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

Tue May 24 10:08:07 UTC 2022

On 20/05/2022 01:52, Zanoni, Paulo R wrote:
> On Tue, 2022-05-17 at 11:32 -0700, Niranjana Vishwanathapura wrote:
>> VM_BIND design document with description of intended use cases.
>>
>> v2: Add more documentation and format as per review comments
>>      from Daniel.
>>
>> Signed-off-by: Niranjana Vishwanathapura <niranjana.vishwanathapura at intel.com>
>> ---
>>
>> diff --git a/Documentation/gpu/rfc/i915_vm_bind.rst b/Documentation/gpu/rfc/i915_vm_bind.rst
>> new file mode 100644
>> index 000000000000..f1be560d313c
>> --- /dev/null
>> +++ b/Documentation/gpu/rfc/i915_vm_bind.rst
>> @@ -0,0 +1,304 @@
>> +==========================================
>> +I915 VM_BIND feature design and use cases
>> +==========================================
>> +
>> +VM_BIND feature
>> +================
>> +DRM_I915_GEM_VM_BIND/UNBIND ioctls allows UMD to bind/unbind GEM buffer
>> +objects (BOs) or sections of a BOs at specified GPU virtual addresses on a
>> +specified address space (VM). These mappings (also referred to as persistent
>> +mappings) will be persistent across multiple GPU submissions (execbuff calls)
>> +issued by the UMD, without user having to provide a list of all required
>> +mappings during each submission (as required by older execbuff mode).
>> +
>> +VM_BIND/UNBIND ioctls will support 'in' and 'out' fences to allow userpace
>> +to specify how the binding/unbinding should sync with other operations
>> +like the GPU job submission. These fences will be timeline 'drm_syncobj's
>> +for non-Compute contexts (See struct drm_i915_vm_bind_ext_timeline_fences).
>> +For Compute contexts, they will be user/memory fences (See struct
>> +drm_i915_vm_bind_ext_user_fence).
>> +
>> +VM_BIND feature is advertised to user via I915_PARAM_HAS_VM_BIND.
>> +User has to opt-in for VM_BIND mode of binding for an address space (VM)
>> +during VM creation time via I915_VM_CREATE_FLAGS_USE_VM_BIND extension.
>> +
>> +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.
>> +
>> +VM_BIND features include:
>> +
>> +* Multiple Virtual Address (VA) mappings can map to the same physical pages
>> +  of an object (aliasing).
>> +* VA mapping can map to a partial section of the BO (partial binding).
>> +* Support capture of persistent mappings in the dump upon GPU error.
>> +* TLB is flushed upon unbind completion. Batching of TLB flushes in some
>> +  use cases will be helpful.
>> +* Asynchronous vm_bind and vm_unbind support with 'in' and 'out' fences.
>> +* Support for userptr gem objects (no special uapi is required for this).
>> +
>> +Execbuff ioctl in VM_BIND mode
>> +-------------------------------
>> +The execbuff ioctl handling in VM_BIND mode differs significantly from the
>> +older method. A VM in VM_BIND mode will not support older execbuff mode of
>> +binding. In VM_BIND mode, execbuff ioctl will not accept any execlist. Hence,
>> +no support for implicit sync. It is expected that the below work will be able
>> +to support requirements of object dependency setting in all use cases:
>> +
>> +"dma-buf: Add an API for exporting sync files"
>> +(https://lwn.net/Articles/859290/)
> I would really like to have more details here. The link provided points
> to new ioctls and we're not very familiar with those yet, so I think
> you should really clarify the interaction between the new additions
> here. Having some sample code would be really nice too.
>
> For Mesa at least (and I believe for the other drivers too) we always
> have a few exported buffers in every execbuf call, and we rely on the
> implicit synchronization provided by execbuf to make sure everything
> works. The execbuf ioctl also has some code to flush caches during
> implicit synchronization AFAIR, so I would guess we rely on it too and
> whatever else the Kernel does. Is that covered by the new ioctls?
>
> In addition, as far as I remember, one of the big improvements of
> vm_bind was that it would help reduce ioctl latency and cpu overhead.
> But if making execbuf faster comes at the cost of requiring additional
> ioctls calls for implicit synchronization, which is required on ever
> execbuf call, then I wonder if we'll even get any faster at all.
> Comparing old execbuf vs plain new execbuf without the new required
> ioctls won't make sense.
> But maybe I'm wrong and we won't need to call these new ioctls around
> every single execbuf ioctl we submit? Again, more clarification and
> some code examples here would be really nice. This is a big change on
> an important part of the API, we should clarify the new expected usage.

