On Fri, Jun 3, 2022 at 6:52 PM Niranjana Vishwanathapura < niranjana.vishwanathapura@intel.com> wrote:
On Fri, Jun 03, 2022 at 10:20:25AM +0300, Lionel Landwerlin wrote:
On 02/06/2022 23:35, Jason Ekstrand wrote:
On Thu, Jun 2, 2022 at 3:11 PM Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com> wrote: On Wed, Jun 01, 2022 at 01:28:36PM -0700, Matthew Brost wrote: >On Wed, Jun 01, 2022 at 05:25:49PM +0300, Lionel Landwerlin wrote: >> On 17/05/2022 21:32, Niranjana Vishwanathapura wrote: >> > +VM_BIND/UNBIND ioctl will immediately start binding/unbinding the mapping in an >> > +async worker. The binding and unbinding will work like aspecial
GPU engine. >> > +The binding and unbinding operations are serialized and will wait on specified >> > +input fences before the operation and will signal the output fences upon the >> > +completion of the operation. Due to serialization,completion of
an operation >> > +will also indicate that all previous operations are also complete. >> >> I guess we should avoid saying "will immediately start binding/unbinding" if >> there are fences involved. >> >> And the fact that it's happening in an async worker seem toimply
it's not >> immediate. >> Ok, will fix. This was added because in earlier design binding was deferred until next execbuff. But now it is non-deferred (immediate in that sense). But yah,this is
confusing and will fix it. >> >> I have a question on the behavior of the bind operation when no input fence >> is provided. Let say I do : >> >> VM_BIND (out_fence=fence1) >> >> VM_BIND (out_fence=fence2) >> >> VM_BIND (out_fence=fence3) >> >> >> In what order are the fences going to be signaled? >> >> In the order of VM_BIND ioctls? Or out of order? >> >> Because you wrote "serialized I assume it's : in order >> Yes, in the order of VM_BIND/UNBIND ioctls. Note that bind andunbind
will use the same queue and hence are ordered. >> >> One thing I didn't realize is that because we only get one "VM_BIND" engine, >> there is a disconnect from the Vulkan specification. >> >> In Vulkan VM_BIND operations are serialized but per engine. >> >> So you could have something like this : >> >> VM_BIND (engine=rcs0, in_fence=fence1, out_fence=fence2) >> >> VM_BIND (engine=ccs0, in_fence=fence3, out_fence=fence4) >> >> >> fence1 is not signaled >> >> fence3 is signaled >> >> So the second VM_BIND will proceed before the first VM_BIND. >> >> >> I guess we can deal with that scenario in userspace by doing the wait >> ourselves in one thread per engines. >> >> But then it makes the VM_BIND input fences useless. >> >> >> Daniel : what do you think? Should be rework this or just dealwith
wait >> fences in userspace? >> > >My opinion is rework this but make the ordering via an engineparam
optional. > >e.g. A VM can be configured so all binds are ordered within the VM > >e.g. A VM can be configured so all binds accept an engine argument (in >the case of the i915 likely this is a gem context handle) andbinds
>ordered with respect to that engine. > >This gives UMDs options as the later likely consumes more KMD resources >so if a different UMD can live with binds being ordered withinthe VM
>they can use a mode consuming less resources. > I think we need to be careful here if we are looking for some outof
(submission) order completion of vm_bind/unbind. In-order completion means, in a batch of binds and unbinds to be completed in-order, user only needs to specify in-fence for the first bind/unbind call and the our-fence for the last bind/unbind call. Also, the VA released by an unbind call can be re-used by any subsequent bind call in that in-order batch. These things will break if binding/unbinding were to be allowed to go out of order (of submission) and user need to be extra careful not to run into pre-mature triggereing of out-fence and bindfailing
as VA is still in use etc. Also, VM_BIND binds the provided mapping on the specified address space (VM). So, the uapi is not engine/context specific. We can however add a 'queue' to the uapi which can be one from the pre-defined queues, I915_VM_BIND_QUEUE_0 I915_VM_BIND_QUEUE_1 ... I915_VM_BIND_QUEUE_(N-1) KMD will spawn an async work queue for each queue which will only bind the mappings on that queue in the order of submission. User can assign the queue to per engine or anything like that. But again here, user need to be careful and not deadlock these queues with circular dependency of fences. I prefer adding this later an as extension based on whether it is really helping with the implementation. I can tell you right now that having everything on a single in-order queue will not get us the perf we want. What vulkan really wants isone
of two things: 1. No implicit ordering of VM_BIND ops. They just happen inwhatever
their dependencies are resolved and we ensure ordering ourselves by having a syncobj in the VkQueue. 2. The ability to create multiple VM_BIND queues. We need at least2
but I don't see why there needs to be a limit besides the limits the i915 API already has on the number of engines. Vulkan could expose multiple sparse binding queues to the client if it's not arbitrarily limited.Thanks Jason, Lionel.
