Comment 51 for bug 1671360

OK - I found a good mention of the problem here. He says “There's a bug relating to the Gaming 5 board where dmesg gets flooded with errors about interrupts unless you disable CONFIG_PINCTRL_AMD.”
https://www.reddit.com/r/VFIO/comments/63igfa/horrible_amd_ryzen_performance/

That prompted me to compile a new kernel without the pinctrl-amd option. That option is found when in the kernel configuration utility makeconfig, in the section called Device Drivers/Pin control/AMD GPIO pin control.

I used the compiling instructions found here:
http://askubuntu.com/questions/163298/whats-a-simple-way-to-recompile-the-kernel

I used GIT to download the latest kernel source (4.11.0-rc6) I made no changes to the default kernel configuration other than to check off the pinctrl-amd option. Compiling took over 2 hours, running on only one core!

The new kernel unstalled with no problems. acpi=off is no longer necessary to boot, and 16 cores are visible and active in KDE sysguard. No crashes.

After installing the new kernel, I tried compiling with the multicore switch, 'make -j 16'. It's pretty impressive to see 16 threads running at 97%.

Of course, this is just a temporary fix.