Based on the following passage from Wikipedia (and other common versions of the myth), I don't believe that Sisyphus knew in advance the futility of his efforts:
"As a punishment for his trickery, King Sisyphus was made to endlessly roll a huge boulder up a steep hill. The maddening nature of the punishment was reserved for King Sisyphus due to his hubristic belief that his cleverness surpassed that of Zeus himself. Zeus accordingly displayed his own cleverness by enchanting the boulder into rolling away from King Sisyphus before he reached the top which ended up consigning Sisyphus to an eternity of useless efforts and unending frustration. Thus it came to pass that pointless or interminable activities are sometimes described as Sisyphean."
I believe that the entire point of the punishment was to trick him to believe that he was going to succeed. Moreover, I don't think that he had any choice in the matter, it wasn't his desire to roll the boulder. It resembles Hell in Christianity, those who suffer don't have their say in the matter.