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

["Christian König" <christian.koenig@amd.com>]

On 6/11/25 17:29, Tvrtko Ursulin wrote:
> 
> 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.

Yeah, it's mostly for extra security I think.

> 
> 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?

Like a macro which uses an array on the stack for small (<4) number of values and k(v)malloc() for large ones.

IIRC there is also a relatively new functionality which allows releasing the memory automatically when we leave the function.

Regards,
Christian.

> 
> 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;
>>>   }
>>
>