I've been seeing constant CPU utilization on my AMD A10-7800:
23991 root 20 0 0 0 0 R 68.2 0.0 547:48.94 kworker/3:2
29 root 20 0 0 0 0 S 31.5 0.0 254:01.85 ksoftirqd/3

I tracked down an extremely recent bug report in Ubuntu's Launchpad.

My system differs from it on software quite a bit, and this bug appears to go back at least to 3.19 since that is the one being patched.

My system:
cat /proc/version
Linux version 4.1.6-200.fc22.x86_64 ([email protected]) (gcc version 5.1.1 20150618 (Red Hat 5.1.1-4) (GCC) ) #1 SMP Mon Aug 17 19:54:31 UTC 2015

cat /etc/redhat-release
Fedora release 22 (Twenty Two)

Results seem to be from perf, usage for perf is detailed in an answer (3rd one at time of post) here:

Michael et al, have you seen anything like this on your kernel tests (it may require something using/attempting to use USB 3.0)?

Additionally, adding a modprobe conf file seems to temporarily remove the problem by disabling power management on XHCI (and probably EHCI/UHCI/OHCI):

cat /etc/modprobe.d/usbdisablepm.conf
options usbcore autosuspend=0