Re: [PATCH 10/42] drm/i915: Defer active reference until required

[Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>]

On 07/10/2016 10:46, Chris Wilson wrote:
> 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?

I really dislike this.  Where there was elegance with obj/vma_put, it is 
now replaced with out of place looking 
__i915_gem_object_release_unless_active. I don't see why would higher 
level layers have to concern themselves with calling something with such 
a low-level sounding name.

How much does this influence performance and in what cases? If 
significant, could we try to come up with something similar but more 
elegant?

Regards,

Tvrtko

> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Joonas Lahtinen <joonas.lahtinen@xxxxxxxxxxxxxxx>
> ---
>   drivers/gpu/drm/i915/i915_drv.h              | 28 ++++++++++++++++++++++++++++
>   drivers/gpu/drm/i915/i915_gem.c              | 22 +++++++++++++++++++++-
>   drivers/gpu/drm/i915/i915_gem_batch_pool.c   |  2 +-
>   drivers/gpu/drm/i915/i915_gem_context.c      |  2 +-
>   drivers/gpu/drm/i915/i915_gem_execbuffer.c   |  2 --
>   drivers/gpu/drm/i915/i915_gem_gtt.c          |  7 ++++++-
>   drivers/gpu/drm/i915/i915_gem_render_state.c |  3 ++-
>   drivers/gpu/drm/i915/intel_ringbuffer.c      | 15 ++++++++++++---
>   8 files changed, 71 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
> index ee25e265416f..fee5cc92e2f2 100644
> --- a/drivers/gpu/drm/i915/i915_drv.h
> +++ b/drivers/gpu/drm/i915/i915_drv.h
> @@ -2232,6 +2232,12 @@ struct drm_i915_gem_object {
>   	((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
>   	 */
> @@ -2399,6 +2405,28 @@ 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)
>   {
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 7fa5cb764739..b560263bf446 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -2618,7 +2618,10 @@ 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)
> @@ -2889,6 +2892,12 @@ 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);
>   }
>   
> @@ -4365,6 +4374,17 @@ 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);
> diff --git a/drivers/gpu/drm/i915/i915_gem_batch_pool.c b/drivers/gpu/drm/i915/i915_gem_batch_pool.c
> index ed989596d9a3..cb25cad3318c 100644
> --- a/drivers/gpu/drm/i915/i915_gem_batch_pool.c
> +++ b/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]);
>   	}
> diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
> index df10f4e95736..1d2ab73a8f43 100644
> --- a/drivers/gpu/drm/i915/i915_gem_context.c
> +++ b/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);
> diff --git a/drivers/gpu/drm/i915/i915_gem_execbuffer.c b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> index 72c7c1855e70..0deecd4e3b6c 100644
> --- a/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> +++ b/drivers/gpu/drm/i915/i915_gem_execbuffer.c
> @@ -1299,8 +1299,6 @@ 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);
>   
> diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
> index 2d846aa39ca5..1c95da8424cb 100644
> --- a/drivers/gpu/drm/i915/i915_gem_gtt.c
> +++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
> @@ -3712,11 +3712,16 @@ 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);
>   }
> diff --git a/drivers/gpu/drm/i915/i915_gem_render_state.c b/drivers/gpu/drm/i915/i915_gem_render_state.c
> index 95b7e9afd5f8..09cf4874c45f 100644
> --- a/drivers/gpu/drm/i915/i915_gem_render_state.c
> +++ b/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;
>   }
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
> index b60c6f09fbfd..f3dfb7ca625d 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.c
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
> @@ -1763,14 +1763,19 @@ 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)
> @@ -1968,7 +1973,11 @@ 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);
>   }
>   

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