When I upgraded to Sierra, it lost all my face names.
It started scanning but little ver half way through, it stopped scanning faces.
You need to laugh Activity Monitor
Kill all the agents under Photos
Kill Photos and Photos Agent
I have nearly 120,000 photos in my Photos. When I upgraded to Mac OS X Sierra, it lost all the faces I entered and it started scanning faces again. Crap.
After it scanned 63,000 photos, I still had 57,000 unscanned photos. But it stopped scanning. I rebooted the system several times but no success. Then I learned the trick on the Apple support forum.
I had to kill Photos agents. After I killed them, it looks like started to scanning again. It is a good thing it started scanning but still Sierra is a very problematic. I wish I did not upgraded the OS. I am feeling Apple is loosing it's touch.