[Intel-gfx] [PATCH 2/6] drm/i915/gem: Avoid gem_context->mutex for simple vma lookup

[Tvrtko Ursulin]

On 19/03/2020 09:19, Chris Wilson wrote:
> As we store the handle lookup inside a radix tree, we do not need the
> gem_context->mutex except until we need to insert our lookup into the
> common radix tree. This takes a small bit of rearranging to ensure that
> the lut we insert into the tree is ready prior to actually inserting it
> (as soon as it is exposed via the radixtree, it is visible to any other
> submission).
> 
> v2: For brownie points, remove the goto spaghetti.
> v3: Tighten up the closed-handle checks.
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> ---
>   .../gpu/drm/i915/gem/i915_gem_execbuffer.c    | 136 +++++++++++-------
>   1 file changed, 87 insertions(+), 49 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> index d3f4f28e9468..042a9ccf348f 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c
> @@ -481,7 +481,7 @@ eb_add_vma(struct i915_execbuffer *eb,
>   
>   	GEM_BUG_ON(i915_vma_is_closed(vma));
>   
> -	ev->vma = i915_vma_get(vma);
> +	ev->vma = vma;
>   	ev->exec = entry;
>   	ev->flags = entry->flags;
>   
> @@ -728,77 +728,117 @@ static int eb_select_context(struct i915_execbuffer *eb)
>   	return 0;
>   }
>   
> -static int eb_lookup_vmas(struct i915_execbuffer *eb)
> +static int __eb_add_lut(struct i915_execbuffer *eb,
> +			u32 handle, struct i915_vma *vma)
>   {
> -	struct radix_tree_root *handles_vma = &eb->gem_context->handles_vma;
> -	struct drm_i915_gem_object *obj;
> -	unsigned int i, batch;
> +	struct i915_gem_context *ctx = eb->gem_context;
> +	struct i915_lut_handle *lut;
>   	int err;
>   
> -	if (unlikely(i915_gem_context_is_closed(eb->gem_context)))
> -		return -ENOENT;
> +	lut = i915_lut_handle_alloc();
> +	if (unlikely(!lut))
> +		return -ENOMEM;
>   
> -	INIT_LIST_HEAD(&eb->relocs);
> -	INIT_LIST_HEAD(&eb->unbound);
> +	i915_vma_get(vma);
> +	if (!atomic_fetch_inc(&vma->open_count))
> +		i915_vma_reopen(vma);
> +	lut->handle = handle;
> +	lut->ctx = ctx;
> +
> +	/* Check that the context hasn't been closed in the meantime */
> +	err = -EINTR;
> +	if (!mutex_lock_interruptible(&ctx->mutex)) {
> +		err = -ENOENT;
> +		if (likely(!i915_gem_context_is_closed(ctx)))
> +			err = radix_tree_insert(&ctx->handles_vma, handle, vma);
> +		if (err == 0) { /* And nor has this handle */
> +			struct drm_i915_gem_object *obj = vma->obj;
> +
> +			i915_gem_object_lock(obj);
> +			if (idr_find(&eb->file->object_idr, handle) == obj) {
> +				list_add(&lut->obj_link, &obj->lut_list);
> +			} else {
> +				radix_tree_delete(&ctx->handles_vma, handle);
> +				err = -ENOENT;
> +			}
> +			i915_gem_object_unlock(obj);
> +		}
> +		mutex_unlock(&ctx->mutex);
> +	}
> +	if (unlikely(err))
> +		goto err;
>   
> -	batch = eb_batch_index(eb);
> +	return 0;
>   
> -	for (i = 0; i < eb->buffer_count; i++) {
> -		u32 handle = eb->exec[i].handle;
> -		struct i915_lut_handle *lut;
> +err:
> +	atomic_dec(&vma->open_count);
> +	i915_vma_put(vma);
> +	i915_lut_handle_free(lut);
> +	return err;
> +}
> +
> +static struct i915_vma *eb_lookup_vma(struct i915_execbuffer *eb, u32 handle)
> +{
> +	do {
> +		struct drm_i915_gem_object *obj;
>   		struct i915_vma *vma;
> +		int err;
>   
> -		vma = radix_tree_lookup(handles_vma, handle);
> +		rcu_read_lock();
> +		vma = radix_tree_lookup(&eb->gem_context->handles_vma, handle);

radix_tree_lookup is documented to be RCU safe okay. How about freeing 
VMAs - is that done after a RCU grace period?

