On Wed, Jun 8, 2022 at 4:44 PM Niranjana Vishwanathapura < niranjana.vishwanathap...@intel.com> wrote:
> On Wed, Jun 08, 2022 at 08:33:25AM +0100, Tvrtko Ursulin wrote: > > > > > >On 07/06/2022 22:32, Niranjana Vishwanathapura wrote: > >>On Tue, Jun 07, 2022 at 11:18:11AM -0700, Niranjana Vishwanathapura > wrote: > >>>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.vishwanathap...@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.vishwanathap...@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. > >> > >>Ah, I think you are waking about I915_EXEC_RING_MASK (0x3f) which > execbuf3 > Yup! That's exactly the limit I was talking about. > >>will also have. So, we can simply define in vm_bind/unbind structures, > >> > >>#define I915_VM_BIND_MAX_QUEUE 64 > >> __u32 queue; > >> > >>I think that will keep things simple. > > > >Hmmm? What does execbuf2 limit has to do with how many engines > >hardware can have? I suggest not to do that. > > > >Change with added this: > > > > if (set.num_engines > I915_EXEC_RING_MASK + 1) > > return -EINVAL; > > > >To context creation needs to be undone and so let users create engine > >maps with all hardware engines, and let execbuf3 access them all. > > > > Earlier plan was to carry I915_EXEC_RING_MAP (0x3f) to execbuff3 also. > Hence, I was using the same limit for VM_BIND queues (64, or 65 if we > make it N+1). > But, as discussed in other thread of this RFC series, we are planning > to drop this I915_EXEC_RING_MAP in execbuff3. So, there won't be > any uapi that limits the number of engines (and hence the vm_bind queues > need to be supported). > > If we leave the number of vm_bind queues to be arbitrarily large > (__u32 queue_idx) then, we need to have a hashmap for queue (a wq, > work_item and a linked list) lookup from the user specified queue index. > Other option is to just put some hard limit (say 64 or 65) and use > an array of queues in VM (each created upon first use). I prefer this. > I don't get why a VM_BIND queue is any different from any other queue or userspace-visible kernel object. But I'll leave those details up to danvet or whoever else might be reviewing the implementation. --Jason > > Niranjana > > >Regards, > > > >Tvrtko > > > >> > >>Niranjana > >> > >>> > >>>> 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 > >>>> > >> > >>>> > >> >