On the Defectiveness of SCHED_DEADLINE w.r.t. Tardiness and Affinities, and a Partial Fix.

SCHED_DEADLINE (DL for short) is an Earliest-Deadline-First (EDF) scheduler included in the Linux kernel. A question motivated by DL is how EDF should be implemented in the presence of CPU affinities to maintain optimal bounded tardiness guarantees. Recent works have shown that under arbitrary affinities, DL does not maintain such guarantees. Such works have also shown that repairing DL to maintain these guarantees would likely require an impractical overhaul of the existing code. In this work, we show that for the special case where affinities are semi-partitioned, DL can be modified to maintain tardiness guarantees with minor changes. We also draw attention to the fact that admission control is already broken in several respects in the existing DL implementation.