I had smoke through out the cabin and cockpit one day flying south out of Miami in a 757 at about 20,000 ft over the keys heading south to TGU Honduras. Shut off non essential busses, declared an emergency and returned and landed, got another plane and continued our trip. It was about a year after Swisair 111 had the electrical problem off Halifax. Our procedure hadn't been changed and said to do the checklist, shut off unessential busses and continue and see if smoke decreased.
I shut down nonessential busses, declared an emergency descended and headed back to MIA and landed. Two weeks later they changed the checklist to what I did after I wrote it up.