Just encounter a strange high cpu problem of kernel_task in Sierra on my rMBP. It means that the kernel_task process is having more than 80% CPU usage in the process monitor. At first, it seems like some software problems, especially when atom editor is opened with a super long starting time. After closing atom, it does lower the cpu for a period, but running another software afterwards will also boost the kernel_task cpu percentage to 80%. It strongly suggests some underlying hardware problems there.
Comments in this post give a reason: temperature and fan control. Since it’s now summer and super hot, the 10.12 OS X on my mbp is quite possibly reducing cpu speed for temperature compensation, causing the kernel task taking more cpu to consume. Resetting SMC in this article brings the fan control back and fixes the kernel_task cpu problem.
And installing some fan speed monitor/control software is also a good idea to keep an eye on it.