Work-in-Progress: Function-as-Subtask API Replacing Publish/Subscribe for OS-Native DAG Scheduling

TL;DR

This paper introduces the Function-as-Subtask API to enforce DAG semantics in real-time OS scheduling, replacing the unreliable publish/subscribe model in ROS 2, and demonstrates its implementation and evaluation in a Rust-based kernel.

Contribution

The paper proposes the FasS API that guarantees DAG semantics at the API level, improving reliability over existing publish/subscribe approaches.

Findings

FasS API enforces DAG constraints at the API level.

Implementation in Rust demonstrates semantic fidelity.

Guidelines for applying FasS to Linux are outlined.

Abstract

The Directed Acyclic Graph (DAG) task model for real-time scheduling finds its primary practical target in Robot Operating System 2 (ROS 2). However, ROS 2's publish/subscribe API leaves DAG precedence constraints unenforced: a callback may publish mid-execution, and multi-input callbacks let developers choose topic-matching policies. Thus preserving DAG semantics relies on conventions; once violated, the model collapses. We propose the Function-as-Subtask (FasS) API, which expresses each subtask as a function whose arguments/return values are the subtask's incoming/outgoing edges. By minimizing description freedom, DAG semantics is guaranteed at the API rather than by programmer discipline. We implement a DAG-native scheduler using FasS on a Rust-based experimental kernel and evaluate its semantic fidelity, and we outline design guidelines for applying FasS to Linux Linux sched_ext.