Re: [PATCH] virtio-fs: Improved request latencies when Virtio queue is full

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, May 22, 2023 at 03:19:15PM +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@gootzen.net>
> ---
>  fs/fuse/virtio_fs.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/fuse/virtio_fs.c b/fs/fuse/virtio_fs.c
> index 4d8d4f16c727..8af9d3dc61d3 100644
> --- a/fs/fuse/virtio_fs.c
> +++ b/fs/fuse/virtio_fs.c
> @@ -347,6 +347,8 @@ static void virtio_fs_hiprio_done_work(struct work_struct *work)
>  		}
>  	} while (!virtqueue_enable_cb(vq) && likely(!virtqueue_is_broken(vq)));
>  	spin_unlock(&fsvq->lock);
> +
> +	schedule_delayed_work(&fsvq->dispatch_work, 0);

This avoids scheduling work when there is nothing queued and uses
schedule_work() since there is no timeout value:

  if (!list_empty(&fsvq->queued_reqs)) {
      schedule_work(&fsvq->dispatch_work);
  }
  spin_unlock(&fsvq->lock);

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