Jason, what are you referring to when you say "limits the i915 API already has on the number of engines"? I am not sure if there is such an uapi today.
There's a limit of something like 64 total engines today based on the number of bits we can cram into the exec flags in execbuffer2. I think someone had an extended version that allowed more but I ripped it out because no one was using it. Of course, execbuffer3 might not have that problem at all.
I am trying to see how many queues we need and don't want it to be
arbitrarily large and unduely blow up memory usage and complexity in i915 driver.
I expect a Vulkan driver to use at most 2 in the vast majority of cases. I could imagine a client wanting to create more than 1 sparse queue in which case, it'll be N+1 but that's unlikely. As far as complexity goes, once you allow two, I don't think the complexity is going up by allowing N. As for memory usage, creating more queues means more memory. That's a trade-off that userspace can make. Again, the expected number here is 1 or 2 in the vast majority of cases so I don't think you need to worry.
Why? Because Vulkan has two basic kind of bind operations and wedon't
want any dependencies between them: 1. Immediate. These happen right after BO creation or maybe aspart of
vkBindImageMemory() or VkBindBufferMemory(). These don't happen on a queue and we don't want them serialized with anything. Tosynchronize
with submit, we'll have a syncobj in the VkDevice which is signaledby
all immediate bind operations and make submits wait on it. 2. Queued (sparse): These happen on a VkQueue which may be the sameas
a render/compute queue or may be its own queue. It's up to us whatwe
want to advertise. From the Vulkan API PoV, this is like any other queue. Operations on it wait on and signal semaphores. If we have a VM_BIND engine, we'd provide syncobjs to wait and signal just likewe do
in execbuf(). The important thing is that we don't want one type of operation toblock
on the other. If immediate binds are blocking on sparse binds, it's going to cause over-synchronization issues. In terms of the internal implementation, I know that there's goingto be
a lock on the VM and that we can't actually do these things in parallel. That's fine. Once the dma_fences have signaled and we'reThats correct. It is like a single VM_BIND engine with multiple queues feeding to it.
Right. As long as the queues themselves are independent and can block on dma_fences without holding up other queues, I think we're fine.
unblocked to do the bind operation, I don't care if there's a bit of synchronization due to locking. That's expected. What we can'tafford
to have is an immediate bind operation suddenly blocking on a sparse operation which is blocked on a compute job that's going to run for another 5ms.As the VM_BIND queue is per VM, VM_BIND on one VM doesn't block the VM_BIND on other VMs. I am not sure about usecases here, but just wanted to clarify.
Yes, that's what I would expect.
--Jason
Niranjana
For reference, Windows solves this by allowing arbitrarily manypaging
queues (what they call a VM_BIND engine/queue). That design works pretty well and solves the problems in question. Again, we couldjust
make everything out-of-order and require using syncobjs to orderthings
as userspace wants. That'd be fine too. One more note while I'm here: danvet said something on IRC aboutVM_BIND
queues waiting for syncobjs to materialize. We don't reallywant/need
this. We already have all the machinery in userspace to handle wait-before-signal and waiting for syncobj fences to materialize and that machinery is on by default. It would actually take MORE work in Mesa to turn it off and take advantage of the kernel being able towait
for syncobjs to materialize. Also, getting that right isridiculously
hard and I really don't want to get it wrong in kernel space. Whenwe
do memory fences, wait-before-signal will be a thing. We don't needto
try and make it a thing for syncobj. --JasonThanks Jason,
I missed the bit in the Vulkan spec that we're allowed to have a sparse queue that does not implement either graphics or compute operations :
"While some implementations may include VK_QUEUE_SPARSE_BINDING_BIT support in queue families that also include graphics and compute support, other implementations may only exposea
VK_QUEUE_SPARSE_BINDING_BIT-only queue family."So it can all be all a vm_bind engine that just does bind/unbind operations.
But yes we need another engine for the immediate/non-sparse operations.
-Lionel
Daniel, any thoughts? Niranjana >Matt > >> >> Sorry I noticed this late. >> >> >> -Lionel >> >>