Peter-Jan Gootzen
2023-May-31 20:34 UTC
[PATCH V2] virtio-fs: Improved request latencies when Virtio queue is full
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?
Vivek Goyal
2023-May-31 20:49 UTC
[PATCH V2] virtio-fs: Improved request latencies when Virtio queue is full
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? Thanks Vivek