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On a Windows 10 16299.192 desktop system task manager always shows 99% usage. If I try killing the highest usage program all others suddenly show their CPU usage increasing to fill the gap, as if the CPU usage is being calculated as % of CPU cycles currently used instead of % CPU cycles available like normal.
I tried editing a registry key to allow me to disable the processor idle state (as has worked in the past) but no change even after a reboot.
Process Explorer does show the correct CPU usage numbers, with System Idle Process filling in the remainder of (100% - Current Usage) like normal.
Intel i3-4170 CPU. Let me know if there's any more information you might need.