Modern Embedded Systems Programming: Beyond the RTOS

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Millicent Elliott
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1 Modern Embedded Systems Programming: Beyond the RTOS Miro Samek Quantum Leaps, LLC 1

2 Presentation Outline A quick introduction to RTOS and the perils of blocking Active objects State machines ~40 min Active object frameworks for deeply embedded systems Demonstrations Q&A ~10 min ~10 min 2

$pinmode(led_pin, OUTPUT); // setup: set the LED pin as output while (1) { // endless loop$ digitalwrite(led_pin, HIGH); // turn LED on delay(1000); // wait for 1000ms digitalwrite(led_pin,

digitalwrite(led_pin, HIGH); // turn LED on delay(1000); // wait for 1000ms digitalwrite(led_pin,

3 In the beginning was the Superloop // adapted from the Arduino Blink Tutorial (*) void main() { pinmode(led_pin, OUTPUT); // setup: set the LED pin as output while (1) { // endless loop digitalwrite(led_pin, HIGH); // turn LED on delay(1000); // wait for 1000ms digitalwrite(led_pin, LOW); // turn LED off delay(1000); // wait for 1000ms (*) Arduino Blink Tutorial: 3

4 RTOS Multithreading: Multiple Superloops void thread_alarm() { // RTOS thread routine pinmode(sw_pin, INPUT); // setup: set the Switch pin as input while (1) { // endless loop if (digitalread(sw_pin) == HIGH) { // is the switch depressed? digitalwrite(alarm_pin, HIGH); // start the alarm void thread_blink() { // RTOS thread routine pinmode(led_pin, OUTPUT); // setup: set the LED pin else { while (1) { // endless loop the alarm digitalwrite(alarm_pin, LOW); // stop digitalwrite(led_pin, HIGH); // turn the LED on void thread_blink() { // RTOS thread routine RTOS_delay(1000);// wait // for wait100ms for 1000ms RTOS_delay(100); pinmode(led_pin, OUTPUT); // setup: set pin as output digitalwrite(led_pin, LOW); // turn the LED off while (1) { // endless loop RTOS_delay(1000); // wait for 1000ms digitalwrite(led_pin, HIGH); // turn the LED on RTOS_delay(1000); // wait for 1000ms digitalwrite(led_pin, LOW); // turn the LED off RTOS_delay(1000); // wait for 1000ms 4

5 Thread Context & Context Switch TCB sp... Thread Control Blocks (TCBs) TCB sp stack stack void thread_blink() { per-thread stacks pinmode(led_pin, OUTPUT); While (1) { // endless loop digitalwrite(led_pin, HIGH); Memory pc RTOS_delay(1000); digitalwrite(led_pin, LOW); CPU registers pc RTOS_delay(1000); status register sr sp stack pointer context save... pc context restore program counter 5

6 priority Thread Blocking Thread makes a blocking call, e.g, RTOS_delay() Thread-A runs RTOS kernel Thread-B blocked K Clock tick interrupt ISR K RTOS kernel context switch Thread-A blocked (6b) Thread-A runs context switch Thread-B runs Thread-B blocked time 6

7 RTOS Benefits 1) Divide and conquer strategy Multiple threads are easier to develop than one kitchen sink superloop 2) More efficient CPU use Threads that are efficiently blocked don't consume CPU cycles 3) Threads can be decoupled in the time domain Under a preemptive, priority-based scheduler, changes in low-priority threads have no impact on the timing of high-priority threads (Rate Monotonic Analysis (RMA)) 7

8 Perils of Blocking Starvation Shared-state concurrency Race conditions Mutual Exclusion Failure Deadlock Missed deadlines Blocking Priority inversion Synchronization by blocking Unresponsive threads More threads Architectural decay 8

9 Best Practices of Concurrent Programming(*) Don't block inside your code Communicate and synchronize threads asynchronously via event objects Don't share data or resources among threads Keep data isolated and bound to threads (strict encapsulation) Structure your threads as message pumps (*) Herb Sutter Prefer Using Active Objects Instead of Naked Threads 9

10 Best Practices: RTOS Implementation Event object Event queue Event object Private thread BLOCKING ISR Event object Event queue Event queue start Private data and resources Wait for event Process event Event loop NO BLOCKING void thread_handler(ao_type *ao) { // AO thread routine... // setup while (1) { // event loop // pend on the event queue (BLOCKING!) Event e = RTOS_queuePend(ao->queue); ao->handle(e); // handle event (NON-BLOCKING!) 10

11 Active Object (Actor) Design Pattern Active Objects (Actors) are event-driven, strictly encapsulated software objects running in their own threads and communicating asynchronously by means of events. Not a novelty. Carl Hewitt's actors 1970s. ROOM actors 1990s. Adapted from ROOM into UML as active objects ROOM actors and UML active objects use hierarchical state machines (UML statecharts) to specify the behavior of event-driven active objects. 11

