Re: [Intel-xe] [RFC PATCH] Documentation/gpu: Draft VM_BIND locking document

[Rodrigo Vivi]

On Fri, Jun 30, 2023 at 06:44:52PM +0200, Thomas Hellström wrote:
> Add the first version of the VM_BIND locking document which is
> intended to be part of the xe driver upstreaming agreement.
> 
> The document describes and discuss the locking used during exec-
> functions, evicton and for userptr gmvas. Intention is to be using the
> same nomenclature as the drm-vm-bind-async.rst, but to keep naming a
> little shorter, use gvm and gmva instead of gpu_vm and gpu_vma which
> is used in the previous document, with an intention to modify also
> that document.

I preferred the gpu_vm and gpu_vma as written in the async doc.
Much easier to read imho.

> 
> Signed-off-by: Thomas Hellström <thomas.hellstrom@xxxxxxxxxxxxxxx>
> ---
>  Documentation/gpu/drm-vm-bind-locking.rst | 339 ++++++++++++++++++++++
>  1 file changed, 339 insertions(+)
>  create mode 100644 Documentation/gpu/drm-vm-bind-locking.rst
> 
> diff --git a/Documentation/gpu/drm-vm-bind-locking.rst b/Documentation/gpu/drm-vm-bind-locking.rst
> new file mode 100644
> index 000000000000..f5d1a40a2906
> --- /dev/null
> +++ b/Documentation/gpu/drm-vm-bind-locking.rst
> @@ -0,0 +1,339 @@
> +===============
> +VM_BIND locking
> +===============
> +
> +This document attempts to describe what's needed to get VM_BIND locking right,
> +including the userptr mmu_notifier locking and it will also discuss some
> +optimizations to get rid of the looping through of all userptr mappings and
> +external / shared object mappings that is needed in the simplest
> +implementation. It will also discuss some implications for faulting gvms.
> +
> +Nomenclature
> +============
> +
> +* ``Context``: GPU execution context.
> +* ``gvm``: Abstraction of a GPU address space with meta-data. Typically
> +  one per client (DRM file-private), or one per context. 
> +* ``gvma``: Abstraction of a GPU address range within a gvma with

within a gpu_vm you meant?

> +  associated meta-data. The backing storage of a gvma can either be
> +  a gem buffer object or anonymous pages mapped also into the CPU
> +  address space for the process.
> +* ``userptr gvma or just userptr``: A gvma, the backing store of
> +  which is anonymous pages as described above.
> +* ``revalidating``: Revalidating a gvma means making the latest version
> +  of the backing store resident and making sure the gvma's
> +  page-table entries point to that backing store.
> +* ``dma_fence``: A struct dma_fence that is similar to a struct completion
> +  and which tracks GPU activity. When the GPU activity is finished,
> +  the dma_fence signals.
> +* ``dma_resv``: A struct dma_resv (AKA reservation object) that is used
> +  to track GPU activity in the form of multiple dma_fences on a
> +  gvm or a gem buffer object. The dma_resv contains an array / list
> +  of dma_fences and a lock that needs to be held when adding
> +  additional dma_fences to the dma_resv. The lock is of a type that
> +  allows deadlock-safe locking of multiple dma_resvs in arbitrary order.
> +* ``exec function``: An exec function is a function that revalidates all
> +  affected gvmas, submits a GPU command batch and registers the
> +  dma_fence representing the GPU command's activity with all affected
> +  dma_resvs. For completeness, although not covered by this document,
> +  it's worth mentioning that an exec function may also be the
> +  revalidation worker that is used by some drivers in compute /
> +  long-running mode.
> +* ``local object``: A GEM object which is local to a gvm. Shared gem
> +  objects also share the gvm's dma_resv.
> +* ``shared object``: AKA external object: A GEM object which may be shared
> +  by multiple gvms and whose backing storage may be shared with
> +  other drivers.
> +
> +
> +Introducing the locks
> +=====================
> +
> +One of the benefits of VM_BIND is that local GEM objects share the gvm's
> +dma_resv object and hence the dma_resv lock. So even with a huge
> +number of local GEM objects, only one lock is needed to make the exec
> +sequence atomic.
> +
> +The following locks and locking orders are used:
> +
> +* The ``gvm->lock`` (optionally an rwsem). Protects how the gvm is
> +  partitioned into gvmas, protects the gvm's list of external objects,
> +  and can also with some simplification protect the gvm's list of
> +  userptr gvmas. With the CPU mm analogy this would correspond to the
> +  mmap_lock.
> +* The ``userptr_seqlock``. This lock is taken in read mode for each
> +  userptr gvma on the gvm's userptr list, and in write mode during mmu
> +  notifier invalidation.

is this something that exists withing the mmu_notifier or a new lock
when handling the notifier?

