I am looking for any approximation results for unrelated multiprocessor scheduling problem with precedence constraints (minimizing makespan), i.e. $R|prec|C_{max}$, with the constraint that some tasks need to run together on the same processor. I searched a lot and the closest constraint I could find was that some tasks have to run on certain processors. I need to group tasks in classes such that each class must run on the same processor. Note that the precedence constraints prevent collapsing each class into a single big task.I would like to find any literature on the problem  (or any non-trivial relaxation, for example removing the precedence constraints will allow the classes of tasks to be treated as one task).
Approximation results on scheduling under an uncommon constraint
approximation algorithms;approximation hardness;scheduling