Here is the hung task:
Apr 30 15:21:50 michael-desktop-ubuntu kernel: INFO: task nvidia-modeset:243 blocked for more than 120 seconds.
Apr 30 15:21:50 michael-desktop-ubuntu kernel: Tainted: P IOE 4.15.0-20-generic #21-Ubuntu
Apr 30 15:21:50 michael-desktop-ubuntu kernel: "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Apr 30 15:21:50 michael-desktop-ubuntu kernel: nvidia-modeset D 0 243 2 0x80000000
Apr 30 15:21:50 michael-desktop-ubuntu kernel: Call Trace:
Apr 30 15:21:50 michael-desktop-ubuntu kernel: __schedule+0x297/0x8b0
Apr 30 15:21:50 michael-desktop-ubuntu kernel: schedule+0x2c/0x80
Apr 30 15:21:50 michael-desktop-ubuntu kernel: schedule_timeout+0x1cf/0x350
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ? schedule_timeout+0x1cf/0x350
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ? __slab_free+0x14d/0x2c0
Apr 30 15:21:50 michael-desktop-ubuntu kernel: __down+0x91/0xe0
Apr 30 15:21:50 michael-desktop-ubuntu kernel: down+0x41/0x50
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ? down+0x41/0x50
Apr 30 15:21:50 michael-desktop-ubuntu kernel: nvkms_kthread_q_callback+0x65/0xe0 [nvidia_modeset]
Apr 30 15:21:50 michael-desktop-ubuntu kernel: _main_loop+0x76/0x140 [nvidia]
Apr 30 15:21:50 michael-desktop-ubuntu kernel: kthread+0x121/0x140
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ? _raw_q_schedule+0x80/0x80 [nvidia]
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ? kthread_create_worker_on_cpu+0x70/0x70
Apr 30 15:21:50 michael-desktop-ubuntu kernel: ret_from_fork+0x35/0x40
uname:
Linux michael-desktop-ubuntu 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
Drivers:
ii libnvidia-cfg1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA binary OpenGL/GLX configuration library
ii libnvidia-common-390 390.48-0ubuntu3 all Shared files used by the NVIDIA libraries
ii libnvidia-compute-390:amd64 390.48-0ubuntu3 amd64 NVIDIA libcompute package
ii libnvidia-compute-390:i386 390.48-0ubuntu3 i386 NVIDIA libcompute package
ii libnvidia-decode-390:amd64 390.48-0ubuntu3 amd64 NVIDIA Video Decoding runtime libraries
ii libnvidia-decode-390:i386 390.48-0ubuntu3 i386 NVIDIA Video Decoding runtime libraries
ii libnvidia-encode-390:amd64 390.48-0ubuntu3 amd64 NVENC Video Encoding runtime library
ii libnvidia-encode-390:i386 390.48-0ubuntu3 i386 NVENC Video Encoding runtime library
ii libnvidia-fbc1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL-based Framebuffer Capture runtime library
ii libnvidia-fbc1-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL-based Framebuffer Capture runtime library
ii libnvidia-gl-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii libnvidia-gl-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL/GLX/EGL/GLES GLVND libraries and Vulkan ICD
ii libnvidia-ifr1-390:amd64 390.48-0ubuntu3 amd64 NVIDIA OpenGL-based Inband Frame Readback runtime library
ii libnvidia-ifr1-390:i386 390.48-0ubuntu3 i386 NVIDIA OpenGL-based Inband Frame Readback runtime library
ii nvidia-compute-utils-390 390.48-0ubuntu3 amd64 NVIDIA compute utilities
ii nvidia-dkms-390 390.48-0ubuntu3 amd64 NVIDIA DKMS package
ii nvidia-driver-390 390.48-0ubuntu3 amd64 NVIDIA driver metapackage
ii nvidia-headless-no-dkms-390 390.48-0ubuntu3 amd64 NVIDIA headless metapackage - no DKMS
ii nvidia-kernel-common-390 390.48-0ubuntu3 amd64 Shared files used with the kernel module
ii nvidia-kernel-source-390 390.48-0ubuntu3 amd64 NVIDIA kernel source package
ii nvidia-prime 0.8.8 all Tools to enable NVIDIA's Prime
ii nvidia-settings 390.42-0ubuntu1 amd64 Tool for configuring the NVIDIA graphics driver
ii nvidia-utils-390 390.48-0ubuntu3 amd64 NVIDIA driver support binaries
ii xserver-xorg-video-nvidia-390 390.48-0ubuntu3 amd64 NVIDIA binary Xorg driver
I can't reliably reproduce, but it happens fairly often after reboot (circa 2-5 minutes).
Apport report attached