> +* The ``gvm->resv`` lock. Protects the gvm's list of gvmas needing
> +  rebinding, and also the residency of all the gvm's local GEM object.
> +* The ``gvm->userptr_notifier_lock``. This is an rwsem that is taken in read
> +  mode during exec and write mode during a mmu notifier invalidation. In
> +  the absence of a separate page-table lock, this lock can serve
> +  together with the gvm's dma_resv lock as a page-table lock. More on
> +  this below. The userptr notifier lock is per gvm.

is the userptr_seqlock also per gpu_vm?
and what's the difference from this and the other?

> +* The ``gvm->page_table_lock``. Protects the gvm's page-table updates. For
> +  simplicity the gvm's dma_resv lock can be reused as page-table lock.
> +
> +There are certain optimizations described below that require
> +additional locks. More on that later.
> +
> +.. code-block:: C
> +
> +   dma_resv_lock(&gvm->resv);
> +
> +   for_each_gvma_on_revalidate_list(gvm, &gvma) {
> +		revalidate_gvma(&gvma);
> +		remove_from_revalidate_list(&gvma);
> +   }
> +
> +   add_dependencies(&gpu_job, &gvm->resv);
> +   job_dma_fence = gpu_submit(&gpu_job));
> +
> +   add_dma_fence(job_dma_fence, &gvm->resv);
> +   dma_resv_unlock(&gvm->resv);
> +
> +Eviction of one of these local objects will then be something like the
> +following:
> +
> +.. code-block:: C
> +
> +   obj = get_object_from_lru();
> +
> +   dma_resv_lock(obj->resv);
> +   for_each_gvma_of_obj(obj, &gvma);
> +		put_gvma_on_revalidate_list(&gvma);
> +
> +   add_dependencies(&eviction_job, &obj->resv);
> +   job_dma_fence = gpu_submit(&eviction_job);
> +   add_dma_fence(&obj->resv, job_dma_fence);
> +
> +   dma_resv_unlock(&obj->resv);
> +   put_object(obj);
> +
> +Note that since the object is local to the gvm, it will share the gvm's
> +``dma_resv`` lock so that ``obj->resv == gvm->resv``. Invalidated gvmas are put
> +on the gvm's revalidation list, which is protected by ``gvm->resv``, which
> +is always locked while evicting, due to the above equality.
> +
> +Does the gvma need to be unbound before eviction? For VM_BIND gvms
> +the answer is no. Since the eviction blit or copy will wait for GPU
> +idle, any attempt by the GPU to access freed memory through the
> +gvma will be preceded by a new exec function, which will
> +make sure the gvma is revalidated, that is not an issue.

The question opening the phrase made me think this was an open,
but it more like an answer for a common question? Should we rephrase
that to an affirmative note?

