Re: [PATCH v4 28/33] drm/xe: Add SVM VRAM migration

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

On Thu, 2025-01-30 at 08:32 -0800, Matthew Brost wrote:
> On Thu, Jan 30, 2025 at 02:22:55PM +0000, Matthew Auld wrote:
> > On 29/01/2025 19:52, 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)
> > > 
> > > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > > ---
> > >   drivers/gpu/drm/xe/xe_svm.c | 99
> > > +++++++++++++++++++++++++++++++++++--
> > >   drivers/gpu/drm/xe/xe_svm.h |  5 ++
> > >   2 files changed, 100 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/xe/xe_svm.c
> > > b/drivers/gpu/drm/xe/xe_svm.c
> > > index ba1db030bf33..fc030855d078 100644
> > > --- a/drivers/gpu/drm/xe/xe_svm.c
> > > +++ b/drivers/gpu/drm/xe/xe_svm.c
> > > @@ -502,7 +502,6 @@ 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,
> > > @@ -582,6 +581,64 @@ 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_mem_region *tile_to_mr(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 xe_mem_region *mr = tile_to_mr(tile);
> > > +	struct drm_buddy_block *block;
> > > +	struct list_head *blocks;
> > > +	struct xe_bo *bo;
> > > +	ktime_t end = 0;
> > > +	int err;
> > > +
> > > +retry:
> > > +	xe_vm_lock(vm, false);
> > > +	bo = xe_bo_create(tile_to_xe(tile), tile, vm, range-
> > > >base.itree.last + 1 -
> > > +			  range->base.itree.start,
> > > ttm_bo_type_device,
> > > +			  XE_BO_FLAG_VRAM_IF_DGFX(tile) |
> > > +			  XE_BO_FLAG_CPU_ADDR_MIRROR);
> > > +	xe_vm_unlock(vm);
> > 
> > What was the trick again to ensure eviction is not triggered at
> > this point?
> > I thought there was some trick with eviction_valuable() but I can't
> > find it.
> > 
> 
> I dropped that given the hacky nature of how it was implemented. Yes,
> it
> is possible that we allocate VRAM and it is immediately evicted
> before
> the bind occurs but in practice should never really happen given this
> BO
> should be the last entry on the LRU list. Even if this happens, I
> believe this is harmless given the bind will abort and trigger a
> retry.

It might be worth mentioning that in the multidevice series, we create
an external bo and hold on to the lock until the struct mm_struct is
populated with the underlying pages. After that, but before populating
the gpu_vm, eviction can happen. But since now the cpu page-table is
populated, that would trigger a notifier callback and a seqno update
and as Matt says, the bind will be aborted and retried.

/Thomas


> 
> Matt
> 
> > > +	if (IS_ERR(bo)) {
> > > +		err = PTR_ERR(bo);
> > > +		if (xe_vm_validate_should_retry(NULL, err,
> > > &end))
> > > +			goto retry;
> > > +		return bo;
> > > +	}
> > > +
> > > +	drm_gpusvm_devmem_init(&bo->devmem_allocation,
> > > +			       vm->xe->drm.dev, vm-
> > > >svm.gpusvm.mm,
> > > +			       &gpusvm_devmem_ops,
> > > +			       &tile->mem.vram.dpagemap,
> > > +			       range->base.itree.last + 1 -
> > > +			       range->base.itree.start);
> > > +
> > > +	blocks = &to_xe_ttm_vram_mgr_resource(bo->ttm.resource)-
> > > >blocks;
> > > +	list_for_each_entry(block, blocks, link)
> > > +		block->private = mr;
> > > +
> > > +	/*
> > > +	 * 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);
> > > +	if (err) {
> > > +		xe_bo_put(bo);	/* Local ref */
> > > +		xe_bo_put(bo);	/* Creation ref */
> > > +		return ERR_PTR(err);
> > > +	}
> > > +
> > > +	return bo;
> > > +}
> > > +
> > >   /**
> > >    * xe_svm_handle_pagefault() - SVM handle page fault
> > >    * @vm: The VM.
> > > @@ -590,7 +647,8 @@ 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.
> > >    */
> > > @@ -598,11 +656,18 @@ 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;
> > > @@ -610,6 +675,9 @@ 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)
> > > @@ -625,9 +693,31 @@ 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
> > > &&
> > > +	    (range->base.itree.last + 1 - range-
> > > >base.itree.start) >= SZ_64K) {
> > > +		range->migrated = true;
> > > +
> > > +		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);
> > > +			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;
> > > @@ -658,6 +748,7 @@ 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 63daffdfdbf6..4c2576162c39 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.
> > > +	 */
> > > +	u8 migrated	:1;
> > >   };
> > >   int xe_devm_add(struct xe_tile *tile, struct xe_mem_region
> > > *mr);
> >