A task set where each task is such that deadline <= period is EDF feasible if and only if Sum C_i / T_i <= 1. The paper reference is "Liu and Layland, Scheduling algorithms for multiprogramming in a hard real-time environment, 1973".
Actually, it seems that the task model is more restrictive. Each task must be such that D_i = T_i. Maybe it keeps true for D_i <= T_i, but I have no reference.
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
Actually, it seems that the task model is more restrictive. Each task must be such that D_i = T_i. Maybe it keeps true for D_i <= T_i, but I have no reference.