> +
> +Introducing external (or shared) buffer objects
> +===============================================
> +
> +Since shared buffer objects may be shared by multiple gvm's they
> +can't share their reservation object with a single gvm, but will rather
> +have a reservation object of their own. The shared objects bound to a
> +gvm using one or many
> +gvmas are therefore typically put on a per-gvm list which is
> +protected by the gvm lock. One could in theory protect it also with
> +the ``gvm->resv``, but since the list of dma_resvs to take is typically
> +built before the ``gvm->resv`` is locked due to a limitation in
> +the current locking helpers, that is typically not done. Also see
> +below for userptr gvmas.
> +
> +At eviction time we now need to invalidate *all* gvmas of a shared
> +object, but we can no longer be certain that we hold the gvm's
> +dma_resv of all the object's gvmas. We can only be certain that we
> +hold the object's private dma_resv. We can trylock the dma_resvs for
> +the affected gvm's but that might be unnecessarily complex. If we
> +have a ww_acquire context at hand at eviction time we can also perform
> +sleeping locks of those dma_resvs but that could cause expensive
> +rollbacks. One option is to just mark the invalidated gvmas with a bool
> +which is inspected on the next exec function, when the gvm's
> +dma_resv and the object's dma_resv is held, and the invalidated
> +gvmas could then be put on the gvm's list of invalidated
> +gvmas. That bool would then, although being per-gvma formally be
> +protected by the object's dma_resv.
> +
> +The exec function would then look something like the following:
> +
> +.. code-block:: C
> +
> +   read_lock(&gvm->lock);
> +		
> +   dma_resv_lock(&gvm->resv);
> +
> +   // Shared object list is protected by the gvm->lock.
> +   for_each_shared_obj(gvm, &obj) {
> +		dma_resv_lock(&obj->resv);
> +		move_marked_gvmas_to_revalidate_gvma_list(obj, &gvm);
> +   }
> +
> +   for_each_gvma_to_revalidate(gvm, &gvma) {
> +		revalidate_gvma(&gvma);
> +		remove_from_revalidate_list(&gvma);
> +   }
> +
> +   add_dependencies(&gpu_job, &gvm->resv);
> +   job_dma_fence = gpu_submit(&gpu_job));
> +
> +   add_dma_fence(job_dma_fence, &gvm->resv);
> +   for_each_shared_obj(gvm, &obj)
> +          add_dma_fence(job_dma_fence, &obj->resv);
> +   dma_resv_unlock_all_resv_locks();
> +
> +   read_unlock(&gvm->lock);
> +
> +And the corresponding shared-object aware eviction would look like:
> +
> +.. code-block:: C
> +
> +   obj = get_object_from_lru();
> +
> +   dma_resv_lock(obj->resv);
> +   for_each_gvma_of_obj(obj, &gvma);
> +		if (object_is_vm_local(obj))
> +		             put_gvma_on_revalidate_list(&gvma, &gvm);
> +		else
> +		             mark_gvma_for_revalidation(&gvma);
> +
> +   add_dependencies(&eviction_job, &obj->resv);
> +   job_dma_fence = gpu_submit(&eviction_job);
> +   add_dma_fence(&obj->resv, job_dma_fence);
> +
> +   dma_resv_unlock(&obj->resv);
> +   put_object(obj);
> +
> +Yet another option is to put the gvmas to be invalidated on a separate
> +gvm list protected by a lower level lock that can be taken both at eviction
> +time and at transfer-to-revalidate list time. The details are not in
> +this document, but this for reference implemented in the Intel xe
> +driver.

is this part of what we need to rethink for the suspend/resume evictions?

