I wonder how much additional cost is induced by calculating the light bounced off from the wall? Is there infinite of possible light trajectory in that case?

If you were confused about the difference between global illumination and ray-tracing like I was at first - since GL has light rays bounce to redirect light and "create" another light source - I highly recommend skimming through this paper: https://digitalcommons.unf.edu/cgi/viewcontent.cgi?article=1100&context=ojii_volumes#:~:text=%22Global%20illumination%2C%22%20the%20more,it%20produces%20more%20realistic%20images.