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@xxxxxxxxxxx>
> >> ---
> >> 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?
Thanks
Vivek
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