7 Jun
2022
7 Jun
'22
6:18 p.m.
On Tue, Jun 07, 2022 at 12:12:03PM -0500, Jason Ekstrand wrote:
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 a special > 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 to imply > 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 and unbind > 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 deal with > wait > >> fences in userspace? > >> > > > >My opinion is rework this but make the ordering via an engine param > 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) and binds > >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 within the VM > >they can use a mode consuming less resources. > > > > I think we need to be careful here if we are looking for some out of > (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 bind failing > 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 is one > of two things: > 1. No implicit ordering of VM_BIND ops. They just happen in whatever > 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 least 2 > 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.
Thanks Jason. Ok, I am not sure which exec flag is that, but yah, execbuffer3 probably will not have this limiation. So, we need to define a VM_BIND_MAX_QUEUE and somehow export it to user (I am thinking of embedding it in I915_PARAM_HAS_VM_BIND. bits[0]->HAS_VM_BIND, bits[1-3]->'n' meaning 2^n queues.
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.
Ok, will start with n=3 meaning 8 queues. That would require us create 8 workqueues. We can change 'n' later if required.
Niranjana
> Why? Because Vulkan has two basic kind of bind operations and we don't > want any dependencies between them: > 1. Immediate. These happen right after BO creation or maybe as part of > vkBindImageMemory() or VkBindBufferMemory(). These don't happen on a > queue and we don't want them serialized with anything. To synchronize > with submit, we'll have a syncobj in the VkDevice which is signaled by > all immediate bind operations and make submits wait on it. > 2. Queued (sparse): These happen on a VkQueue which may be the same as > a render/compute queue or may be its own queue. It's up to us what we > 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 like we do > in execbuf(). > The important thing is that we don't want one type of operation to block > 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 going to 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're Thats 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't afford > 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 many paging > queues (what they call a VM_BIND engine/queue). That design works > pretty well and solves the problems in question. Again, we could just > make everything out-of-order and require using syncobjs to order things > as userspace wants. That'd be fine too. > One more note while I'm here: danvet said something on IRC about VM_BIND > queues waiting for syncobjs to materialize. We don't really want/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 to wait > for syncobjs to materialize. Also, getting that right is ridiculously > hard and I really don't want to get it wrong in kernel space. When we > do memory fences, wait-before-signal will be a thing. We don't need to > try and make it a thing for syncobj. > --Jason > > Thanks 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 expose a > 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 > >> > >>