Re: [PATCH v2 12/29] drm/xe: Add SVM garbage collector

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

On Tue, 2024-10-15 at 20:25 -0700, Matthew Brost wrote:
> Add basic SVM garbage collector which can destroy an SVM range upon
> an
> MMU UNMAP event.
> 
> v2:
>  - Flush garbage collector in xe_svm_close
> 
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_svm.c      | 87
> +++++++++++++++++++++++++++++++-
>  drivers/gpu/drm/xe/xe_svm.h      |  1 +
>  drivers/gpu/drm/xe/xe_vm.c       |  4 ++
>  drivers/gpu/drm/xe/xe_vm_types.h |  5 ++
>  4 files changed, 95 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_svm.c
> b/drivers/gpu/drm/xe/xe_svm.c
> index a9addaea316d..9c2f44cba166 100644
> --- a/drivers/gpu/drm/xe/xe_svm.c
> +++ b/drivers/gpu/drm/xe/xe_svm.c
> @@ -30,6 +30,7 @@ xe_svm_range_alloc(struct drm_gpusvm *gpusvm)
>  	if (!range)
>  		return ERR_PTR(-ENOMEM);
>  
> +	INIT_LIST_HEAD(&range->garbage_collector_link);
>  	xe_vm_get(gpusvm_to_vm(gpusvm));
>  
>  	return &range->base;
> @@ -46,6 +47,24 @@ static struct xe_svm_range *to_xe_range(struct
> drm_gpusvm_range *r)
>  	return container_of(r, struct xe_svm_range, base);
>  }
>  
> +static void
> +xe_svm_garbage_collector_add_range(struct xe_vm *vm, struct
> xe_svm_range *range,
> +				   const struct mmu_notifier_range
> *mmu_range)
> +{
> +	struct xe_device *xe = vm->xe;
> +
> +	drm_gpusvm_range_set_unmapped(&range->base, mmu_range);
> +
> +	spin_lock(&vm->svm.garbage_collector.lock);
> +	if (list_empty(&range->garbage_collector_link))
> +		list_add_tail(&range->garbage_collector_link,
> +			      &vm-
> >svm.garbage_collector.range_list);
> +	spin_unlock(&vm->svm.garbage_collector.lock);
> +
> +	queue_work(xe_device_get_root_tile(xe)->primary_gt-
> >usm.pf_wq,
> +		   &vm->svm.garbage_collector.work);
> +}
> +
>  static u8
>  xe_svm_range_notifier_event_begin(struct xe_vm *vm, struct
> drm_gpusvm_range *r,
>  				  const struct mmu_notifier_range
> *mmu_range,
> @@ -88,7 +107,9 @@ xe_svm_range_notifier_event_end(struct xe_vm *vm,
> struct drm_gpusvm_range *r,
>  	struct drm_gpusvm_ctx ctx = { .in_notifier = true, };
>  
>  	drm_gpusvm_range_unmap_pages(&vm->svm.gpusvm, r, &ctx);
> -	/* TODO: Add range to garbage collector */
> +	if (mmu_range->event == MMU_NOTIFY_UNMAP)
> +		xe_svm_garbage_collector_add_range(vm,
> to_xe_range(r),
> +						   mmu_range);
>  }
>  
>  static void xe_svm_invalidate(struct drm_gpusvm *gpusvm,
> @@ -184,6 +205,58 @@ static void xe_svm_invalidate(struct drm_gpusvm
> *gpusvm,
>  		xe_svm_range_notifier_event_end(vm, r, mmu_range);
>  }
>  
> +static int __xe_svm_garbage_collector(struct xe_vm *vm,
> +				      struct xe_svm_range *range)
> +{
> +	/* TODO: Do unbind */
> +
> +	drm_gpusvm_range_remove(&vm->svm.gpusvm, &range->base);
> +
> +	return 0;
> +}
> +
> +static int xe_svm_garbage_collector(struct xe_vm *vm)
> +{
> +	struct xe_svm_range *range, *next;
> +	int err;
> +
> +	lockdep_assert_held_write(&vm->lock);
> +
> +	if (xe_vm_is_closed_or_banned(vm))
> +		return -ENOENT;
> +
> +	spin_lock(&vm->svm.garbage_collector.lock);
> +	list_for_each_entry_safe(range, next,
> +				 &vm-
> >svm.garbage_collector.range_list,
> +				 garbage_collector_link) {
> +		list_del(&range->garbage_collector_link);
> +		spin_unlock(&vm->svm.garbage_collector.lock);

This looks broken, what if someone removed the "next" entry here?
You probably want to use list_next_entry_or_null();

