TASP: Enabling Time-Triggered Task Scheduling in TSN-Based Mixed-Criticality Systems

Abstract

Distributed mixed-criticality system (DMCS) has been widely used in various critical domains such as self-driving cars and space crafts. To guarantee the end-to-end QoS (i.e., deadline/jitter requirements) of sensing-controlling-actuating control loops (CL) applications, DMCS adopts Time-Sensitive Networking (TSN), an emerging real-time Ethernet technology, for communication between end systems (ES). TSN provides a synchronized global clock and guarantees bounded delay for time-critical traffic in CLs, making it possible for DMCS to collaboratively schedule the computation (on ES) and communication (in TSN) to meet the Quality of Service (QoS) requirement. However, as modern DMCS tends to use fully-fledged Linux distributions (rather than a custom real-time OS) on ES to enjoy Linux’s mature ecosystem, it is challenging for DMCS to realize TSN-based QoS guarantees because the event-triggered scheduling of Linux on ES is incompatible with TSN.This paper proposes TAsk Scheduling Puppeteer (TASP), a mechanism that schedules CL tasks based on TSN without modifying the Linux OS. The key idea of TASP is to manipulate task scheduling by controlling the timing of CL packet submissions at the interface between TSN and ES. Specifically, TASP extracts two parameters: Fore Guardband (ForeGB) and Back Guardband (BackGB). During the ForeGB period before a CL packet’s submission, TASP forbids any packets’ submission; while during the BackGB period after a CL packet’s submission, TASP forbids any other CL packets’ submission. ForeGB and BackGB can ensure that there is at most one schedulable task on the ES at any time, and thus Linux has no choice but to schedule the only task, making the Linux scheduler a puppet. We have deployed TASP and evaluated it in real-world TSN switches based on an open-source TSN project, OpenTSN. The TASP-enabled ES can achieve task scheduling precisely based on TSN’s global clock, which outperforms the original ES by reducing end-to-end jitter from milliseconds to microseconds.