drm/i915: Defer active reference until required

We only need the active reference to keep the object alive after the
handle has been deleted (so as to prevent a synchronous gem_close). Why
then pay the price of a kref on every execbuf when we can insert that
final active ref just in time for the handle deletion?
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161028125858.23563-6-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_drv.h

@@ -2246,6 +2246,12 @@ struct drm_i915_gem_object {
 #define __I915_BO_ACTIVE(bo) \
 	((READ_ONCE((bo)->flags) >> I915_BO_ACTIVE_SHIFT) & I915_BO_ACTIVE_MASK)
 
+	/**
+	 * Have we taken a reference for the object for incomplete GPU
+	 * activity?
+	 */
+#define I915_BO_ACTIVE_REF (I915_BO_ACTIVE_SHIFT + I915_NUM_ENGINES)
+
 	/**
 	 * This is set if the object has been written to since last bound
 	 * to the GTT
@@ -2407,6 +2413,28 @@ i915_gem_object_has_active_engine(const struct drm_i915_gem_object *obj,
 	return obj->flags & BIT(engine + I915_BO_ACTIVE_SHIFT);
 }
 
+static inline bool
+i915_gem_object_has_active_reference(const struct drm_i915_gem_object *obj)
+{
+	return test_bit(I915_BO_ACTIVE_REF, &obj->flags);
+}
+
+static inline void
+i915_gem_object_set_active_reference(struct drm_i915_gem_object *obj)
+{
+	lockdep_assert_held(&obj->base.dev->struct_mutex);
+	__set_bit(I915_BO_ACTIVE_REF, &obj->flags);
+}
+
+static inline void
+i915_gem_object_clear_active_reference(struct drm_i915_gem_object *obj)
+{
+	lockdep_assert_held(&obj->base.dev->struct_mutex);
+	__clear_bit(I915_BO_ACTIVE_REF, &obj->flags);
+}
+
+void __i915_gem_object_release_unless_active(struct drm_i915_gem_object *obj);
+
 static inline unsigned int
 i915_gem_object_get_tiling(struct drm_i915_gem_object *obj)
 {

drivers/gpu/drm/i915/i915_gem.c

@@ -2661,7 +2661,10 @@ i915_gem_object_retire__read(struct i915_gem_active *active,
 		list_move_tail(&obj->global_list,
 			       &request->i915->mm.bound_list);
 
-	i915_gem_object_put(obj);
+	if (i915_gem_object_has_active_reference(obj)) {
+		i915_gem_object_clear_active_reference(obj);
+		i915_gem_object_put(obj);
+	}
 }
 
 static bool i915_context_is_banned(const struct i915_gem_context *ctx)
@@ -2935,6 +2938,12 @@ void i915_gem_close_object(struct drm_gem_object *gem, struct drm_file *file)
 	list_for_each_entry_safe(vma, vn, &obj->vma_list, obj_link)
 		if (vma->vm->file == fpriv)
 			i915_vma_close(vma);
+
+	if (i915_gem_object_is_active(obj) &&
+	    !i915_gem_object_has_active_reference(obj)) {
+		i915_gem_object_set_active_reference(obj);
+		i915_gem_object_get(obj);
+	}
 	mutex_unlock(&obj->base.dev->struct_mutex);
 }
 
@@ -4475,6 +4484,17 @@ void i915_gem_free_object(struct drm_gem_object *gem_obj)
 	intel_runtime_pm_put(dev_priv);
 }
 
+void __i915_gem_object_release_unless_active(struct drm_i915_gem_object *obj)
+{
+	lockdep_assert_held(&obj->base.dev->struct_mutex);
+
+	GEM_BUG_ON(i915_gem_object_has_active_reference(obj));
+	if (i915_gem_object_is_active(obj))
+		i915_gem_object_set_active_reference(obj);
+	else
+		i915_gem_object_put(obj);
+}
+
 int i915_gem_suspend(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = to_i915(dev);

drivers/gpu/drm/i915/i915_gem_batch_pool.c

@@ -73,7 +73,7 @@ void i915_gem_batch_pool_fini(struct i915_gem_batch_pool *pool)
 		list_for_each_entry_safe(obj, next,
 					 &pool->cache_list[n],
 					 batch_pool_link)
-			i915_gem_object_put(obj);
+			__i915_gem_object_release_unless_active(obj);
 
 		INIT_LIST_HEAD(&pool->cache_list[n]);
 	}

drivers/gpu/drm/i915/i915_gem_context.c

@@ -155,7 +155,7 @@ void i915_gem_context_free(struct kref *ctx_ref)
 		if (ce->ring)
 			intel_ring_free(ce->ring);
 
-		i915_vma_put(ce->state);
+		__i915_gem_object_release_unless_active(ce->state->obj);
 	}
 
 	put_pid(ctx->pid);

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -1290,8 +1290,6 @@ void i915_vma_move_to_active(struct i915_vma *vma,
 	 * add the active reference first and queue for it to be dropped
 	 * *last*.
 	 */
-	if (!i915_gem_object_is_active(obj))
-		i915_gem_object_get(obj);
 	i915_gem_object_set_active(obj, idx);
 	i915_gem_active_set(&obj->last_read[idx], req);
 

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3734,11 +3734,16 @@ void __iomem *i915_vma_pin_iomap(struct i915_vma *vma)
 void i915_vma_unpin_and_release(struct i915_vma **p_vma)
 {
 	struct i915_vma *vma;
+	struct drm_i915_gem_object *obj;
 
 	vma = fetch_and_zero(p_vma);
 	if (!vma)
 		return;
 
+	obj = vma->obj;
+
 	i915_vma_unpin(vma);
-	i915_vma_put(vma);
+	i915_vma_close(vma);
+
+	__i915_gem_object_release_unless_active(obj);
 }

drivers/gpu/drm/i915/i915_gem_render_state.c

@@ -224,7 +224,8 @@ int i915_gem_render_state_init(struct drm_i915_gem_request *req)
 	i915_vma_move_to_active(so.vma, req, 0);
 err_unpin:
 	i915_vma_unpin(so.vma);
+	i915_vma_close(so.vma);
 err_obj:
-	i915_gem_object_put(obj);
+	__i915_gem_object_release_unless_active(obj);
 	return ret;
 }

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1762,14 +1762,19 @@ static void cleanup_phys_status_page(struct intel_engine_cs *engine)
 static void cleanup_status_page(struct intel_engine_cs *engine)
 {
 	struct i915_vma *vma;
+	struct drm_i915_gem_object *obj;
 
 	vma = fetch_and_zero(&engine->status_page.vma);
 	if (!vma)
 		return;
 
+	obj = vma->obj;
+
 	i915_vma_unpin(vma);
-	i915_gem_object_unpin_map(vma->obj);
-	i915_vma_put(vma);
+	i915_vma_close(vma);
+
+	i915_gem_object_unpin_map(obj);
+	__i915_gem_object_release_unless_active(obj);
 }
 
 static int init_status_page(struct intel_engine_cs *engine)
@@ -1967,7 +1972,11 @@ intel_engine_create_ring(struct intel_engine_cs *engine, int size)
 void
 intel_ring_free(struct intel_ring *ring)
 {
-	i915_vma_put(ring->vma);
+	struct drm_i915_gem_object *obj = ring->vma->obj;
+
+	i915_vma_close(ring->vma);
+	__i915_gem_object_release_unless_active(obj);
+
 	kfree(ring);
 }