Just encounter strange high cpu problem of kernel_task in Sierra on my rMBP. At first, it seems like some software problem, especially when opening atom editor, which causes long opening time. After closing atom, it does lower the cpu for a period, but running other task heavy software afterwards will also boost the kernel_task cpu percentage to 80%. It may suggest some underlying hardware problem there.
Comments in this post give the reason: temperature and fan control. Since it’s summer and super hot, the osx on my mbp is reducing cpu speed for temperature compensation, causing kernel task to take more cpu to consume. Resetting SMC in this article brings the fan control back and fixes the kernel_task cpu problem.