Threaded submission & semaphore sharing

[Lionel Landwerlin]

Hey Christian,

The problem boils down to the fact that we don't immediately create dma 
fences when calling vkQueueSubmit().
This is delayed to a thread.

 From a single application thread, you can QueueSubmit() to 2 queues 
from 2 different devices.
Each QueueSubmit to one queue has a dependency on the previous 
QueueSubmit on the other queue through an exported/imported semaphore.

 From the API point of view the state of the semaphore should be changed 
after each QueueSubmit().
The problem is that it's not because of the thread and because you might 
have those 2 submission threads tied to different VkDevice/VkInstance or 
even different applications (synchronizing themselves outside the vulkan 
API).

Hope that makes sense.
It's not really easy to explain by mail, the best explanation is 
probably reading the test : 
https://gitlab.freedesktop.org/mesa/crucible/blob/master/src/tests/func/sync/semaphore-fd.c#L788

Like David mentioned you're not running into that issue right now, 
because you only dispatch to the thread under specific conditions.
But I could build a case to force that and likely run into the same issue.

-Lionel

On 02/08/2019 07:33, Koenig, Christian wrote:
> Hi Lionel,
>
> Well could you describe once more what the problem is?
>
> Cause I don't fully understand why a rather normal tandem submission 
> with two semaphores should fail in any way.
>
> Regards,
> Christian.
>
> Am 02.08.2019 06:28 schrieb Lionel Landwerlin 
> <lionel.g.landwerlin at intel.com>:
> There aren't CTS tests covering the issue I was mentioning.
> But we could add them.
>
> I don't have all the details regarding your implementation but even with
> the "semaphore thread", I could see it running into the same issues.
> What if a mix of binary & timeline semaphores are handed to 
> vkQueueSubmit()?
>
> For example with queueA & queueB from 2 different VkDevice :
>      vkQueueSubmit(queueA, signal semA);
>      vkQueueSubmit(queueA, wait on [semA, timelineSemB]); with
> timelineSemB triggering a wait before signal.
>      vkQueueSubmit(queueB, signal semA);
>
>
> -Lionel
>
> On 02/08/2019 06:18, Zhou, David(ChunMing) wrote:
> > Hi Lionel,
> >
> > By the Queue thread is a heavy thread, which is always resident in 
> driver during application running, our guys don't like that. So we 
> switch to Semaphore Thread, only when waitBeforeSignal of timeline 
> happens, we spawn a thread to handle that wait. So we don't have your 
> this issue.
> > By the way, I already pass all your CTS cases for now. I suggest you 
> to switch to Semaphore Thread instead of Queue Thread as well. It 
> works very well.
> >
> > -David
> >
> > -----Original Message-----
> > From: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > Sent: Friday, August 2, 2019 4:52 AM
> > To: dri-devel <dri-devel at lists.freedesktop.org>; Koenig, Christian 
> <Christian.Koenig at amd.com>; Zhou, David(ChunMing) 
> <David1.Zhou at amd.com>; Jason Ekstrand <jason at jlekstrand.net>
> > Subject: Threaded submission & semaphore sharing
> >
> > Hi Christian, David,
> >
> > Sorry to report this so late in the process, but I think we found an 
> issue not directly related to syncobj timelines themselves but with a 
> side effect of the threaded submissions.
> >
> > Essentially we're failing a test in crucible :
> > func.sync.semaphore-fd.opaque-fd
> > This test create a single binary semaphore, shares it between 2 
> VkDevice/VkQueue.
> > Then in a loop it proceeds to submit workload alternating between 
> the 2 VkQueue with one submit depending on the other.
> > It does so by waiting on the VkSemaphore signaled in the previous 
> iteration and resignaling it.
> >
> > The problem for us is that once things are dispatched to the 
> submission thread, the ordering of the submission is lost.
> > Because we have 2 devices and they both have their own submission 
> thread.
> >
> > Jason suggested that we reestablish the ordering by having 
> semaphores/syncobjs carry an additional uint64_t payload.
> > This 64bit integer would represent be an identifier that submission 
> threads will WAIT_FOR_AVAILABLE on.
> >
> > The scenario would look like this :
> >       - vkQueueSubmit(queueA, signal on semA);
> >           - in the caller thread, this would increment the syncobj 
> additional u64 payload and return it to userspace.
> >           - at some point the submission thread of queueA submits 
> the workload and signal the syncobj of semA with value returned in the 
> caller thread of vkQueueSubmit().
> >       - vkQueueSubmit(queueB, wait on semA);
> >           - in the caller thread, this would read the syncobj additional
> > u64 payload
> >           - at some point the submission thread of queueB will try 
> to submit the work, but first it will WAIT_FOR_AVAILABLE the u64 value 
> returned in the step above
> >
> > Because we want the binary semaphores to be shared across processes 
> and would like this to remain a single FD, the simplest location to 
> store this additional u64 payload would be the DRM syncobj.
> > It would need an additional ioctl to read & increment the value.
> >
> > What do you think?
> >
> > -Lionel
>
>

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