Hi all,

We were having issue with our userspace application which
__sometimes__ result in high CPU sys usage at each execution. The high
sys CPU usage persist until the application is killed.

We simplified the application to just creating a timer and its handler
then does nothing, but looping and sleeping for the timer to be
triggered. With top, the CPU that running the application usually
takes almost 0% for sys usage. But sometimes it will occupy certain
amount of sys usage, up to 100% at most of the time on my embedded
device.

On my laptop, Intel Core i5-4200U, running Ubuntu 14.02.2, Linux
3.13.0-45-generic and 4.4.0-34-generic, the issue can be reproduced
with lower sys usage (7~50%). The same can be reproduced with
buildroot+vanilla kernel 4.7 and 3.13.

Restart the application could temporarily fix the the issue, but there
are chances to happen again.

Googling found one issue that seems related, but no further action.

Keystone II Linux: Random High CPU usage - userspace application using
1 full core - Linux forum - Linux - TI E2E Community
http://e2e.ti.com/support/embedded/linux/f/354/p/433791/1553204

I've checked the /proc/timer_stats, /proc/interrupts, and perf, all
the irq counter, timer counter, timer/irq event didn't show any
abnormal value or useful clue. I'm looking forward for any suggestion
desperately.

Attached the test code and log, and script to detect the issue, which
require dstat and taskset.

Best Regards,
Mac Lin