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([10.249.254.228]) by orsmga005-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 22 Oct 2023 11:02:53 -0700 From: =?utf-8?q?Thomas_Hellstr=C3=B6m?= To: intel-xe@lists.freedesktop.org Subject: [PATCH v3] Documentation/gpu: VM_BIND locking document Date: Sun, 22 Oct 2023 20:02:36 +0200 Message-ID: <20231022180236.5170-1-thomas.hellstrom@linux.intel.com> X-Mailer: git-send-email 2.41.0 MIME-Version: 1.0 X-BeenThere: dri-devel@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Direct Rendering Infrastructure - Development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Matthew Brost , =?utf-8?q?Thomas_Hellstr=C3=B6m?= , Francois Dugast , linux-kernel@vger.kernel.org, Oak Zeng , Boris Brezillon , Danilo Krummrich , dri-devel@lists.freedesktop.org, Rodrigo Vivi Errors-To: dri-devel-bounces@lists.freedesktop.org Sender: "dri-devel" 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 gpu-vmas. Intention is to be using the same nomenclature as the drm-vm-bind-async.rst. v2: - s/gvm/gpu_vm/g (Rodrigo Vivi) - Clarify the userptr seqlock with a pointer to mm/mmu_notifier.c (Rodrigo Vivi) - Adjust commit message accordingly. - Add SPDX license header. v3: - Large update to align with the drm_gpuvm manager locking - Add "Efficient userptr gpu_vma exec function iteration" section - Add "Locking at bind- and unbind time" section. Cc: Rodrigo Vivi Signed-off-by: Thomas Hellström --- Documentation/gpu/drm-vm-bind-locking.rst | 494 ++++++++++++++++++++++ 1 file changed, 494 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..c290ff4287fb --- /dev/null +++ b/Documentation/gpu/drm-vm-bind-locking.rst @@ -0,0 +1,494 @@ +.. SPDX-License-Identifier: (GPL-2.0+ OR MIT) + +=============== +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 gpu_vms. + +Nomenclature +============ + +* ``Context``: GPU execution context. +* ``gpu_vm``: Abstraction of a virtual GPU address space with + meta-data. Typically one per client (DRM file-private), or one per + context. +* ``gpu_vma``: Abstraction of a GPU address range within a gpu_vm with + associated meta-data. The backing storage of a gpu_vma can either be + a GEM object or anonymous pages mapped also into the CPU + address space for the process. +* gpu_vm_bo: Abstracts the association of a GEM object and + a VM. Note that if only one gpu_vma per vm and buffer object were + allowed, the state stored with a gpu_vm_bo could just as well have + been stored with the gpu_vma. For the purpose of this document, each + GEM object maintains a list of gpu_vm_bos, and each gpu_vm_bo + maintains a list of gpu_vmas. +* ``userptr gpu_vma or just userptr``: A gpu_vma, the backing store of + which is anonymous pages as described above. +* ``revalidating``: Revalidating a gpu_vma means making the latest version + of the backing store resident and making sure the gpu_vma'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 + gpu_vm or a GEM 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 gpu_vmas, 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 gpu_vm. Shared gem + objects also share the gpu_vm's dma_resv. +* ``shared object``: AKA external object: A GEM object which may be shared + by multiple gpu_vms and whose backing storage may be shared with + other drivers. + + +Locks used and locking orders +============================= + +One of the benefits of VM_BIND is that local GEM objects share the gpu_vm'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 ``gpu_vm->lock`` (optionally an rwsem). Protects how the gpu_vm is + partitioned into gpu_vmas. It can also protect the gpu_vm's list of + userptr gpu_vmas. With a CPU mm analogy this would correspond to the + mmap_lock. +* The ``userptr_seqlock``. This lock is taken in read mode for each + userptr gpu_vma on the gpu_vm's userptr list, and in write mode during mmu + notifier invalidation. This is not a real seqlock but described in + ``mm/mmu_notifier.c`` as a "Collision-retry read-side/write-side + 'lock' a lot like a seqcount, however this allows multiple + write-sides to hold it at once...". The read side critical section + is enclosed by ``mmu_interval_read_begin() / + mmu_interval_read_retry()`` with ``mmu_interval_read_begin()`` + sleeping if the write side is held. + The write side is held by the core mm while calling mmu interval + invalidation notifiers. +* The ``gpu_vm->resv`` lock. Protects the gpu_vm's list of gpu_vmas needing + rebinding, and also the residency of all the gpu_vm's local GEM object. + Furthermore it typically protects the gpu_vm's list of evicted GEM + objects and external objects. +* The ``gpu_vm->userptr_notifier_lock``. This is an rwsem that is + taken in read mode during