Hey Paulo,

I think in the case of X11/Wayland, we'll be doing 1 or 2 extra ioctls 
per frame which seems pretty reasonable.

Essentially we need to set the dependencies on the buffer we´re going to 
tell the display engine (gnome-shell/kde/bare-display-hw) to use.

In the Vulkan case, we're trading building execbuffer lists of 
potentially thousands of buffers for every single submission versus 1 or 
2 ioctls for a single item when doing vkQueuePresent() (which happens 
less often than we do execbuffer ioctls).

That seems like a good trade off and doesn't look like a lot more work 
than explicit fencing where we would have to send associated fences.

Here is the Mesa MR associated with this : 
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4037

-Lionel

>
>> +
>> +This also means, we need an execbuff extension to pass in the batch
>> +buffer addresses (See struct drm_i915_gem_execbuffer_ext_batch_addresses).
>> +
>> +If at all execlist support in execbuff ioctl is deemed necessary for
>> +implicit sync in certain use cases, then support can be added later.
> IMHO we really need to sort this and check all the assumptions before
> we commit to any interface. Again, implicit synchronization is
> something we rely on during *every* execbuf ioctl for most workloads.
>
>
>> +In VM_BIND mode, VA allocation is completely managed by the user instead of
>> +the i915 driver. Hence all VA assignment, eviction are not applicable in
>> +VM_BIND mode. Also, for determining object activeness, VM_BIND mode will not
>> +be using the i915_vma active reference tracking. It will instead use dma-resv
>> +object for that (See `VM_BIND dma_resv usage`_).
>> +
>> +So, a lot of existing code in the execbuff path like relocations, VA evictions,
>> +vma lookup table, implicit sync, vma active reference tracking etc., are not
>> +applicable in VM_BIND mode. Hence, the execbuff path needs to be cleaned up
>> +by clearly separating out the functionalities where the VM_BIND mode differs
>> +from older method and they should be moved to separate files.
> I seem to recall some conversations where we were told a bunch of
> ioctls would stop working or make no sense to call when using vm_bind.
> Can we please get a complete list of those? Bonus points if the Kernel
> starts telling us we just called something that makes no sense.
>
>> +
>> +VM_PRIVATE objects
>> +-------------------
>> +By default, BOs can be mapped on multiple VMs and can also be dma-buf
>> +exported. Hence these BOs are referred to as Shared BOs.
>> +During each execbuff submission, the request fence must be added to the
>> +dma-resv fence list of all shared BOs mapped on the VM.
>> +
>> +VM_BIND feature introduces an optimization where user can create BO which
>> +is private to a specified VM via I915_GEM_CREATE_EXT_VM_PRIVATE flag during
>> +BO creation. Unlike Shared BOs, these VM private BOs can only be mapped on
>> +the VM they are private to and can't be dma-buf exported.
>> +All private BOs of a VM share the dma-resv object. Hence during each execbuff
>> +submission, they need only one dma-resv fence list updated. Thus, the fast
>> +path (where required mappings are already bound) submission latency is O(1)
>> +w.r.t the number of VM private BOs.
> I know we already discussed this, but just to document it publicly: the
> ideal case for user space would be that every BO is created as private
> but then we'd have an ioctl to convert it to non-private (without the
> need to have a non-private->private interface).
>
> An explanation on why we can't have an ioctl to mark as exported a
> buffer that was previously vm_private would be really appreciated.
>
> Thanks,
> Paulo
>
>
>> +
>> +VM_BIND locking hirarchy
>> +-------------------------
>> +The locking design here supports the older (execlist based) execbuff mode, the
>> +newer VM_BIND mode, the VM_BIND mode with GPU page faults and possible future
