Re: [PATCH v5 6/6] drm/syncobj: Add a fast path to drm_syncobj_array_find

[Tvrtko Ursulin <tvrtko.ursulin@igalia.com>]

On 11/06/2025 15:21, Christian König wrote:
> On 6/11/25 16:00, Tvrtko Ursulin wrote:
>> Running the Cyberpunk 2077 benchmark we can observe that the lookup helper
>> is relatively hot, but the 97% of the calls are for a single object. (~3%
>> for two points, and never more than three points. While a more trivial
>> workload like vkmark under Plasma is even more skewed to single point
>> lookups.)
>>
>> Therefore lets add a fast path to bypass the kmalloc_array/kfree and use a
>> pre-allocated stack array for those cases.
> 
> Have you considered using memdup_user()? That's using a separate bucket IIRC and might give similar performance.

I haven't but I can try it. I would be surprised if it made a (positive) 
difference though.

And I realised I need to repeat the benchmarks anyway, since in v4 I had 
to stop doing access_ok+__get_user, after kernel test robot let me know 
64-bit get_user is a not a thing on all platforms. I thought the gains 
are from avoiding allocations but, as you say, now I need to see if 
copy_from_user doesn't nullify them..

> If that is still not sufficient I'm really wondering if we shouldn't have a macro for doing this. It's a really common use case as far as I can see.

Hmm macro for what exactly?