exec and write mode during a mmu notifier + invalidation. The userptr notifier lock is per gpu_vm. +* The gpu_vm list spinlocks. With some implementations they are needed + to be able to update the gpu_vm evicted- and external object + list. For those implementations, the spinlocks are grabbed when the + lists are manipulated. However to avoid locking order violations + with the dma_resv locks, a special scheme is needed when iterating + over the lists. + +.. _gpu_vma lifetime: + +Protection and lifetime of gpu_vm_bos and gpu_vmas +================================================== + +The GEM object's list of gpu_vm_bos is typically protected by the +GEM object's dma_resv. Each gpu_vm_bo holds a reference counted pointer +to the underlying GEM object, and each gpu_vma holds a reference counted +pointer to the gpu_vm_bo. When iterating over the GEM object's +list of gpu_vm_bos the gem object's dma_resv must thus be held, +but if it needs to be dropped during the iteration, care needs to be +taken so that any gpu_vm_bo, and the gpu_vm, if dereferenced +while the lock is dropped, do not disappear. The easiest way to avoid +this is to take a reference on affected objects while the dma_resv is +still held. If iterating over the gpu_vm_bo's gpu_vmas, even +greater care needs to be taken since the gpu_vmas are not +reference counted. If a driver accesses a gpu_vma obtained from +the gpu_vm_bo's list of gpu_vmas, and the GEM object's +dma_resv is dropped, at the very least, it should be thoroughly +documented how the gpu_vma is kept alive. Otherwise holding the +GEM object's dma_resv lock also around unlinking a gpu_vma from a +gpu_vm_bo will ensure that doesn't happen. + + +Revalidation and eviction of local objects +========================================== + +Revalidation +____________ +With VM_BIND, all local objects need to be resident when the gpu is +executing using the gpu_vm, and the objects need to have valid +gpu_vmas set up pointing to them. Typically each gpu command buffer +submission is therefore preceded with a re-validation section: + +.. code-block:: C + + dma_resv_lock(gpu_vm->resv); + + // Validation section starts here. + for_each_gpu_vm_bo_on_evict_list(&gpu_vm->evict_list, &gpu_vm_bo) { + validate_gem_bo(&gpu_vm_bo->gem_bo); + + // The following list iteration needs the Gem object's + // dma_resv to be held (it protects the gpu_vm_bo's list of + // gpu_vmas, but since local gem objects share the gpu_vm's + // dma_resv, it is already held at this point. + for_each_gpu_vma_of_gpu_vm_bo(&gpu_vm_bo, &gpu_vma) + move_gpu_vma_to_rebind_list(&gpu_vma, &gpu_vm->rebind_list); + } + + for_each_gpu_vma_on_rebind_list(&gpu vm->rebind_list, &gpu_vma) { + rebind_gpu_vma(&gpu_vma); + remove_gpu_vma_from_rebind_list(&gpu_vma); + } + // Validation section ends here, and job submission starts. + + add_dependencies(&gpu_job, &gpu_vm->resv); + job_dma_fence = gpu_submit(&gpu_job)); + + add_dma_fence(job_dma_fence, &gpu_vm->resv); + dma_resv_unlock(gpu_vm->resv); + +The reason for having a separate gpu_vm rebind list is that there +might be userptr gpu_vmas that are not mapping a buffer object that +also need rebinding. + +Eviction +________ + +Eviction of one of these local objects will then look similar to the +following: + +.. code-block:: C + + obj = get_object_from_lru(); + + dma_resv_lock(obj->resv); + for_each_gpu_vm_bo_of_obj(obj, &gpu_vm_bo); + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list); + + 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 gpu_vm, it will share the gpu_vm's +dma_resv lock so that ``obj->resv == gpu_vm->resv``. +The gpu_vm_bos marked for eviction are put on the gpu_vm's evict list, +which is protected by ``gpu_vm->resv``, that is always locked while +evicting, due to the above equality. + +For VM_BIND gpu_vms, gpu_vmas don't need to be unbound before eviction, +Since the eviction blit or copy will wait for GPU idle, any attempt by +the GPU to access freed memory through the gpu_vma will be preceded by +a new exec function, with a revalidation section which will make sure +the gpu_vma is rebound. The eviction code holding the object's dma_resv while +revalidating will ensure a new exec function may not race with the eviction. + +Locking with external (or shared) buffer objects +================================================ + +Since shared buffer objects may be shared by multiple gpu_vm's they +can't share their reservation object with a single gpu_vm, but will rather +have a reservation object of their own. The shared objects bound to a +gpu_vm using one or many gpu_vmas are therefore typically put on a +per-gpu_vm list which is protected by the gpu_vm's dma_resv lock. Once +the gpu_vm's reservation