Under a Communistic system, how would labour be distributed?

Title says it all. My understanding is that in a free market system, labour is distributed based on what is profitable; presumably, under communism, people are less obligated to go into jobs based on financial need and more able to go into jobs for their "love of the work" - in this case, how will critical jobs that are unenjoyable or not in demand be filled? For instance, most people wouldn't want to be a sanitation worker or coal miner without an explicit financial reward - how are people incentivized to go into these jobs rather than more romanticized careers if they are free from financial constraints? I'm not asking this manevolently, I'm jsut curious.