Re: [PATCH v5 27/32] drm/xe: Add SVM VRAM migration

["Ghimiray, Himal Prasad" <himal.prasad.ghimiray@intel.com>]

On 19-02-2025 08:35, Matthew Brost wrote:
> On Wed, Feb 19, 2025 at 08:29:53AM +0530, Ghimiray, Himal Prasad wrote:
>>
>>
>> On 19-02-2025 03:24, Matthew Brost wrote:
>>> On Thu, Feb 13, 2025 at 11:58:27PM +0530, Ghimiray, Himal Prasad wrote:
>>>>
>>>>
>>>> On 13-02-2025 07:41, Matthew Brost wrote:
>>>>> Migration is implemented with range granularity, with VRAM backing being
>>>>> a VM private TTM BO (i.e., shares dma-resv with VM). The lifetime of the
>>>>> TTM BO is limited to when the SVM range is in VRAM (i.e., when a VRAM
>>>>> SVM range is migrated to SRAM, the TTM BO is destroyed).
>>>>>
>>>>> The design choice for using TTM BO for VRAM backing store, as opposed to
>>>>> direct buddy allocation, is as follows:
>>>>>
>>>>> - DRM buddy allocations are not at page granularity, offering no
>>>>>      advantage over a BO.
>>>>> - Unified eviction is required (SVM VRAM and TTM BOs need to be able to
>>>>>      evict each other).
>>>>> - For exhaustive eviction [1], SVM VRAM allocations will almost certainly
>>>>>      require a dma-resv.
>>>>> - Likely allocation size is 2M which makes of size of BO (872)
>>>>>      acceptable per allocation (872 / 2M == .0004158).
>>>>>
>>>>> With this, using TTM BO for VRAM backing store seems to be an obvious
>>>>> choice as it allows leveraging of the TTM eviction code.
>>>>>
>>>>> Current migration policy is migrate any SVM range greater than or equal
>>>>> to 64k once.
>>>>>
>>>>> [1] https://patchwork.freedesktop.org/series/133643/
>>>>>
>>>>> v2:
>>>>>     - Rebase on latest GPU SVM
>>>>>     - Retry page fault on get pages returning mixed allocation
>>>>>     - Use drm_gpusvm_devmem
>>>>> v3:
>>>>>     - Use new BO flags
>>>>>     - New range structure (Thomas)
>>>>>     - Hide migration behind Kconfig
>>>>>     - Kernel doc (Thomas)
>>>>>     - Use check_pages_threshold
>>>>> v4:
>>>>>     - Don't evict partial unmaps in garbage collector (Thomas)
>>>>>     - Use %pe to print errors (Thomas)
>>>>>     - Use %p to print pointers (Thomas)
>>>>> v5:
>>>>>     - Use range size helper (Thomas)
>>>>>     - Make BO external (Thomas)
>>>>>     - Set tile to NULL for BO creation (Thomas)
>>>>>     - Drop BO mirror flag (Thomas)
>>>>>     - Hold BO dma-resv lock across migration (Auld, Thomas)
>>>>>
>>>>> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
>>>>> ---
>>>>>     drivers/gpu/drm/xe/xe_svm.c | 111 ++++++++++++++++++++++++++++++++++--
>>>>>     drivers/gpu/drm/xe/xe_svm.h |   5 ++
>>>>>     2 files changed, 112 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/drivers/gpu/drm/xe/xe_svm.c b/drivers/gpu/drm/xe/xe_svm.c
>>>>> index 0a78a838508c..2e1e0f31c1a8 100644
>>>>> --- a/drivers/gpu/drm/xe/xe_svm.c
>>>>> +++ b/drivers/gpu/drm/xe/xe_svm.c
>>>>> @@ -32,6 +32,11 @@ static unsigned long xe_svm_range_end(struct xe_svm_range *range)
>>>>>     	return drm_gpusvm_range_end(&range->base);
>>>>>     }
>>>>> +static unsigned long xe_svm_range_size(struct xe_svm_range *range)
>>>>> +{
>>>>> +	return drm_gpusvm_range_size(&range->base);
>>>>> +}
>>>>> +
>>>>>     static void *xe_svm_devm_owner(struct xe_device *xe)
>>>>>     {
>>>>>     	return xe;
>>>>> @@ -512,7 +517,6 @@ static int xe_svm_populate_devmem_pfn(struct drm_gpusvm_devmem *devmem_allocatio
>>>>>     	return 0;
>>>>>     }
>>>>> -__maybe_unused
>>>>>     static const struct drm_gpusvm_devmem_ops gpusvm_devmem_ops = {
>>>>>     	.devmem_release = xe_svm_devmem_release,
>>>>>     	.populate_devmem_pfn = xe_svm_populate_devmem_pfn,
>>>>> @@ -592,6 +596,71 @@ static bool xe_svm_range_is_valid(struct xe_svm_range *range,
>>>>>     	return (range->tile_present & ~range->tile_invalidated) & BIT(tile->id);
>>>>>     }
>>>>> +static struct xe_vram_region *tile_to_vr(struct xe_tile *tile)
>>>>> +{
>>>>> +	return &tile->mem.vram;
>>>>> +}
>>>>> +
>>>>> +static struct xe_bo *xe_svm_alloc_vram(struct xe_vm *vm, struct xe_tile *tile,
>>>>> +				       struct xe_svm_range *range,
>>>>> +				       const struct drm_gpusvm_ctx *ctx)
>>>>> +{
>>>>> +	struct mm_struct *mm = vm->svm.gpusvm.mm;
>>>>> +	struct xe_vram_region *vr = tile_to_vr(tile);
>>>>> +	struct drm_buddy_block *block;
>>>>> +	struct list_head *blocks;
>>>>> +	struct xe_bo *bo;
>>>>> +	ktime_t end = 0;
>>>>> +	int err;
