I'm still seeing this problem in stock amd64 16.04 (kernel 4.4.0-21):

Apr 28 13:46:37 rincewind kernel: [  108.345612] NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [QQmlThread:1159]
Apr 28 13:46:37 rincewind kernel: [  108.347049] Modules linked in: pci_stub vboxpci(OE) vboxnetadp(OE) vboxnetflt(OE) vboxdrv(OE) ftdi_sio pl2303 usbserial snd_usb_audio snd_usbmidi_lib snd_hda_codec_hdmi nvidia_uvm(POE) coretemp kvm_intel snd_hda_codec_realtek snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep kvm snd_pcm snd_seq_midi snd_seq_midi_event snd_rawmidi snd_seq snd_seq_device snd_timer snd gpio_ich dcdbas dell_smm_hwmon input_leds shpchp soundcore serio_raw mei_me mei irqbypass i7core_edac 8250_fintek edac_core lpc_ich mac_hid parport_pc ppdev lp parport autofs4 hid_generic usbhid hid broadcom bcm_phy_lib nvidia_drm(POE) nvidia_modeset(POE) nvidia(POE) drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops tg3 drm ahci ptp libahci pps_core fjes
Apr 28 13:46:37 rincewind kernel: [  108.347082] CPU: 0 PID: 1159 Comm: QQmlThread Tainted: P           OE   4.4.0-21-generic #37-Ubuntu
Apr 28 13:46:37 rincewind kernel: [  108.347084] Hardware name: Dell Inc. Vostro 430/054KM3, BIOS 2.2.0 07/06/2010
Apr 28 13:46:37 rincewind kernel: [  108.347085] task: ffff8804264b8000 ti: ffff880429d98000 task.ti: ffff880429d98000
Apr 28 13:46:37 rincewind kernel: [  108.347086] RIP: 0010:[<ffffffff81823f35>]  [<ffffffff81823f35>] _raw_spin_unlock_irqrestore+0x15/0x20
Apr 28 13:46:37 rincewind kernel: [  108.347092] RSP: 0018:ffff880429d9b910  EFLAGS: 00000282
Apr 28 13:46:37 rincewind kernel: [  108.347093] RAX: 0000000000000009 RBX: 0000000000000000 RCX: 0000000000000051
Apr 28 13:46:37 rincewind kernel: [  108.347094] RDX: ffffffffc0621d16 RSI: 0000000000000282 RDI: 0000000000000282
Apr 28 13:46:37 rincewind kernel: [  108.347095] RBP: ffff880429d9b910 R08: 0000000000000000 R09: 0000000000000020
Apr 28 13:46:37 rincewind kernel: [  108.347096] R10: ffff880427f9df18 R11: ffffffffc0651de0 R12: ffffffffc0a5a638
Apr 28 13:46:37 rincewind kernel: [  108.347097] R13: 00000000ffffffff R14: 0000000000000001 R15: 0000000000000001
Apr 28 13:46:37 rincewind kernel: [  108.347099] FS:  00007f10badef700(0000) GS:ffff88043fc00000(0000) knlGS:0000000000000000
Apr 28 13:46:37 rincewind kernel: [  108.347100] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Apr 28 13:46:37 rincewind kernel: [  108.347101] CR2: 0000000001154a70 CR3: 0000000001e0a000 CR4: 00000000000006f0
Apr 28 13:46:37 rincewind kernel: [  108.347102] Stack:
Apr 28 13:46:37 rincewind kernel: [  108.347103]  ffff880429d9b920 ffffffffc015ee4a ffff880427f9df40 ffffffffc063b959
Apr 28 13:46:37 rincewind kernel: [  108.347104]  ffff8800b9034008 ffff880429064008 0000000000000000 ffff88042ac1c2a0
Apr 28 13:46:37 rincewind kernel: [  108.347106]  00000000c1d00060 ffffffffc0621d16 ffffffffc09b8b08 00000000c1d00060
Apr 28 13:46:37 rincewind kernel: [  108.347108] Call Trace:
Apr 28 13:46:37 rincewind kernel: [  108.347209]  [<ffffffffc015ee4a>] os_release_spinlock+0x1a/0x20 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347295]  [<ffffffffc063b959>] _nv016600rm+0x5f9/0x6e0 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347383]  [<ffffffffc0621d16>] ? _nv000746rm+0x596/0x2940 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347470]  [<ffffffffc0621629>] ? _nv000710rm+0x1b09/0x1bc0 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347558]  [<ffffffffc0621732>] ? _nv016605rm+0x52/0x80 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347650]  [<ffffffffc05f6d35>] ? _nv000141rm+0x55/0x70 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347727]  [<ffffffffc068d853>] ? _nv000734rm+0x103/0x340 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347805]  [<ffffffffc0681bda>] ? rm_disable_adapter+0x6a/0x130 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347810]  [<ffffffff810c9d00>] ? down_trylock+0x10/0x40
Apr 28 13:46:37 rincewind kernel: [  108.347866]  [<ffffffffc0161f73>] ? nv_uvm_notify_stop_device+0x63/0x80 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347920]  [<ffffffffc015377d>] ? nv_close_device+0xed/0x130 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347974]  [<ffffffffc0157157>] ? nvidia_dev_put+0x27/0x40 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.347991]  [<ffffffffc0bc6b17>] ? nvkms_close_gpu+0x57/0x70 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348002]  [<ffffffffc0bfdaa5>] ? _nv001953kms+0xf5/0x180 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348010]  [<ffffffffc0bdae7d>] ? _nv001859kms+0x3d/0x80 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348017]  [<ffffffffc0bc8253>] ? _nv000189kms+0x53/0x130 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348025]  [<ffffffffc0bc83ac>] ? nvKmsClose+0x7c/0x120 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348032]  [<ffffffffc0bc6cd1>] ? nvkms_close_common+0x21/0x60 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348040]  [<ffffffffc0bc6d2a>] ? nvkms_close+0x1a/0x30 [nvidia_modeset]
Apr 28 13:46:37 rincewind kernel: [  108.348093]  [<ffffffffc015339c>] ? nvidia_frontend_close+0x2c/0x50 [nvidia]
Apr 28 13:46:37 rincewind kernel: [  108.348096]  [<ffffffff8120e514>] ? __fput+0xe4/0x220
Apr 28 13:46:37 rincewind kernel: [  108.348098]  [<ffffffff8120e68e>] ? ____fput+0xe/0x10
Apr 28 13:46:37 rincewind kernel: [  108.348101]  [<ffffffff8109e943>] ? task_work_run+0x73/0x90
Apr 28 13:46:37 rincewind kernel: [  108.348104]  [<ffffffff81083ba4>] ? do_exit+0x2e4/0xae0
Apr 28 13:46:37 rincewind kernel: [  108.348106]  [<ffffffff81084423>] ? do_group_exit+0x43/0xb0
Apr 28 13:46:37 rincewind kernel: [  108.348108]  [<ffffffff810904d2>] ? get_signal+0x292/0x600
Apr 28 13:46:37 rincewind kernel: [  108.348111]  [<ffffffff8102e517>] ? do_signal+0x37/0x6f0
Apr 28 13:46:37 rincewind kernel: [  108.348114]  [<ffffffff81102e3e>] ? do_futex+0x34e/0x500
Apr 28 13:46:37 rincewind kernel: [  108.348117]  [<ffffffff811c8df4>] ? mprotect_fixup+0x154/0x240
Apr 28 13:46:37 rincewind kernel: [  108.348120]  [<ffffffff8100320c>] ? exit_to_usermode_loop+0x8c/0xd0
Apr 28 13:46:37 rincewind kernel: [  108.348122]  [<ffffffff81003c5e>] ? syscall_return_slowpath+0x4e/0x60
Apr 28 13:46:37 rincewind kernel: [  108.348124]  [<ffffffff81824650>] ? int_ret_from_sys_call+0x25/0x8f
Apr 28 13:46:37 rincewind kernel: [  108.348125] Code: 90 66 66 90 eb c6 31 c0 eb ca e8 27 ce 85 ff 90 90 90 90 90 90 90 66 66 66 66 90 55 48 89 e5 c6 07 00 66 66 66 90 48 89 f7 57 9d <66> 66 90 66 90 5d c3 0f 1f 40 00 66 66 66 66 90 55 48 89 e5 c6 

Am now running 4.5.2 to see if the problem persists.