> +
> +		err = __xe_svm_garbage_collector(vm, range);
> +		if (err) {
> +			drm_warn(&vm->xe->drm,
> +				 "Garbage collection failed: %d\n",
> err);
> +			xe_vm_kill(vm, true);
> +			return err;
> +		}
> +
> +		spin_lock(&vm->svm.garbage_collector.lock);
> +	}
> +	spin_unlock(&vm->svm.garbage_collector.lock);
> +
> +	return 0;
> +}
> +
> +static void xe_svm_garbage_collector_work_func(struct work_struct
> *w)
> +{
> +	struct xe_vm *vm = container_of(w, struct xe_vm,
> +					svm.garbage_collector.work);
> +
> +	down_write(&vm->lock);
> +	xe_svm_garbage_collector(vm);
> +	up_write(&vm->lock);
> +}
> +
>  static const struct drm_gpusvm_ops gpusvm_ops = {
>  	.range_alloc = xe_svm_range_alloc,
>  	.range_free = xe_svm_range_free,
> @@ -198,6 +271,11 @@ static const u64 fault_chunk_sizes[] = {
>  
>  int xe_svm_init(struct xe_vm *vm)
>  {
> +	spin_lock_init(&vm->svm.garbage_collector.lock);
> +	INIT_LIST_HEAD(&vm->svm.garbage_collector.range_list);
> +	INIT_WORK(&vm->svm.garbage_collector.work,
> +		  xe_svm_garbage_collector_work_func);
> +
>  	return drm_gpusvm_init(&vm->svm.gpusvm, "Xe SVM", &vm->xe-
> >drm,
>  			       current->mm, NULL, 0, vm->size,
>  			       SZ_512M, &gpusvm_ops,
> fault_chunk_sizes,
> @@ -211,6 +289,8 @@ void xe_svm_close(struct xe_vm *vm)
>  	/* Flush running notifiers making xe_vm_close() visable */
>  	xe_svm_notifier_lock(vm);
>  	xe_svm_notifier_unlock(vm);
> +
> +	flush_work(&vm->svm.garbage_collector.work);
>  }
>  
>  void xe_svm_fini(struct xe_vm *vm)
> @@ -241,7 +321,10 @@ int xe_svm_handle_pagefault(struct xe_vm *vm,
> struct xe_vma *vma,
>  	lockdep_assert_held_write(&vm->lock);
>  
>  retry:
> -	/* TODO: Run garbage collector */
> +	/* Always process UNMAPs first so view SVM ranges is current
> */
> +	err = xe_svm_garbage_collector(vm);
> +	if (err)
> +		return err;
>  
>  	r = drm_gpusvm_range_find_or_insert(&vm->svm.gpusvm,
> fault_addr,
>  					    xe_vma_start(vma),
> xe_vma_end(vma),
> diff --git a/drivers/gpu/drm/xe/xe_svm.h
> b/drivers/gpu/drm/xe/xe_svm.h
> index ee0bd1ae655b..06d90d0f71a6 100644
> --- a/drivers/gpu/drm/xe/xe_svm.h
> +++ b/drivers/gpu/drm/xe/xe_svm.h
> @@ -17,6 +17,7 @@ struct xe_vma;
>  
>  struct xe_svm_range {
>  	struct drm_gpusvm_range base;
> +	struct list_head garbage_collector_link;
>  	u8 tile_present;
>  	u8 tile_invalidated;
>  };
> diff --git a/drivers/gpu/drm/xe/xe_vm.c b/drivers/gpu/drm/xe/xe_vm.c
> index 63aa0a25d3b7..399cbbdbddd5 100644
> --- a/drivers/gpu/drm/xe/xe_vm.c
> +++ b/drivers/gpu/drm/xe/xe_vm.c
> @@ -3071,6 +3071,10 @@ int xe_vm_bind_ioctl(struct drm_device *dev,
> void *data, struct drm_file *file)
>  		goto put_exec_queue;
>  	}
>  
> +	/* Ensure all UNMAPs visable */
> +	if (xe_vm_in_fault_mode(vm))
> +		flush_work(&vm->svm.garbage_collector.work);

Hmm, what is someone added an UNMAP here?

Thanks,
Thomas

> +
>  	err = down_write_killable(&vm->lock);
>  	if (err)
>  		goto put_vm;
> diff --git a/drivers/gpu/drm/xe/xe_vm_types.h
> b/drivers/gpu/drm/xe/xe_vm_types.h
> index b736e53779d2..2eae3575c409 100644
> --- a/drivers/gpu/drm/xe/xe_vm_types.h
> +++ b/drivers/gpu/drm/xe/xe_vm_types.h
> @@ -146,6 +146,11 @@ struct xe_vm {
>  	struct {
>  		/** @svm.gpusvm: base GPUSVM used to track fault
> allocations */
>  		struct drm_gpusvm gpusvm;
> +		struct {
> +			spinlock_t lock;
> +			struct list_head range_list;
> +			struct work_struct work;
> +		} garbage_collector;
>  	} svm;
>  
>  	struct xe_device *xe;