>> +system allocator support (See `Shared Virtual Memory (SVM) support`_).
>> +The older execbuff mode and the newer VM_BIND mode without page faults manages
>> +residency of backing storage using dma_fence. The VM_BIND mode with page faults
>> +and the system allocator support do not use any dma_fence at all.
>> +
>> +VM_BIND locking order is as below.
>> +
>> +1) Lock-A: A vm_bind mutex will protect vm_bind lists. This lock is taken in
>> +   vm_bind/vm_unbind ioctl calls, in the execbuff path and while releasing the
>> +   mapping.
>> +
>> +   In future, when GPU page faults are supported, we can potentially use a
>> +   rwsem instead, so that multiple page fault handlers can take the read side
>> +   lock to lookup the mapping and hence can run in parallel.
>> +   The older execbuff mode of binding do not need this lock.
>> +
>> +2) Lock-B: The object's dma-resv lock will protect i915_vma state and needs to
>> +   be held while binding/unbinding a vma in the async worker and while updating
>> +   dma-resv fence list of an object. Note that private BOs of a VM will all
>> +   share a dma-resv object.
>> +
>> +   The future system allocator support will use the HMM prescribed locking
>> +   instead.
>> +
>> +3) Lock-C: Spinlock/s to protect some of the VM's lists like the list of
>> +   invalidated vmas (due to eviction and userptr invalidation) etc.
>> +
>> +When GPU page faults are supported, the execbuff path do not take any of these
>> +locks. There we will simply smash the new batch buffer address into the ring and
>> +then tell the scheduler run that. The lock taking only happens from the page
>> +fault handler, where we take lock-A in read mode, whichever lock-B we need to
>> +find the backing storage (dma_resv lock for gem objects, and hmm/core mm for
>> +system allocator) and some additional locks (lock-D) for taking care of page
>> +table races. Page fault mode should not need to ever manipulate the vm lists,
>> +so won't ever need lock-C.
>> +
>> +VM_BIND LRU handling
>> +---------------------
>> +We need to ensure VM_BIND mapped objects are properly LRU tagged to avoid
>> +performance degradation. We will also need support for bulk LRU movement of
>> +VM_BIND objects to avoid additional latencies in execbuff path.
>> +
>> +The page table pages are similar to VM_BIND mapped objects (See
>> +`Evictable page table allocations`_) and are maintained per VM and needs to
>> +be pinned in memory when VM is made active (ie., upon an execbuff call with
>> +that VM). So, bulk LRU movement of page table pages is also needed.
>> +
>> +The i915 shrinker LRU has stopped being an LRU. So, it should also be moved
>> +over to the ttm LRU in some fashion to make sure we once again have a reasonable
>> +and consistent memory aging and reclaim architecture.
>> +
>> +VM_BIND dma_resv usage
>> +-----------------------
>> +Fences needs to be added to all VM_BIND mapped objects. During each execbuff
>> +submission, they are added with DMA_RESV_USAGE_BOOKKEEP usage to prevent
>> +over sync (See enum dma_resv_usage). One can override it with either
>> +DMA_RESV_USAGE_READ or DMA_RESV_USAGE_WRITE usage during object dependency
>> +setting (either through explicit or implicit mechanism).
>> +
>> +When vm_bind is called for a non-private object while the VM is already
>> +active, the fences need to be copied from VM's shared dma-resv object
>> +(common to all private objects of the VM) to this non-private object.
>> +If this results in performance degradation, then some optimization will
>> +be needed here. This is not a problem for VM's private objects as they use
>> +shared dma-resv object which is always updated on each execbuff submission.
>> +
>> +Also, in VM_BIND mode, use dma-resv apis for determining object activeness
>> +(See dma_resv_test_signaled() and dma_resv_wait_timeout()) and do not use the
