I think the proper course of action is several loud announcements at the door. Failing that, you can close the door, which is a no-brainer. How far need you enter to close the door, though, and if you've been knocking and yelling for a few minutes, one would assume you can smell the marijuana from outside. Convincing a judge that it was necessary to enter beyond reaching an arm in to close the door and that the marijuana odor was not plainly apparent outside, but was just inside the threshold sounds like an uphill battle.
This gets even more complicated with the recent decision on dog sniffs on the curtilage and evaluating whether you could even be at the door (in this case I think you're 100% solid since the purpose of being at the door was not solely to conduct the sniff). The body of case law on the topic is clear that you can enter and secure a residence to obtain a search warrant if you can articulate that it will prevent the destruction of evidence, but what proof do you have anybody even knows, or reasonably should have known you were ever at their door?
There are a lot of variables here.
"Logic is rarely the engine that propels a police department forward."
-David Simon in "Homicide"