Can not use GPU for cycles rendering

[thongpv87]

I have this script to enable GPU render on my machine (Nvidia GTX 1080 Ti with lastest driver and CUDA toolkit):

import bpy
scene=bpy.context.scene
scene.render.filepath="//render/IMG.PNG"
scene.render.image_settings.file_format="PNG"
scene.render.engine="CYCLES"
bpy.context.scene.cycles.samples=50
scene.cycles.tile_order="BOTTOM_TO_TOP"
scene.cycles.debug_use_spatial_splits=True
scene.cycles.use_progressive_refine=False
scene.render.use_save_buffers=False
scene.render.threads_mode="AUTO"
scene.render.tile_x=16
scene.render.tile_y=16
scene.render.resolution_x=1920
scene.render.resolution_y=1080
scene.render.resolution_percentage=100
scene.frame_set(1)
scene.render.use_compositing=False
scene.render.use_sequencer=False
scene.render.engine="CYCLES"
gpus=[gpu for gpu in bpy.context.user_preferences.addons["cycles"].preferences.devices]
for gpu in gpus:
    if gpu.type == "CUDA":
        gpu.use=True
        print('Use GPU: ', gpu.name)
    else:
        gpu.use=False
        print('Disable GPU: ', gpu.name)
bpy.ops.render.render(write_still=True)

I run blender with command:
blender -b pavillon_barcelone_v1.2.blend -P script.py

It show me
Disable GPU: Intel Core Processor (Broadwell) Use GPU: GeForce GTX 1080 Ti

But when I check with nvidia-smi tool, I found that blender did not use my GPU for rendering.

`$ nvidia-smi
Mon Apr 2 12:43:16 2018
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 387.26 Driver Version: 387.26 |
|-------------------------------+----------------------+----------------------+
| GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC |
| Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. |
|===============================+======================+======================|
| 0 GeForce GTX 108... Off | 00000000:05:00.0 Off | N/A |
| 21% 26C P0 57W / 250W | 18MiB / 11172MiB | 0% Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes: GPU Memory |
| GPU PID Type Process name Usage |
|=============================================================================|
| No running processes found |
+-----------------------------------------------------------------------------+
`

My blender version is v2.79