object is locked, it is safe to traverse the +external object list and lock the dma_resvs of all external objects. + +At eviction time we now need to put the gpu_vm_bos of *all* gpu_vms a +shared object is bound to on the gpu_vm's evict list, but we can no longer +be certain that we hold the gpu_vm's dma_resv of all the gpu_vms the +object is bound to, since at eviction time we only hold the object's +private dma_resv. If we have a ww_acquire context at hand at eviction +time we could grab the those dma_resvs but that could cause +expensive ww_mutex rollbacks. A simple option is to just mark the +gpu_vm_bos of the evicted gem object with an ``evicted`` bool that +is inspected the next time the corresponding gpu_vm evicted list needs +to be traversed. At that time the gpu_vm's dma_resv and the object's +dma_resv is held, and the gpu_vm_bo marked evicted, can then be added +to the gpu_vm's list of evicted gpu_vm_bos. The ``evicted`` bool would +then be protected by the object's dma_resv. + +The exec function would then become + +.. code-block:: C + + dma_resv_lock(gpu_vm->resv); + + // External object list is protected by the gpu_vm->resv lock. + for_each_gpu_vm_bo_on_extobj_list(gpu_vm, &gpu_vm_bo) { + dma_resv_lock(gpu_vm_bo.gem_obj->resv); + if (gpu_vm_bo_marked_evicted(&gpu_vm_bo)) + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list); + } + + for_each_gpu_vm_bo_on_evict_list(&gpu_vm->evict_list, &gpu_vm_bo) { + validate_gem_bo(&gpu_vm_bo->gem_bo); + + for_each_gpu_vma_of_gpu_vm_bo(&gpu_vm_bo, &gpu_vma) + move_gpu_vma_to_rebind_list(&gpu_vma, &gpu_vm->rebind_list); + } + + for_each_gpu_vma_on_rebind_list(&gpu vm->rebind_list, &gpu_vma) { + rebind_gpu_vma(&gpu_vma); + remove_gpu_vma_from_rebind_list(&gpu_vma); + } + + add_dependencies(&gpu_job, &gpu_vm->resv); + job_dma_fence = gpu_submit(&gpu_job)); + + add_dma_fence(job_dma_fence, &gpu_vm->resv); + for_each_shared_obj(gpu_vm, &obj) + add_dma_fence(job_dma_fence, &obj->resv); + dma_resv_unlock_all_resv_locks(); + +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_gpu_vm_bo_of_obj(obj, &gpu_vm_bo) + if (object_is_vm_local(obj)) + add_gpu_vm_bo_to_evict_list(&gpu_vm_bo, &gpu_vm->evict_list); + else + mark_gpu_vm_bo_evicted(&gpu_vm_bo); + + 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); + +.. _Spinlock iteration: + +Accessing the gpu_vm's lists without the dma_resv lock held +=========================================================== + +Many drivers will not need to access the gpu_vm's evict- and +external objects lists without holding the gpu_vm's dma_resv lock, +but some drivers do, for example due to asynchronous state updates +from within the dma_fence signalling critical path. In such case a +spinlock can be used to protect manipulation of the lists. However, +since higher level sleeping locks needs to be taken for each list item +while iterating over the lists, the items already iterated over needs +to be temporarily moved to a private list and the spinlock released +while processing each item: + +.. code block:: C + + struct list_head still_in_list; + + INIT_LIST_HEAD(&still_in_list); + + spin_lock(&gpu_vm->list_lock); + do { + struct list_head *entry = list_first_entry_or_null(&gpu_vm->list, head); + + if (!entry) + break; + + list_move_tail(&entry->head, &still_in_list); + list_entry_get_unless_zero(entry); + spin_unlock(&gpu_vm->list_lock); + + process(entry); + + spin_lock(&gpu_vm->list_lock); + list_entry_put(entry); + } while (true); + + list_splice_tail(&still_in_list, &gpu_vm->list); + spin_unlock(&gpu_vm->list_lock); + +However, due to the additional locking and atomic operations, drivers that *can* +avoid accessing the gpu_vm's list outside of the dma_resv lock +might want to avoid this iteration scheme, if the driver anticipates a +large number of list items. For lists where the anticipated number of +list items is small, list iteration doesn't happen very often, or +there is a significant additional cost associated with each iteration, +the atomic operation overhead associated with this type of iteration +is, however, probably negligible. Note that if this scheme is +used, it is necessary to make sure this list iteration is protected by +an outer level lock or semaphore, since list items are temporarily +pulled off the list while iterating. + +TODO: Pointer to the gpuvm code implementation if this iteration and +how to choose either iteration scheme. + +userptr gpu_vmas +================ + +A userptr gpu_vma is a gpu_vma 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, +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 has proven difficult to exclude certain notifications. + +Using a MMU notifier for device DMA (and other methods) is described