Re: [Intel-gfx] [RFC v3 1/3] drm/doc/rfc: VM_BIND feature design document

[Niranjana Vishwanathapura <niranjana.vishwanathapura@intel.com>]

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.

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.

>     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.

>     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.

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
>       >>
>       >>