> +
> +Introducing userptr gvmas
> +=========================
> +
> +A userptr gvma is a gvma that, instead of mapping a buffer object to a
> +GPU virtual address range, directly maps a CPU mm range of anonymous-
> +or file page-cache pages.
> +A very simple approach would be to just pin the pages using
> +pin_user_pages() at bind time and unpin them at unbind time, but this
> +creates a Denial-Of-Service vector since a single user-space process
> +would be able to pin down all of system memory, which is not
> +desirable. (For special use-cases and with proper accounting pinning might
> +still be a desirable feature, though). What we need to do in the general case is
> +to obtain a reference to the desired pages, make sure we are notified
> +using a MMU notifier just before the CPU mm unmaps the pages, dirty
> +them if they are not mapped read-only to the GPU, and then drop the reference.
> +When we are notified by the MMU notifier that CPU mm is about to drop the
> +pages, we need to stop GPU access to the pages,
> +GPU page-table and make sure that before the next time the GPU tries to access
> +whatever is now present in the CPU mm range, we unmap the old pages
> +from the GPU page tables and repeat the process of obtaining new page
> +references. Note that when the core mm decides to laundry pages, we get such
> +an unmap MMU notification and can mark the pages dirty again before the
> +next GPU access. We also get similar MMU notifications for NUMA accounting
> +which the GPU driver doesn't really need to care about, but so far
> +it's proven difficult to exclude certain notifications.
> +
> +Using a MMU notifier for device DMA (and other methods) is described in
> +`this document 
> +<https://docs.kernel.org/core-api/pin_user_pages.html#case-3-mmu-notifier-registration-with-or-without-page-faulting-hardware>`_.
> +
> +Now the method of obtaining struct page references using
> +get_user_pages() unfortunately can't be used under a dma_resv lock
> +since that would violate the locking order of the dma_resv lock vs the
> +mmap_lock that is grabbed when resolving a CPU pagefault. This means the gvm's
> +list of userptr gvmas needs to be protected by an outer lock, and this
> +is the first time we strictly need the gvm->lock. While it was
> +previously used also to protect the list of the gvm's shared objects,
> +we could in theory have used the gvm->resv for that.
> +
> +The MMU interval seqlock for a userptr gvma is used in the following
> +way:
> +
> +.. code-block:: C
> +
> +   down_read(&gvm->lock);
> +
> +   retry:
> +
> +   // Note: mmu_interval_read_begin() blocks until there is no
> +   // invalidation notifier running anymore.
> +   seq = mmu_interval_read_begin(&gvma->userptr_interval);
> +   if (seq != gvma->saved_seq) {
> +           obtain_new_page_pointers(&gvma);
> +	   dma_resv_lock(&gvm->resv);
> +	   put_gvma_on_revalidate_list(&gvma, &gvm);
> +	   dma_resv_unlock(&gvm->resv);
> +	   gvma->saved_seq = seq;
> +   }
> +
> +   // The usual revalidation goes here.
> +
> +   // Final userptr sequence validation may not happen before the
> +   // submission dma_fence is added to the gvm's resv, from the POW
> +   // of the MMU invalidation notifier. Hence the
> +   // userptr_notifier_lock that will make them appear atomic.
> +   
> +   add_dependencies(&gpu_job, &gvm->resv);
> +   down_read(&gvm->userptr_notifier_lock);
> +   if (mmu_interval_read_retry(&gvma->userptr_interval, gvma->saved_seq)) {
> +          up_read(&gvm->userptr_notifier_lock);
> +	  goto retry;
> +   }
> +
> +   job_dma_fence = gpu_submit(&gpu_job));
> +
> +   add_dma_fence(job_dma_fence, &gvm->resv);
> +
> +   for_each_shared_obj(gvm, &obj)
> +          add_dma_fence(job_dma_fence, &obj->resv);
> +
> +   dma_resv_unlock_all_resv_locks();
> +   up_read(&gvm->userptr_notifier_lock);
> +   up_read(&gvm->lock);
> +
> +The code between ``mmu_interval_read_begin()`` and the
> +``mmu_interval_read_retry()`` marks the read side critical section of
> +what we call the ``userptr_seqlock``. In reality the gvm's userptr
> +gvma list is looped through, and the check is done for *all* of its
> +userptr gvmas, although we only show a single one here.
> +
> +The userptr gvma MMU invalidation notifier might be called from
> +reclaim context and, again to avoid locking order violations, we can't
> +take any dma_resv lock nor the gvm->lock from within it.
> +
> +.. code-block:: C
> +
> +  bool gvma_userptr_invalidate(userptr_interval, cur_seq)
> +  {
> +          // Make sure the exec function either sees the new sequence
> +	  // and backs off or we wait for the dma-fence:
> +	  
> +          down_write(&gvm->userptr_notifier_lock);
> +	  mmu_interval_set_seq(userptr_interval, cur_seq);
> +	  up_write(&gvm->userptr_notifier_lock);
> +
> +	  dma_resv_wait_timeout(&gvm->resv, DMA_RESV_USAGE_BOOKKEEP,
> +		                false, MAX_SCHEDULE_TIMEOUT);
> +	  return true;
> +  }
> +
> +When this invalidation notifier returns, the GPU can no longer be
> +accessing the old pages of the userptr gvma and needs to redo the page-binding
> +before a new GPU submission can succeed.
> +
> +Optimizing gvma iteration
> +-------------------------
> +
> +Iterating through all of a gvm's userptr gvmas to check the validity
> +on each exec function may be very costly. There is a scheme to avoid
> +this and only iterate through the userptr gvmas that actually saw an
> +invalidation notifier call since the last exec. T
> +
> +TODO: describe that scheme here. It's implemented in the xe driver.
> +
> +Locking for page-table updates at bind- and unbind time
> +=======================================================
> +
> +TODO.
> +
> +Recoverable page-fault implications
> +===================================
> +
> +TODO.
> -- 
> 2.40.1
>