Not good at all.

When I was doing a rebuilds on a trucks engines we had the same problem with the carbon building up around the piston rings and oil scraper.Sometimes we would reuse the old piston if they were still in spec and would soke the top part in handy and (pink bottle) and all the carbon would just brush off after about twenty minutes.

Change the oil we were using to one that had a carbon cleaning product in it and it work wonders on the new engines.Most diesel engine oil have it in it now.