>>>>> +
>>>>> +	if (!mmget_not_zero(mm))
>>>>> +		return ERR_PTR(-EFAULT);
>>>>> +	mmap_read_lock(mm);
>>>>> +
>>>>> +retry:
>>>>> +	bo = xe_bo_create_locked(tile_to_xe(tile), NULL, NULL,
>>>>> +				 xe_svm_range_size(range),
>>>>> +				 ttm_bo_type_device,
>>>>> +				 XE_BO_FLAG_VRAM_IF_DGFX(tile));
>>>>> +	if (IS_ERR(bo)) {
>>>>> +		err = PTR_ERR(bo);
>>>>> +		if (xe_vm_validate_should_retry(NULL, err, &end))
>>>>> +			goto retry;
>>>>> +		goto unlock;
>>>>> +	}
>>>>> +
>>>>> +	drm_gpusvm_devmem_init(&bo->devmem_allocation,
>>>>> +			       vm->xe->drm.dev, mm,
>>>>> +			       &gpusvm_devmem_ops,
>>>>> +			       &tile->mem.vram.dpagemap,
>>>>> +			       xe_svm_range_size(range));
>>>>> +
>>>>> +	blocks = &to_xe_ttm_vram_mgr_resource(bo->ttm.resource)->blocks;
>>>>> +	list_for_each_entry(block, blocks, link)
>>>>> +		block->private = vr;
>>>>> +
>>>>> +	/*
>>>>> +	 * Take ref because as soon as drm_gpusvm_migrate_to_devmem succeeds the
>>>>> +	 * creation ref can be dropped upon CPU fault or unmap.
>>>>> +	 */
>>>>> +	xe_bo_get(bo);
>>>>> +
>>>>> +	err = drm_gpusvm_migrate_to_devmem(&vm->svm.gpusvm, &range->base,
>>>>> +					   &bo->devmem_allocation, ctx);
>>>>> +	xe_bo_unlock(bo);
>>>>> +	if (err) {
>>>>> +		xe_bo_put(bo);	/* Local ref */
>>>>> +		xe_bo_put(bo);	/* Creation ref */
>>>>> +		bo = ERR_PTR(err);
>>>>> +	}
>>>>> +
>>>>> +unlock:
>>>>> +	mmap_read_unlock(mm);
>>>>> +	mmput(mm);
>>>>> +
>>>>> +	return bo;
>>>>> +}
>>>>> +
>>>>>     /**
>>>>>      * xe_svm_handle_pagefault() - SVM handle page fault
>>>>>      * @vm: The VM.
>>>>> @@ -600,7 +669,8 @@ static bool xe_svm_range_is_valid(struct xe_svm_range *range,
>>>>>      * @fault_addr: The GPU fault address.
>>>>>      * @atomic: The fault atomic access bit.
>>>>>      *
>>>>> - * Create GPU bindings for a SVM page fault.
>>>>> + * Create GPU bindings for a SVM page fault. Optionally migrate to device
>>>>> + * memory.
>>>>>      *
>>>>>      * Return: 0 on success, negative error code on error.
>>>>>      */
>>>>> @@ -608,11 +678,18 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>>>>>     			    struct xe_tile *tile, u64 fault_addr,
>>>>>     			    bool atomic)
>>>>>     {
>>>>> -	struct drm_gpusvm_ctx ctx = { .read_only = xe_vma_read_only(vma), };
>>>>> +	struct drm_gpusvm_ctx ctx = {
>>>>> +		.read_only = xe_vma_read_only(vma),
>>>>> +		.devmem_possible = IS_DGFX(vm->xe) &&
>>>>> +			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR),
>>>>> +		.check_pages_threshold = IS_DGFX(vm->xe) &&
>>>>> +			IS_ENABLED(CONFIG_DRM_XE_DEVMEM_MIRROR) ? SZ_64K : 0,
>>>>> +	};
>>>>>     	struct xe_svm_range *range;
>>>>>     	struct drm_gpusvm_range *r;
>>>>>     	struct drm_exec exec;
>>>>>     	struct dma_fence *fence;
>>>>> +	struct xe_bo *bo = NULL;
>>>>>     	ktime_t end = 0;
>>>>>     	int err;
>>>>> @@ -620,6 +697,9 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>>>>>     	xe_assert(vm->xe, xe_vma_is_cpu_addr_mirror(vma));
>>>>>     retry:
>>>>> +	xe_bo_put(bo);
>>>>> +	bo = NULL;
>>>>> +
>>>>>     	/* Always process UNMAPs first so view SVM ranges is current */
>>>>>     	err = xe_svm_garbage_collector(vm);
>>>>>     	if (err)
>>>>> @@ -635,9 +715,31 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>>>>>     	if (xe_svm_range_is_valid(range, tile))
>>>>>     		return 0;
>>>>> +	/* XXX: Add migration policy, for now migrate range once */
>>>>> +	if (!range->migrated && range->base.flags.migrate_devmem &&
>>>>> +	    xe_svm_range_size(range) >= SZ_64K) {
>>>>> +		range->migrated = true;
>>>>
>>>>
>>>> shouldn't this be set to true, only once xe_svm_alloc_vram is successfull ?
>>>> In case of bo alloc failure retry wont enter here.
>>>>
>>>
>>> No. The point of this is try to migrate once to avoid a live lock case
>>> of concurrent CPU and GPU access. Once we have migration policy, I'd
>>> suspect memory marked as prefered VRAM only try once too. If memory is
>>> marked as VRAM only we'd retry to a limit and if reached kill the app.
>>> We can work of those details when that code lands. Until then, trying
>>> once seems reasonable.
>>
>> I understand what we are trying to achieve here, and it functions well.
>> However, my only concern is member name "migrated" gives the impression that
>> it will be set to true only if the range has been successfully migrated to
>> VRAM.
>>
> 
> s/migrated/skip_migrate ?