in +`this document +`_. + +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 gpu_vm's list of userptr gpu_vmas needs to be protected by an +outer lock. + +The MMU interval seqlock for a userptr gpu_vma is used in the following +way: + +.. code-block:: C + + // Exclusive locking mode here is strictly needed only if there are + // invalidated userptr vmas present, to avoid multiple userptr + // revalidations. + down_write(&gpu_vm->lock); + retry: + + // Note: mmu_interval_read_begin() blocks until there is no + // invalidation notifier running anymore. + seq = mmu_interval_read_begin(&gpu_vma->userptr_interval); + if (seq != gpu_vma->saved_seq) { + obtain_new_page_pointers(&gpu_vma); + dma_resv_lock(&gpu_vm->resv); + add_gpu_vma_top_revalidate_list(&gpu_vma, &gpu_vm); + dma_resv_unlock(&gpu_vm->resv); + gpu_vma->saved_seq = seq; + } + + // The usual revalidation goes here. + + // Final userptr sequence validation may not happen before the + // submission dma_fence is added to the gpu_vm'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, &gpu_vm->resv); + down_read(&gpu_vm->userptr_notifier_lock); + if (mmu_interval_read_retry(&gpu_vma->userptr_interval, gpu_vma->saved_seq)) { + up_read(&gpu_vm->userptr_notifier_lock); + goto retry; + } + + job_dma_fence = gpu_submit(&gpu_job)); + + add_dma_fence(job_dma_fence, &gpu_vm->resv); + + for_each_shared_obj(gpu_vm, &obj) + add_dma_fence(job_dma_fence, &obj->resv); + + dma_resv_unlock_all_resv_locks(); + up_read(&gpu_vm->userptr_notifier_lock); + up_write(&gpu_vm->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 gpu_vm's userptr +gpu_vma list is looped through, and the check is done for *all* of its +userptr gpu_vmas, although we only show a single one here. + +The userptr gpu_vma 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 gpu_vm->lock from within it. + +.. code-block:: C + + bool gpu_vma_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(&gpu_vm->userptr_notifier_lock); + mmu_interval_set_seq(userptr_interval, cur_seq); + up_write(&gpu_vm->userptr_notifier_lock); + + // At this point, the exec function can't succeed in + // submitting a new job, because cur_seq is an invalid + // sequence number and will always cause a retry. When all + // invalidation callbacks, the mmu notifier core will flip + // the sequence number to a valid one. However we need to + // stop gpu access to the old pages here. + + dma_resv_wait_timeout(&gpu_vm->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 gpu_vma and needs to redo the +page-binding before a new GPU submission can succeed. + +Efficient userptr gpu_vma exec_function iteration +_________________________________________________ + +If the gpu_vm's list of userptr gpu_vmas becomes large, it's +inefficient to iterate through the complete lists of userptrs on each +exec function to check whether each userptr gpu_vma's saved +sequence number is invalid or stale. A solution to this is to put all +*invalidated* userptr gpu_vmas on a separate gpu_vm list and +only those gpu_vmas on the list are actually checked on each exec +function. This list will then lend itself very-well to the spinlock +locking scheme that is +:ref:`described in the spinlock iteration section `, since +in the mmu notifier, where we add the invalidated gpu_vmas to the +list, it's not possible to take any outer locks like the +``gpu_vm->lock`` or the ``gpu_vm->resv`` lock. Note that the +``gpu_vm->lock`` still needs to be taken while iterating to ensure the list is +complete, as also mentioned in that section. + +If using an invalidated userptr list like this, the retry check in the +exec function trivially becomes a check for invalidated list empty. + +Locking at bind- and unbind time +================================ + +At bind time, assuming a GEM object backed gpu_vma, each +gpu_vma needs to be associated with a gpu_vm_bo and that +gpu_vm_bo in turn needs to be added to the GEM object's +gpu_vm_bo list, and possibly to the gpu_vm's external object +list. This is referred to as *linking* the gpu_vma, and typically +requires that the ``gpu_vm->resv`` and the GEM object's dma_resv are +held. When unlinking a gpu_vma the same locks are typically held, +and that ensures, as briefly discussed +:ref:`previously `, that when iterating over +``gpu_vmas`, either under the ``gpu_vm->resv`` or the GEM +object's dma_resv, that the gpu_vmas stay alive as long +as the lock under which we iterate are not is not released. For +userptr gpu_vmas it's similarly required that during unlink, the +outer ``gpu_vm->lock`` is held, since otherwise when iterating over +the invalidated userptr list as described in the previous section, +there is nothing keeping those userptr gpu_vmas alive. +