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

[axie]

On 2017-06-16 01:48 PM, Christian König wrote:
> 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.

amdgpu_bo_list_free() is called by amdgpu_driver_postclose_kms in file 
amdgpu_kms.c too.
For amdgpu_driver_postclose_kms, there is no need to call kfree_rcu. It 
is more suitable to call kfree.

So we need two different release/free functions. Then I created a new 
function for kref_put.

>
>>   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)
>
>