>> +older i915_vma active reference tracking which is deprecated. This should be
>> +easier to get it working with the current TTM backend. We can remove the
>> +i915_vma active reference tracking fully while supporting TTM backend for igfx.
>> +
>> +Evictable page table allocations
>> +---------------------------------
>> +Make pagetable allocations evictable and manage them similar to VM_BIND
>> +mapped objects. Page table pages are similar to persistent mappings of a
>> +VM (difference here are that the page table pages will not have an i915_vma
>> +structure and after swapping pages back in, parent page link needs to be
>> +updated).
>> +
>> +Mesa use case
>> +--------------
>> +VM_BIND can potentially reduce the CPU overhead in Mesa (both Vulkan and Iris),
>> +hence improving performance of CPU-bound applications. It also allows us to
>> +implement Vulkan's Sparse Resources. With increasing GPU hardware performance,
>> +reducing CPU overhead becomes more impactful.
>> +
>> +
>> +VM_BIND Compute support
>> +========================
>> +
>> +User/Memory Fence
>> +------------------
>> +The idea is to take a user specified virtual address and install an interrupt
>> +handler to wake up the current task when the memory location passes the user
>> +supplied filter. User/Memory fence is a <address, value> pair. To signal the
>> +user fence, specified value will be written at the specified virtual address
>> +and wakeup the waiting process. User can wait on a user fence with the
>> +gem_wait_user_fence ioctl.
>> +
>> +It also allows the user to emit their own MI_FLUSH/PIPE_CONTROL notify
>> +interrupt within their batches after updating the value to have sub-batch
>> +precision on the wakeup. Each batch can signal a user fence to indicate
>> +the completion of next level batch. The completion of very first level batch
>> +needs to be signaled by the command streamer. The user must provide the
>> +user/memory fence for this via the DRM_I915_GEM_EXECBUFFER_EXT_USER_FENCE
>> +extension of execbuff ioctl, so that KMD can setup the command streamer to
>> +signal it.
>> +
>> +User/Memory fence can also be supplied to the kernel driver to signal/wake up
>> +the user process after completion of an asynchronous operation.
>> +
>> +When VM_BIND ioctl was provided with a user/memory fence via the
>> +I915_VM_BIND_EXT_USER_FENCE extension, it will be signaled upon the completion
>> +of binding of that mapping. All async binds/unbinds are serialized, hence
>> +signaling of user/memory fence also indicate the completion of all previous
>> +binds/unbinds.
>> +
>> +This feature will be derived from the below original work:
>> +https://patchwork.freedesktop.org/patch/349417/
>> +
>> +Long running Compute contexts
>> +------------------------------
>> +Usage of dma-fence expects that they complete in reasonable amount of time.
>> +Compute on the other hand can be long running. Hence it is appropriate for
>> +compute to use user/memory fence and dma-fence usage will be limited to
>> +in-kernel consumption only. This requires an execbuff uapi extension to pass
>> +in user fence (See struct drm_i915_vm_bind_ext_user_fence). Compute must opt-in
>> +for this mechanism with I915_CONTEXT_CREATE_FLAGS_LONG_RUNNING flag during
>> +context creation. The dma-fence based user interfaces like gem_wait ioctl and
>> +execbuff out fence are not allowed on long running contexts. Implicit sync is
>> +not valid as well and is anyway not supported in VM_BIND mode.
>> +
>> +Where GPU page faults are not available, kernel driver upon buffer invalidation
>> +will initiate a suspend (preemption) of long running context with a dma-fence
>> +attached to it. And upon completion of that suspend fence, finish the
>> +invalidation, revalidate the BO and then resume the compute context. This is
>> +done by having a per-context preempt fence (also called suspend fence) proxying