Regards,

Tvrtko

>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
>> Reviewed-by: Maíra Canal <mcanal@igalia.com>
>> ---
>> v2:
>>   * Added comments describing how the fast path arrays were sized.
>>   * Make container freeing criteria clearer by using a boolean.
>> ---
>>   drivers/gpu/drm/drm_syncobj.c | 56 +++++++++++++++++++++++++++--------
>>   1 file changed, 44 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
>> index be5905dca87f..65c301852f0d 100644
>> --- a/drivers/gpu/drm/drm_syncobj.c
>> +++ b/drivers/gpu/drm/drm_syncobj.c
>> @@ -1259,6 +1259,8 @@ EXPORT_SYMBOL(drm_timeout_abs_to_jiffies);
>>   static int drm_syncobj_array_find(struct drm_file *file_private,
>>   				  u32 __user *handles,
>>   				  uint32_t count,
>> +				  struct drm_syncobj **stack_syncobjs,
>> +				  u32 stack_count,
>>   				  struct drm_syncobj ***syncobjs_out)
>>   {
>>   	struct drm_syncobj **syncobjs;
>> @@ -1268,9 +1270,13 @@ static int drm_syncobj_array_find(struct drm_file *file_private,
>>   	if (!access_ok(handles, count * sizeof(*handles)))
>>   		return -EFAULT;
>>   
>> -	syncobjs = kmalloc_array(count, sizeof(*syncobjs), GFP_KERNEL);
>> -	if (!syncobjs)
>> -		return -ENOMEM;
>> +	if (count > stack_count) {
>> +		syncobjs = kmalloc_array(count, sizeof(*syncobjs), GFP_KERNEL);
>> +		if (!syncobjs)
>> +			return -ENOMEM;
>> +	} else {
>> +		syncobjs = stack_syncobjs;
>> +	}
>>   
>>   	for (i = 0; i < count; i++) {
>>   		u32 handle;
>> @@ -1292,25 +1298,31 @@ static int drm_syncobj_array_find(struct drm_file *file_private,
>>   err_put_syncobjs:
>>   	while (i-- > 0)
>>   		drm_syncobj_put(syncobjs[i]);
>> -	kfree(syncobjs);
>> +
>> +	if (syncobjs != stack_syncobjs)
>> +		kfree(syncobjs);
>>   
>>   	return ret;
>>   }
>>   
>>   static void drm_syncobj_array_free(struct drm_syncobj **syncobjs,
>> -				   uint32_t count)
>> +				   uint32_t count,
>> +				   bool free_container)
>>   {
>>   	uint32_t i;
>>   
>>   	for (i = 0; i < count; i++)
>>   		drm_syncobj_put(syncobjs[i]);
>> -	kfree(syncobjs);
>> +
>> +	if (free_container)
>> +		kfree(syncobjs);
>>   }
>>   
>>   int
>>   drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
>>   		       struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_wait *args = data;
>>   	ktime_t deadline, *pdeadline = NULL;
>>   	u32 count = args->count_handles;
>> @@ -1336,6 +1348,8 @@ drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     count,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1354,7 +1368,7 @@ drm_syncobj_wait_ioctl(struct drm_device *dev, void *data,
>>   						 &first,
>>   						 pdeadline);
>>   
>> -	drm_syncobj_array_free(syncobjs, count);
>> +	drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
>>   
>>   	if (timeout < 0)
>>   		return timeout;
>> @@ -1368,6 +1382,7 @@ int
>>   drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, void *data,
>>   				struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_timeline_wait *args = data;
>>   	ktime_t deadline, *pdeadline = NULL;
>>   	u32 count = args->count_handles;
>> @@ -1394,6 +1409,8 @@ drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     count,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1412,7 +1429,7 @@ drm_syncobj_timeline_wait_ioctl(struct drm_device *dev, void *data,
>>   						 &first,
>>   						 pdeadline);
>>   
>> -	drm_syncobj_array_free(syncobjs, count);
>> +	drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
>>   
>>   	if (timeout < 0)
>>   		return timeout;
>> @@ -1529,6 +1546,7 @@ int
>>   drm_syncobj_reset_ioctl(struct drm_device *dev, void *data,
>>   			struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_array *args = data;
>>   	struct drm_syncobj **syncobjs;
>>   	uint32_t i;
>> @@ -1546,6 +1564,8 @@ drm_syncobj_reset_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     args->count_handles,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1553,7 +1573,8 @@ drm_syncobj_reset_ioctl(struct drm_device *dev, void *data,
>>   	for (i = 0; i < args->count_handles; i++)
>>   		drm_syncobj_replace_fence(syncobjs[i], NULL);
>>   
>> -	drm_syncobj_array_free(syncobjs, args->count_handles);
>> +	drm_syncobj_array_free(syncobjs, args->count_handles,
>> +			       syncobjs != stack_syncobjs);
>>   
>>   	return 0;
>>   }
>> @@ -1562,6 +1583,7 @@ int
>>   drm_syncobj_signal_ioctl(struct drm_device *dev, void *data,
>>   			 struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_array *args = data;
>>   	struct drm_syncobj **syncobjs;
>>   	uint32_t i;
>> @@ -1579,6 +1601,8 @@ drm_syncobj_signal_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     args->count_handles,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1589,7 +1613,8 @@ drm_syncobj_signal_ioctl(struct drm_device *dev, void *data,
>>   			break;
>>   	}
>>   
>> -	drm_syncobj_array_free(syncobjs, args->count_handles);
>> +	drm_syncobj_array_free(syncobjs, args->count_handles,
>> +			       syncobjs != stack_syncobjs);
>>   
>>   	return ret;
>>   }
>> @@ -1598,6 +1623,7 @@ int
>>   drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>>   				  struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_timeline_array *args = data;
>>   	uint64_t __user *points = u64_to_user_ptr(args->points);
>>   	uint32_t i, j, count = args->count_handles;
>> @@ -1617,6 +1643,8 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     count,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1653,7 +1681,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>>   err_chains:
>>   	kfree(chains);
>>   out:
>> -	drm_syncobj_array_free(syncobjs, count);
>> +	drm_syncobj_array_free(syncobjs, count, syncobjs != stack_syncobjs);
>>   
>>   	return ret;
>>   }
>> @@ -1661,6 +1689,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data,
>>   int drm_syncobj_query_ioctl(struct drm_device *dev, void *data,
>>   			    struct drm_file *file_private)
>>   {
>> +	struct drm_syncobj *stack_syncobjs[DRM_SYNCOBJ_FAST_PATH_ENTRIES];
>>   	struct drm_syncobj_timeline_array *args = data;
>>   	struct drm_syncobj **syncobjs;
>>   	uint64_t __user *points = u64_to_user_ptr(args->points);
>> @@ -1679,6 +1708,8 @@ int drm_syncobj_query_ioctl(struct drm_device *dev, void *data,
>>   	ret = drm_syncobj_array_find(file_private,
>>   				     u64_to_user_ptr(args->handles),
>>   				     args->count_handles,
>> +				     stack_syncobjs,
>> +				     ARRAY_SIZE(stack_syncobjs),
>>   				     &syncobjs);
>>   	if (ret < 0)
>>   		return ret;
>> @@ -1722,7 +1753,8 @@ int drm_syncobj_query_ioctl(struct drm_device *dev, void *data,
>>   		if (ret)
>>   			break;
>>   	}
>> -	drm_syncobj_array_free(syncobjs, args->count_handles);
>> +	drm_syncobj_array_free(syncobjs, args->count_handles,
>> +			       syncobjs != stack_syncobjs);
>>   
>>   	return ret;
>>   }
>