I upgraded KDE to Plasma-workspaces version 5. (Gentoo system)
Now, sometimes when my system has been idle for a while, it appears
"unresponsive" when I come back (screen doesn't wake up on mouse, mouse pointer
doesn't work if I switch off screen sleep, no reaction to any keys, etc.). The
system doesn't crash, but the nouveau graphics output does, and I cannot reach
a console using the keyboard, either.

I found the below output in /var/log/messages (a number of instances of
observing the bug; ... indicates that I had to restart the system).

Versions used:
x11-drivers/xf86-video-nouveau-1.0.11
x11-base/xorg-server-1.16.4
media-libs/mesa-10.3.7-r1
x11-libs/libdrm-2.4.59
x11-base/xorg-drivers-1.16
Kernel: 4.0.5 (self-compiled)

I filed a bug against Plasma 5 but I'm not sure if it's their bug. See
https://bugs.kde.org/show_bug.cgi?id=352605

I also have an apitrace record of plasmashell, replaying which (like a mini
film) also freezes the nouveau. But it's a rather large file (1.4 GB) and
contains private photos.

From /var/log/messages:

Sep  6 16:31:09 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] read fault
at 0x0004042000 [PTE] from GR/GPC0/T1_1 on channel 0x0
03f7af000 [plasmashell[6790]]
Sep  6 16:31:09 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 10, recovering...
Sep  6 16:33:20 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] read fault
at 0x0000023000 [PTE] from PBDMA0/HOST_CPU on channel 
0x003fbe0000 [unknown]
[...]
Sep  7 23:30:25 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] write fault
at 0x0003d49000 [PTE] from GR/GPC0/PROP_0 on channel 
0x003f8ef000 [plasmashell[19341]]
Sep  7 23:30:25 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 8, recovering...
[...]
Sep  8 23:21:29 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] write fault
at 0x0003e02000 [PTE] from GR/GPC0/PROP_0 on channel 
0x003f8ef000 [plasmashell[4952]]
Sep  8 23:21:29 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 8, recovering...
[...]
Sep 10 02:00:03 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] read fault
at 0x0001740000 [PTE] from GR/GPC0/T1_0 on channel 0x0
03f8ef000 [plasmashell[4709]]
Sep 10 02:00:03 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 8, recovering...
[...]
Sep 11 03:07:57 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] read fault
at 0x0003508000 [PTE] from GR/GPC0/T1_0 on channel 0x0
03f84f000 [plasmashell[6720]]
Sep 11 03:07:57 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 9, recovering...
[...]
Sep 12 00:19:50 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] write fault
at 0x0003ef5000 [PTE] from GR/GPC0/PROP_0 on channel 
0x003facd000 [plasmashell[20804]]
Sep 12 00:19:50 localhost kernel: nouveau E[   PFIFO][0000:02:00.0] PGR engine
fault on channel 5, recovering...