2018-10-05 : Because your upkeep step is after your untap step, your lands will be untapped to help you activate Charnel Troll’s last ability during your upkeep. However, because your upkeep step is before your draw step, you won’t have drawn a card for the turn yet.

2018-10-05 : You can activate Charnel Troll’s last ability during your upkeep before its triggered ability resolves.

2018-10-05 : You can’t choose not to exile a creature card from your graveyard if you have one to exile.