sounds good to me.

> 
>>>
>>> Matt
>>>
>>>>> +
>>>>> +		bo = xe_svm_alloc_vram(vm, tile, range, &ctx);
>>>>> +		if (IS_ERR(bo)) {
>>>>> +			drm_info(&vm->xe->drm,
>>>>> +				 "VRAM allocation failed, falling back to retrying, asid=%u, errno %pe\n",
>>>>> +				 vm->usm.asid, bo);
>>
>> This log is also misleading, we dont retry vram allocation in case of first
>> failure.
>>
> 
> How about...
> 
> "VRAM allocation failed, falling back to retrying fault,"

Yup.

> 
> Matt
> 
>>>>> +			bo = NULL;
>>>>> +			goto retry;
>>>>> +		}
>>>>> +	}
>>>>> +
>>>>>     	err = drm_gpusvm_range_get_pages(&vm->svm.gpusvm, r, &ctx);
>>>>> -	if (err == -EFAULT || err == -EPERM)	/* Corner where CPU mappings have changed */
>>>>> +	/* Corner where CPU mappings have changed */
>>>>> +	if (err == -EOPNOTSUPP || err == -EFAULT || err == -EPERM) {
>>>>> +		if (err == -EOPNOTSUPP)
>>>>> +			drm_gpusvm_range_evict(&vm->svm.gpusvm, &range->base);
>>>>> +		drm_info(&vm->xe->drm,
>>>>> +			 "Get pages failed, falling back to retrying, asid=%u, gpusvm=%p, errno %pe\n",
>>>>> +			 vm->usm.asid, &vm->svm.gpusvm, ERR_PTR(err));
>>>>>     		goto retry;
>>>>> +	}
>>>>>     	if (err)
>>>>>     		goto err_out;
>>>>> @@ -668,6 +770,7 @@ int xe_svm_handle_pagefault(struct xe_vm *vm, struct xe_vma *vma,
>>>>>     	dma_fence_put(fence);
>>>>>     err_out:
>>>>> +	xe_bo_put(bo);
>>>>>     	return err;
>>>>>     }
>>>>> diff --git a/drivers/gpu/drm/xe/xe_svm.h b/drivers/gpu/drm/xe/xe_svm.h
>>>>> index 0fa525d34987..984a61651d9e 100644
>>>>> --- a/drivers/gpu/drm/xe/xe_svm.h
>>>>> +++ b/drivers/gpu/drm/xe/xe_svm.h
>>>>> @@ -35,6 +35,11 @@ struct xe_svm_range {
>>>>>     	 * range. Protected by GPU SVM notifier lock.
>>>>>     	 */
>>>>>     	u8 tile_invalidated;
>>>>> +	/**
>>>>> +	 * @migrated: Range has been migrated to device memory, protected by
>>>>> +	 * GPU fault handler locking.
>>
>> The range is not necessarily migrated to device memory if migrated is true.
>>
>>>>> +	 */
>>>>> +	u8 migrated	:1;
>>>>>     };
>>>>>     int xe_devm_add(struct xe_tile *tile, struct xe_vram_region *vr);
>>>>
>>