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

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

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 4334 bytes --]

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@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.

Stefan

[-- Attachment #1.2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

[-- Attachment #2: Type: text/plain, Size: 183 bytes --]

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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

[Peter-Jan Gootzen via Virtualization]

On 01/06/2023 16:08, 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@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.
> 
> Stefan

Then I propose only handling -ENOSPC as a special case and letting all
other errors go through to userspace.

Noob Linux contributor question: how often should I send in a new revision of
the patch? Should I wait for more comments or send in a V3 with that fix now?

Best,
Peter-Jan
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 5098 bytes --]

On Thu, Jun 01, 2023 at 04:49:06PM +0200, Peter-Jan Gootzen wrote:
> On 01/06/2023 16:08, 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@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.
> > 
> > Stefan
> 
> Then I propose only handling -ENOSPC as a special case and letting all
> other errors go through to userspace.
> 
> Noob Linux contributor question: how often should I send in a new revision of
> the patch? Should I wait for more comments or send in a V3 with that fix now?

You can send v3. If it were a long patch series with multiple
maintainers then maybe allowing the others additional time for review
would avoid churn, but in this case I think it's fine to send the next
revision.

Stefan

[-- Attachment #1.2: signature.asc --]
[-- Type: application/pgp-signature, Size: 488 bytes --]

[-- Attachment #2: Type: text/plain, Size: 183 bytes --]

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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

[Vivek Goyal]

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@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

_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization

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

[Peter-Jan Gootzen via Virtualization]

On 01/06/2023 20:45, Vivek Goyal wrote:
> 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@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
> 

When following the code path, -ENOSPC is only returned when the queue is
full. So that assumption is correct.

As for race conditions where a request might be lost. If -ENOSPC is
returned and in the mean time the queue has become full again, the 
worker will simply fail and be re-executed when something is taken
off the queue again. It is possible that this will happen indefinitely,
but that's also possible in the current version of the driver.

Peter-Jan
_______________________________________________
Virtualization mailing list
Virtualization@lists.linux-foundation.org
https://lists.linuxfoundation.org/mailman/listinfo/virtualization