Regards,

Tvrtko

> +		if (likely(vma))
> +			vma = i915_vma_tryget(vma);
> +		rcu_read_unlock();
>   		if (likely(vma))
> -			goto add_vma;
> +			return vma;
>   
>   		obj = i915_gem_object_lookup(eb->file, handle);
> -		if (unlikely(!obj)) {
> -			err = -ENOENT;
> -			goto err_vma;
> -		}
> +		if (unlikely(!obj))
> +			return ERR_PTR(-ENOENT);
>   
>   		vma = i915_vma_instance(obj, eb->context->vm, NULL);
>   		if (IS_ERR(vma)) {
> -			err = PTR_ERR(vma);
> -			goto err_obj;
> +			i915_gem_object_put(obj);
> +			return vma;
>   		}
>   
> -		lut = i915_lut_handle_alloc();
> -		if (unlikely(!lut)) {
> -			err = -ENOMEM;
> -			goto err_obj;
> -		}
> +		err = __eb_add_lut(eb, handle, vma);
> +		if (likely(!err))
> +			return vma;
>   
> -		err = radix_tree_insert(handles_vma, handle, vma);
> -		if (unlikely(err)) {
> -			i915_lut_handle_free(lut);
> -			goto err_obj;
> -		}
> +		i915_gem_object_put(obj);
> +		if (err != -EEXIST)
> +			return ERR_PTR(err);
> +	} while (1);
> +}
>   
> -		/* transfer ref to lut */
> -		if (!atomic_fetch_inc(&vma->open_count))
> -			i915_vma_reopen(vma);
> -		lut->handle = handle;
> -		lut->ctx = eb->gem_context;
> +static int eb_lookup_vmas(struct i915_execbuffer *eb)
> +{
> +	unsigned int batch = eb_batch_index(eb);
> +	unsigned int i;
> +	int err = 0;
>   
> -		i915_gem_object_lock(obj);
> -		list_add(&lut->obj_link, &obj->lut_list);
> -		i915_gem_object_unlock(obj);
> +	INIT_LIST_HEAD(&eb->relocs);
> +	INIT_LIST_HEAD(&eb->unbound);
> +
> +	for (i = 0; i < eb->buffer_count; i++) {
> +		struct i915_vma *vma;
> +
> +		vma = eb_lookup_vma(eb, eb->exec[i].handle);
> +		if (IS_ERR(vma)) {
> +			err = PTR_ERR(vma);
> +			break;
> +		}
>   
> -add_vma:
>   		err = eb_validate_vma(eb, &eb->exec[i], vma);
> -		if (unlikely(err))
> -			goto err_vma;
> +		if (unlikely(err)) {
> +			i915_vma_put(vma);
> +			break;
> +		}
>   
>   		eb_add_vma(eb, i, batch, vma);
>   	}
>   
> -	return 0;
> -
> -err_obj:
> -	i915_gem_object_put(obj);
> -err_vma:
>   	eb->vma[i].vma = NULL;
>   	return err;
>   }
> @@ -1494,9 +1534,7 @@ static int eb_relocate(struct i915_execbuffer *eb)
>   {
>   	int err;
>   
> -	mutex_lock(&eb->gem_context->mutex);
>   	err = eb_lookup_vmas(eb);
> -	mutex_unlock(&eb->gem_context->mutex);
>   	if (err)
>   		return err;
>   
>