Reducing the Cost of Real-Time Software through a Cyclic Task Abstraction for Ada

211 Pages·2017·11.09 MB·English

Reducing the Cost of Real-Time
Software through a
Cyclic Task Abstraction for Ada
Assisted by Acton: the novel Ada real-time executive
Patrick Bernardi
A thesis submitted for the degree of
Doctor of Philosophy
The Australian National University
February 2017

Declaration
I hereby declare this thesis is my own original work carried out exclusively
for the degree of Doctor of Philosophy at the Australian National University.
Contribution by others though feedback, editing and prior work are acknowl-
edged in the customary manner.
Patrick Bernardi
February 9, 2017
© Patrick Bernardi 2017
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Abstract
Ada is unique as a software engineering language, facilitating
the development of reliable and maintainable software through clear, unam-
biguous, modular code enforced to a specification. However, despite compre-
hensive real-time systems support, Ada lacks the abstraction at the heart of
these systems: cyclic tasks. Without it, users resort to solutions decomposing
their designs: introducing unnecessary complexity to a real-time systems prim-
itive. Furthermore, the only Ada executive for microcontrollers, GNAT for Bare
Boards, impedes the timing analysis of tasks and schedulability analysis of
systems. Consequently, Ada and its environments do not reach their potential
of producing low-cost, reliable and maintainable real-time systems.
This thesis unlocks this latent potential through the development of the Cyclic
Task Specification and Acton. The Specification extends Ada to incorporate cyclic
tasks within Ada’s existing task abstraction using a simple but powerful model.
Acton, on the other hand, is a new Ada executive built from the ground up
to support real-time systems, forgoing an existing Ada tasking run-time in
favour of building the tasking semantics natively within its kernel. The result
is a flexible and portable Ada executive which correctly apportions execution
time and enforces cyclic task attributes. Furthermore, tasks only perform user
assigned activities and lower priority tasks cannot interrupt a running task.
Their contributions come at the cost of a more complex language and larger
execution time overheads for kernel operations. For Acton, the up to order of
magnitude larger overheads is the cost of simplifying task timing and system
schedulability analysis. For the Cyclic Task Specification, the added complexity
simplifies the expression of cyclic task patterns for a wide range of systems. Its
simple model centred on task attributes permits model expansion outside the
formal language, addressing aversions to incorporating high-level abstractions.
Together, the Cyclic Task Specification and Acton advance real-time systems by
providing the clarity, structure and simplicity needed to express cyclic tasks and
have their real-time constraints enforced at run-time. Furthermore, real-time
analysis tools can now use the same cyclic task semantic information as the
compiler to provide automatic timing verification and run-time enforceable
cyclic constraints. Consequently, the contributions of this thesis allow real-time
systems to use less code and have less scope for faults: reducing the life-cycle
cost of real-time systems through quicker development and maintenance.
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Acknowledgements
I would not have embarked down the long journey of a postgrad-
uate degree if not for my supervisor Uwe Zimmer. His convincing arguments
for a topic on autonomous model helicopters proved irresistible compared to
my then favoured option of working in telecommunication industry. For the
reader this will be puzzling because, as presented, this thesis has nothing to
do with autonomous model helicopters. However, when push came to shove
on a failing autonomous model helicopter project, I switched to a question
that was gnawing at me more than the failed autonomous electronic hardware
I had in my hands:
“Why was there no Ada tasking run-time for my rather powerful mi-
crocontroller and why does Ada not support the high-level real-time
abstractions I was taught and used in my designs?
”
The origin of this question lies very much with Uwe, but from his role as a
teacher rather than as a supervisor. Uwe taught Ada and real-time systems in
two courses I took as an undergraduate. His style of teaching led me to ap-
preciate and value the power Ada and real-time systems bring to the building
of reliable, maintainable and predicable systems. However, the hallmark of a
great teacher is not the knowledge imparted but the inspiration and curiosity
planted to seize on the gained knowledge. Uwe achieved this, instilling the
expectation of having the right tools for the task at hand. Uwe is a teacher par
excellence, whom I thank for his teaching, and his ongoing advice, appraisal
and support throughout the thesis.
I am also grateful to acknowledge another teacher par excellence from my youth,
Frank O’Shea, for his influence and passion for mathematics.
This thesis, however, needed more than determination, inspiration and support
to make it possible: it needed a high quality Ada environment from which my
ideas could integrate. AdaCore delivered this in the form of the GNAT Ada en-
vironment and I thank the many people who developed it for producing such
a high-quality and accessible open-source Ada environment. It is a testament
to its design that someone without any experience developing compilers can
step-in and learn not just how the GNAT compiler works, but also how com-
pilers work generally. Furthermore, exposure to GNAT has taught me how to
properly write software, particularly for large systems, for which I am thankful.
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Nevertheless, I cannot diminish the immense support by others. To my fellow
PhD students Florian Poppa and Ben Coughlan I thank for the amazing of-
fice atmosphere and friendships. Our office was one of engaging discussions,
encouragement and support: providing a valuable environment for sounding
board ideas.
To the participants of IRTAW-17 I thank for the warm welcome I received to
the small community, and for their invaluable input to refine the rough edges
of the Cyclic Task Specification.
I would like to end by thanking my friends and family who provided vast
support and encouragement over the course of my thesis; each in their own
cherished way. In particular, special mention goes to Paul Altin and Steve Pe-
terson. To my brother James and sister Maria I thank them for reading sections
of my thesis and making my life more fulfilling.
Finally, to my parents Linda and John, I thank them for their unwavering sup-
port and generosity. Without them, I would not be where I am today.
Contents
Declaration i
Abstract iii
Acknowledgements v
Contents vii
Chapter 1 Introduction 1
Task Abstraction 2
Task Abstraction Implementation 3
Outline 5
Thesis Conventions 6
Chapter 2 Cyclic Tasks and Ada 9
Cyclic Task Definitions and Properties 11
Ada’s Real-Time Facilities 15
Implementing Cyclic Tasks in Ada 16
Prior Work Incorporating Cyclic Tasks into Ada 30
Cyclic Tasks in Other Languages 32
Conclusion 38
Chapter 3 The Cyclic Task Specification 41
The Specification 42
Examples 51
Implementation 54
Ada Semantic Interface Specification 56
Language Impact and Flexibility 62
Comparison with Existing Ada Approaches 65
Conclusion 72
Chapter 4 Review of GNAT for Bare Boards for Real-Time
Systems 75
GNAT 76
GNAT for Bare Boards 79
Real-Time Problems in GNAT for Bare Boards 84
Conclusion 89
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Chapter 5 Acton 91
Rationale 92
Architecture 93
Oak Resources 96
The Oak Run-Loop 112
Scheduling 116
Task Management 123
Cyclic Tasks 128
Protected Objects 134
Interrupt Handling 143
Conclusion 148
Chapter 6 Review of Acton 151
Platforms and Setup 152
Kernel Design 154
Scheduling and Task Dispatching 158
Idle State 163
Protected Objects 164
Interrupt Handling 165
Memory Footprint 166
Portability 169
Language Cover 169
Conclusion 172
Chapter 7 Conclusion 175
Future Work 179
Appendix A Cyclic Task Libraries 181
Appendix B References 189