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

Vivek Goyal
Thu, 01 Jun 2023 18:45:24 -0000

On Thu, Jun 01, 2023 at 10:08:50AM -0400, Stefan Hajnoczi
wrote:
> On Wed, May 31, 2023 at 04:49:39PM -0400, Vivek Goyal wrote:
> > On Wed, May 31, 2023 at 10:34:15PM +0200, Peter-Jan Gootzen wrote:
> > > On 31/05/2023 21:18, Vivek Goyal wrote:
> > > > 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
> > > >
> > >
> > > Good catch that will deadlock.
> > >
> > > Is default kernel behavior to indefinitely retry a file system
> > > request until memory is available?
> >
> > As of now that seems to be the behavior. I think I had copied this
> > code from another driver.
> >
> > But I guess one can argue that if memory is not available, then
> > return -ENOMEM to user space instead of retrying in kernel.
> >
> > Stefan, Miklos, WDYT?
>
> My understanding is that file system syscalls may return ENOMEM, so this
> is okay.

Ok. Fair enough. Thanks.

One more question. How do we know virtqueue is full. Is -ENOSPC is the
correct error code to check and retry indefinitely. Are there other
situations where -ENOSPC can be returned. Peter's current patch rely
on the fact that there is atleast one completion happening after
queuing of request which will kick the worker thread and submit the
request later.

We need to watch out for race conditions very closely. If that assumption
is not valid in some cases or there are races between getting -ENOSPC
and request completions, we will have a deadlock scenario.

Thanks
Vivek