Re: [PATCH v2] drm/i915: Track the last-active inside the i915_vma

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 04/07/2018 09:34, Chris Wilson wrote:
> Using a VMA on more than one timeline concurrently is the exception
> rather than the rule (using it concurrently on multiple engines). As we
> expect to only use one active tracker, store the most recently used
> tracker inside the i915_vma itself and only fallback to the rbtree if
> we need a second or more concurrent active trackers.
> 
> v2: Comments on how we overwrite any existing last_active cache.
> 
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>   drivers/gpu/drm/i915/i915_vma.c | 50 +++++++++++++++++++++++++++++++--
>   drivers/gpu/drm/i915/i915_vma.h |  1 +
>   2 files changed, 49 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> index cd94ffc7f079..33925e00f7e8 100644
> --- a/drivers/gpu/drm/i915/i915_vma.c
> +++ b/drivers/gpu/drm/i915/i915_vma.c
> @@ -119,6 +119,12 @@ i915_vma_retire(struct i915_gem_active *base, struct i915_request *rq)
>   	__i915_vma_retire(active->vma, rq);
>   }
>   
> +static void
> +i915_vma_last_retire(struct i915_gem_active *base, struct i915_request *rq)
> +{
> +	__i915_vma_retire(container_of(base, struct i915_vma, last_active), rq);
> +}
> +
>   static struct i915_vma *
>   vma_create(struct drm_i915_gem_object *obj,
>   	   struct i915_address_space *vm,
> @@ -136,6 +142,7 @@ vma_create(struct drm_i915_gem_object *obj,
>   
>   	vma->active = RB_ROOT;
>   
> +	init_request_active(&vma->last_active, i915_vma_last_retire);
>   	init_request_active(&vma->last_fence, NULL);
>   	vma->vm = vm;
>   	vma->ops = &vm->vma_ops;
> @@ -895,6 +902,22 @@ static struct i915_gem_active *lookup_active(struct i915_vma *vma, u64 idx)
>   {
>   	struct i915_vma_active *active;
>   	struct rb_node **p, *parent;
> +	struct i915_request *old;
> +
> +	/*
> +	 * We track the most recently used timeline to skip a rbtree search
> +	 * for the common case, under typical loads we never need the rbtree
> +	 * at all. We can reuse the last_active slot if it is empty, that is
> +	 * after the previous activity has been retired, or if the active
> +	 * matches the current timeline.
> +	 */
> +	old = i915_gem_active_raw(&vma->last_active,
> +				  &vma->vm->i915->drm.struct_mutex);
> +	if (!old || old->fence.context == idx)
> +		goto out;
> +
> +	/* Move the currently active fence into the rbtree */
> +	idx = old->fence.context;
>   
>   	parent = NULL;
>   	p = &vma->active.rb_node;
> @@ -903,7 +926,7 @@ static struct i915_gem_active *lookup_active(struct i915_vma *vma, u64 idx)
>   
>   		active = rb_entry(parent, struct i915_vma_active, node);
>   		if (active->timeline == idx)
> -			return &active->base;
> +			goto replace;
>   
>   		if (active->timeline < idx)
>   			p = &parent->rb_right;
> @@ -922,7 +945,25 @@ static struct i915_gem_active *lookup_active(struct i915_vma *vma, u64 idx)
>   	rb_link_node(&active->node, parent, p);
>   	rb_insert_color(&active->node, &vma->active);
>   
> -	return &active->base;
> +replace:
> +	/*
> +	 * Overwrite the previous active slot in the rbtree with last_active,
> +	 * leaving last_active zeroed. If the previous slot is still active,
> +	 * we must be careful as we now only expect to recieve one retire

typo in receive

> +	 * callback not two, and so much undo the active counting for the
> +	 * overwritten slot.
> +	 */
> +	if (i915_gem_active_isset(&active->base)) {
> +		__list_del_entry(&active->base.link);
> +		vma->active_count--;
 > +		GEM_BUG_ON(!vma->active_count);

I still don't get this. The cache is exclusive, so when transferring a 
record from rbtree to last_active, why do we need to decrement the 
vma->active_count here? Don't get the part in the comment about two 
retires - do you really sometimes expect two - ie cache is not exclusive?

But the fact that lookup of a cached entry is a straight return, meaning 
vma->active_count is manipulated elsewhere, makes me think it is 
avoidable messing with it on this path as well.

Maybe the separation of duties between the callers and this function 
needs to be stronger.

Regards,

Tvrtko

> +	}
> +	GEM_BUG_ON(list_empty(&vma->last_active.link));
> +	list_replace_init(&vma->last_active.link, &active->base.link);
> +	active->base.request = fetch_and_zero(&vma->last_active.request);
> +
> +out:
> +	return &vma->last_active;
>   }
>   
>   int i915_vma_move_to_active(struct i915_vma *vma,
> @@ -1002,6 +1043,11 @@ int i915_vma_unbind(struct i915_vma *vma)
>   		 */
>   		__i915_vma_pin(vma);
>   
> +		ret = i915_gem_active_retire(&vma->last_active,
> +					     &vma->vm->i915->drm.struct_mutex);
> +		if (ret)
> +			goto unpin;
> +
>   		rbtree_postorder_for_each_entry_safe(active, n,
>   						     &vma->active, node) {
>   			ret = i915_gem_active_retire(&active->base,
> diff --git a/drivers/gpu/drm/i915/i915_vma.h b/drivers/gpu/drm/i915/i915_vma.h
> index c297b0a0dc47..f06d66377107 100644
> --- a/drivers/gpu/drm/i915/i915_vma.h
> +++ b/drivers/gpu/drm/i915/i915_vma.h
> @@ -97,6 +97,7 @@ struct i915_vma {
>   
>   	unsigned int active_count;
>   	struct rb_root active;
> +	struct i915_gem_active last_active;
>   	struct i915_gem_active last_fence;
>   
>   	/**
> 
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