Stack-based scheduling of realtime processes

Abstract

The Priority Ceiling Protocol (PCP) of Sha, Rajkumar and Lehoczky is a policy for locking binary semaphores that bounds priority inversion (i.e., the blocking of a job while a lower priority job executes), and thereby improves schedulability under fixed priority preemptive scheduling. We show how to extend the PCP to handle: multiunit resources, which subsume binary semaphores and reader-writer locks; dynamic priority schemes, such as earliest-deadline-first (EDF), that use static “preemption levels”; sharing of runtime stack space between jobs. These extensions can be applied independently, or together.

The Stack Resource Policy (SRP) is a variant of the SRP that incorporates the three extensions mentioned above, plus the conservative assumption that each job may require the use of a shared stack. This avoids unnecessary context switches and allows the SRP to be implemented very simply using a stack. We prove a schedulability result for EDF scheduling with the SRP that is tighter than the one proved previously for EDF with a dynamic version of the PCP.

The Minimal SRP (MSRP) is a slightly more complex variant of the SRP, which has similar properties, but imposes less blocking. The MSRP is optimal for stack sharing systems, in the sense that it is the least restrictive policy that strictly bounds priority inversion and prevents deadlock for rate monotone (RM) and earliest-deadline-first (EDF) scheduling.