kernelmanagerd using 50% of the cpu constantly
I found the cause of the problem, it is Karabiner-Elements.
Funny thing was I have installed Karabiner-Elements in my mac when it ran macOS High Sierra but I immediately uninstalled (I was trying something with it) but some residue was left behind.
Today when exploring the activity monitor, I noticed some karabiner process was there, so i installed the latest version of Karabiner. Instantly i noticed kernelmanagerd CPU usage became 0.0.
Then I uninstalled Karabiner as in this document
Now my MacBook Pro is normal.