>> +as i915_request fence. This suspend fence is enabled when someone tries to wait
>> +on it, which then triggers the context preemption.
>> +
>> +As this support for context suspension using a preempt fence and the resume work
>> +for the compute mode contexts can get tricky to get it right, it is better to
>> +add this support in drm scheduler so that multiple drivers can make use of it.
>> +That means, it will have a dependency on i915 drm scheduler conversion with GuC
>> +scheduler backend. This should be fine, as the plan is to support compute mode
>> +contexts only with GuC scheduler backend (at least initially). This is much
>> +easier to support with VM_BIND mode compared to the current heavier execbuff
>> +path resource attachment.
>> +
>> +Low Latency Submission
>> +-----------------------
>> +Allows compute UMD to directly submit GPU jobs instead of through execbuff
>> +ioctl. This is made possible by VM_BIND is not being synchronized against
>> +execbuff. VM_BIND allows bind/unbind of mappings required for the directly
>> +submitted jobs.
>> +
>> +Other VM_BIND use cases
>> +========================
>> +
>> +Debugger
>> +---------
>> +With debug event interface user space process (debugger) is able to keep track
>> +of and act upon resources created by another process (debugged) and attached
>> +to GPU via vm_bind interface.
>> +
>> +GPU page faults
>> +----------------
>> +GPU page faults when supported (in future), will only be supported in the
>> +VM_BIND mode. While both the older execbuff mode and the newer VM_BIND mode of
>> +binding will require using dma-fence to ensure residency, the GPU page faults
>> +mode when supported, will not use any dma-fence as residency is purely managed
>> +by installing and removing/invalidating page table entries.
>> +
>> +Page level hints settings
>> +--------------------------
>> +VM_BIND allows any hints setting per mapping instead of per BO.
>> +Possible hints include read-only mapping, placement and atomicity.
>> +Sub-BO level placement hint will be even more relevant with
>> +upcoming GPU on-demand page fault support.
>> +
>> +Page level Cache/CLOS settings
>> +-------------------------------
>> +VM_BIND allows cache/CLOS settings per mapping instead of per BO.
>> +
>> +Shared Virtual Memory (SVM) support
>> +------------------------------------
>> +VM_BIND interface can be used to map system memory directly (without gem BO
>> +abstraction) using the HMM interface. SVM is only supported with GPU page
>> +faults enabled.
>> +
>> +
>> +Broder i915 cleanups
>> +=====================
>> +Supporting this whole new vm_bind mode of binding which comes with its own
>> +use cases to support and the locking requirements requires proper integration
>> +with the existing i915 driver. This calls for some broader i915 driver
>> +cleanups/simplifications for maintainability of the driver going forward.
>> +Here are few things identified and are being looked into.
>> +
>> +- Remove vma lookup cache (eb->gem_context->handles_vma). VM_BIND feature
>> +  do not use it and complexity it brings in is probably more than the
>> +  performance advantage we get in legacy execbuff case.
>> +- Remove vma->open_count counting
>> +- Remove i915_vma active reference tracking. VM_BIND feature will not be using
>> +  it. Instead use underlying BO's dma-resv fence list to determine if a i915_vma
>> +  is active or not.
>> +
>> +
>> +VM_BIND UAPI
>> +=============
>> +
>> +.. kernel-doc:: Documentation/gpu/rfc/i915_vm_bind.h
>> diff --git a/Documentation/gpu/rfc/index.rst b/Documentation/gpu/rfc/index.rst
>> index 91e93a705230..7d10c36b268d 100644
>> --- a/Documentation/gpu/rfc/index.rst
>> +++ b/Documentation/gpu/rfc/index.rst
>> @@ -23,3 +23,7 @@ host such documentation:
>>   .. toctree::
>>   
>>
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>>
>>
>>
>>       i915_scheduler.rst
>> +
>> +.. toctree::
>> +
>> +    i915_vm_bind.rst