[PATCH 2/2] drm/amdgpu: Optimize mutex usage (v2)

[Christian König]

Am 16.06.2017 um 07:03 schrieb Alex Xie:
> Use rw_semaphore instead of mutex for bo_lists.
>
> In original function amdgpu_bo_list_get, the waiting
> for result->lock can be quite long while mutex
> bo_list_lock was holding. It can make other tasks
> waiting for bo_list_lock for long period too.
> Change bo_list_lock to rw_semaphore can avoid most of
> such long waiting.
>
> Secondly, this patch allows several tasks(readers of idr)
> to proceed at the same time.
>
> v2: use rcu and kref (Dave Airlie and Christian König)
>
> Signed-off-by: Alex Xie <AlexBin.Xie at amd.com>
> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu.h         |  2 ++
>   drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c | 40 ++++++++++++++++++++---------
>   2 files changed, 30 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu.h b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> index 063fc73..e9b3981 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu.h
> @@ -871,6 +871,8 @@ struct amdgpu_fpriv {
>   
>   struct amdgpu_bo_list {
>   	struct mutex lock;
> +	struct rcu_head rhead;
> +	struct kref refcount;
>   	struct amdgpu_bo *gds_obj;
>   	struct amdgpu_bo *gws_obj;
>   	struct amdgpu_bo *oa_obj;
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> index 5af956f..efa6903 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_bo_list.c
> @@ -41,6 +41,20 @@ static int amdgpu_bo_list_set(struct amdgpu_device *adev,
>   				     struct drm_amdgpu_bo_list_entry *info,
>   				     unsigned num_entries);
>   
> +static void amdgpu_bo_list_release_rcu(struct kref *ref)
> +{
> +	unsigned i;
> +	struct amdgpu_bo_list *list = container_of(ref, struct amdgpu_bo_list,
> +						   refcount);
> +
> +	for (i = 0; i < list->num_entries; ++i)
> +		amdgpu_bo_unref(&list->array[i].robj);
> +
> +	mutex_destroy(&list->lock);
> +	drm_free_large(list->array);
> +	kfree_rcu(list, rhead);
> +}
> +

I'm probably missing something here: Why a new function and not change 
the existing amdgpu_bo_list_free() to use kfree_rcu instead?

Apart from that the patch looks good to me,
Christian.

>   static int amdgpu_bo_list_create(struct amdgpu_device *adev,
>   				 struct drm_file *filp,
>   				 struct drm_amdgpu_bo_list_entry *info,
> @@ -57,7 +71,7 @@ static int amdgpu_bo_list_create(struct amdgpu_device *adev,
>   
>   	/* initialize bo list*/
>   	mutex_init(&list->lock);
> -
> +	kref_init(&list->refcount);
>   	r = amdgpu_bo_list_set(adev, filp, list, info, num_entries);
>   	if (r) {
>   		kfree(list);
> @@ -83,14 +97,9 @@ static void amdgpu_bo_list_destroy(struct amdgpu_fpriv *fpriv, int id)
>   
>   	mutex_lock(&fpriv->bo_list_lock);
>   	list = idr_remove(&fpriv->bo_list_handles, id);
> -	if (list) {
> -		/* Another user may have a reference to this list still */
> -		mutex_lock(&list->lock);
> -		mutex_unlock(&list->lock);
> -		amdgpu_bo_list_free(list);
> -	}
> -
>   	mutex_unlock(&fpriv->bo_list_lock);
> +	if (list)
> +		kref_put(&list->refcount, amdgpu_bo_list_release_rcu);
>   }
>   
>   static int amdgpu_bo_list_set(struct amdgpu_device *adev,
> @@ -185,11 +194,17 @@ amdgpu_bo_list_get(struct amdgpu_fpriv *fpriv, int id)
>   {
>   	struct amdgpu_bo_list *result;
>   
> -	mutex_lock(&fpriv->bo_list_lock);
> +	rcu_read_lock();
>   	result = idr_find(&fpriv->bo_list_handles, id);
> -	if (result)
> -		mutex_lock(&result->lock);
> -	mutex_unlock(&fpriv->bo_list_lock);
> +
> +	if (result) {
> +		if (kref_get_unless_zero(&result->refcount))
> +			mutex_lock(&result->lock);
> +		else
> +			result = NULL;
> +	}
> +	rcu_read_unlock();
> +
>   	return result;
>   }
>   
> @@ -227,6 +242,7 @@ void amdgpu_bo_list_get_list(struct amdgpu_bo_list *list,
>   void amdgpu_bo_list_put(struct amdgpu_bo_list *list)
>   {
>   	mutex_unlock(&list->lock);
> +	kref_put(&list->refcount, amdgpu_bo_list_release_rcu);
>   }
>   
>   void amdgpu_bo_list_free(struct amdgpu_bo_list *list)