}
>  	} while (!virtqueue_enable_cb(vq) &&
likely(!virtqueue_is_broken(vq)));
> +
> +	if (!list_empty(&fsvq->queued_reqs))
> +		schedule_work(&fsvq->dispatch_work);
>  	spin_unlock(&fsvq->lock);
>  }
>  
> @@ -353,7 +356,7 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
>  {
>  	struct fuse_req *req;
>  	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
> -						 dispatch_work.work);
> +						 dispatch_work);
>  	int ret;
>  
>  	pr_debug("virtio-fs: worker %s called.\n", __func__);
> @@ -388,8 +391,6 @@ static void virtio_fs_request_dispatch_work(struct
work_struct *work)
>  			if (ret == -ENOMEM || ret == -ENOSPC) {
>  				spin_lock(&fsvq->lock);
>  				list_add_tail(&req->list, &fsvq->queued_reqs);
> -				schedule_delayed_work(&fsvq->dispatch_work,
> -						      msecs_to_jiffies(1));
Virtqueue being full is only one of the reasons for failure to queue
the request. What if virtqueue is empty but we could not queue the
request because lack of memory (-ENOMEM). In that case we will queue
the request and it might not be dispatched because there is no completion.
(Assume there is no further new request coming). That means deadlock?

Thanks
Vivek
>  				spin_unlock(&fsvq->lock);
>  				return;
>  			}
> @@ -436,8 +437,6 @@ static int send_forget_request(struct virtio_fs_vq
*fsvq,
>  			pr_debug("virtio-fs: Could not queue FORGET: err=%d. Will try
later\n",
>  				 ret);
>  			list_add_tail(&forget->list, &fsvq->queued_reqs);
> -			schedule_delayed_work(&fsvq->dispatch_work,
> -					      msecs_to_jiffies(1));
>  			if (!in_flight)
>  				inc_in_flight_req(fsvq);
>  			/* Queue is full */
> @@ -469,7 +468,7 @@ static void virtio_fs_hiprio_dispatch_work(struct
work_struct *work)
>  {
>  	struct virtio_fs_forget *forget;
>  	struct virtio_fs_vq *fsvq = container_of(work, struct virtio_fs_vq,
> -						 dispatch_work.work);
> +						 dispatch_work);
>  	pr_debug("virtio-fs: worker %s called.\n", __func__);
>  	while (1) {
>  		spin_lock(&fsvq->lock);
> @@ -647,6 +646,11 @@ static void virtio_fs_requests_done_work(struct
work_struct *work)
>  			virtio_fs_request_complete(req, fsvq);
>  		}
>  	}
> +
> +	spin_lock(&fsvq->lock);
> +	if (!list_empty(&fsvq->queued_reqs))
> +		schedule_work(&fsvq->dispatch_work);
> +	spin_unlock(&fsvq->lock);
>  }
>  
>  /* Virtqueue interrupt handler */
> @@ -670,12 +674,12 @@ static void virtio_fs_init_vq(struct virtio_fs_vq
*fsvq, char *name,
>  
>  	if (vq_type == VQ_REQUEST) {
>  		INIT_WORK(&fsvq->done_work, virtio_fs_requests_done_work);
> -		INIT_DELAYED_WORK(&fsvq->dispatch_work,
> -				  virtio_fs_request_dispatch_work);
> +		INIT_WORK(&fsvq->dispatch_work,
> +			  virtio_fs_request_dispatch_work);
>  	} else {
>  		INIT_WORK(&fsvq->done_work, virtio_fs_hiprio_done_work);
> -		INIT_DELAYED_WORK(&fsvq->dispatch_work,
> -				  virtio_fs_hiprio_dispatch_work);
> +		INIT_WORK(&fsvq->dispatch_work,
> +			  virtio_fs_hiprio_dispatch_work);
>  	}
>  }
>  
> @@ -1254,8 +1258,6 @@ __releases(fiq->lock)
>  			spin_lock(&fsvq->lock);
>  			list_add_tail(&req->list, &fsvq->queued_reqs);
>  			inc_in_flight_req(fsvq);
> -			schedule_delayed_work(&fsvq->dispatch_work,
> -						msecs_to_jiffies(1));
>  			spin_unlock(&fsvq->lock);
>  			return;
>  		}
> @@ -1265,7 +1267,7 @@ __releases(fiq->lock)
>  		/* Can't end request in submission context. Use a worker */
>  		spin_lock(&fsvq->lock);
>  		list_add_tail(&req->list, &fsvq->end_reqs);
> -		schedule_delayed_work(&fsvq->dispatch_work, 0);
> +		schedule_work(&fsvq->dispatch_work);
>  		spin_unlock(&fsvq->lock);
>  		return;
>  	}
> -- 
> 2.34.1
>