12 Active Object Framework Implement the Active Object pattern as a framework void thread_handler(ao_type *ao) { // AO thread routine... // setup while (1) { // event loop // pend on the event queue (BLOCKING!) Event e = RTOS_queuePend(ao->queue); ao->handle(e); // handle event (NON-BLOCKING!) Inversion of control (main difference from RTOS) automates and enforces the best practices (safer design) brings conceptual integrity and consistency to the applications 12

13 Paradigm Shift: Sequential Event-Driven Sequential programming with RTOS No blocking No use for most RTOS mechanisms! Event-driven active object framework Paradigm Shift Events Semaphores Threads Mutexes Event Flags void thread_blink() { pinmode(led_pin, OUTPUT); delay() while (1) { digitalwrite(led_pin, HIGH); Callback RTOS_delay(1000); // NOT allowed!timers... Message Queues* Active Objects Event Posting Publish/ Subscribe Memory Pools Time Events State Machines 13

Reduce Spaghetti Code with State Machines Finite State Machines the best known spaghetti reducers State captures only the relevant aspects of the system's history Natural fit for event-driven

14 Reduce Spaghetti Code with State Machines Finite State Machines the best known spaghetti reducers State captures only the relevant aspects of the system's history Natural fit for event-driven programming, where the code cannot block and must return to the event-loop after each event Minimal context (a single state-variable) instead of the whole call stack default ANY_KEY / send_lower_case_scan_code(); internal transitions CAPS_LOCK CAPS_LOCK caps_locked ANY_KEY / send_upper_case_scan_code(); trigger list of actions 14

15 State Machines are not Flowcharts Statechart (event-driven) represents all states of a system driven by explicit events processing happens on arcs (transitions) no notion of progression (a) Flowchart (sequential) represents stages of processing in a system gets from node to node upon completion processing happens in nodes progresses from start to finish (b) s1 E1 / action1(); do X s2 E2 / action2(); do Y do Z s3 E3 / action3(); do W 15

Hierarchical State Machines Traditional FSMs explode due to repetitions C (a) State hierarchy eliminates repetitions programming-by-difference (b) OFF on operand1 OPER C C

16 Hierarchical State Machines Traditional FSMs explode due to repetitions C (a) State hierarchy eliminates repetitions programming-by-difference (b) OFF on operand1 OPER C C OPER operand1 DIGIT_0_9, C POINT OPER result OFF operand2 DIGIT_0_9, POINT OFF C result EQUALS opentered OPER DIGIT_0_9, POINT OFF opentered EQUALS operand2 OFF DIGIT_0_9, POINT 16

17 AO Frameworks for Deeply Embedded Systems 17

18 AO Frameworks can be smaller than RTOS kernels, because they don't need blocking RAM (data) AO Frameworks vs. RTOS kernels Linux, Windows XP 1MB Windows CE VxWorks 100KB A typical small preemptive RTOS 10KB QP/C, QP/C++ 1KB QP-nano 100B 10B 1KB 2KB 10KB 100KB 1MB 10MB ROM (code) 18

19 AO Framework Software Bus direct event posting Active Object 1 Active Object 2 Active Object N... multicasting a published event publish-subscribe software bus ISR_1() ISR_2() 19

20 Coding Hierarchical State Machines top on entry / exit / C ready OFF QState Calc_on(Calc * const me, QEvt const *e) { QState status; switch (e->sig) { case Q_ENTRY_SIG: /* entry action */... status = Q_HANDLED(); break; case Q_EXIT_SIG: /* exit action */... status = Q_HANDLED(); break; case Q_INIT_SIG: /* initial transition */ status = Q_TRAN(&Calc_ready); break; case C_SIG: /* state transition */ BSP_clear(); /* clear the display */ status = Q_TRAN(&Calc_on); break; case OFF_SIG: /* state transition */ status = Q_TRAN(&Calc_final); break; default: status = Q_SUPER(&QHsm_top); /* superstate */ break; return status; 20

21 Cooperative Kernel (QV) find highest-priority non-empty queue priority = n all queues empty (idle condition) vanilla scheduler priority = n-1 priority = 1... idle processing e = queue.get(); e = queue.get();... e = queue.get(); dispatch(e); dispatch(e);... dispatch(e); priority = 0 21

22 Preemptive, Non-Blocking Kernel (QK) Synchronous Preemption priority low priority task high priority task function call (3) interrupt entry/exit RTC scheduler (4) (2) (1) 0 task preempted time Asynchronous Preemption priority interrupt 10 (5) (3) (4) interrupt call (5) (6) interrupt return (7) low priority task high priority task function call (8) (9) interrupt entry/exit (10) RTC scheduler (2) (1) 0 task preempted (11) time 22

23 Graphical Modeling and Code Generation Active Objects enable you to effectively apply UML modeling A modeling tool needs an AO framework as a target for automatic code generation 23

24 Summary Experts use the Active Object design pattern instead of naked RTOS AO framework is an ideal fit for deeply embedded real-time systems AO framework requires a paradigm shift (sequential event-driven) Compared to RTOS, AO framework opens new possibilities: Safer architecture and state-machine design method (functional safety) Simpler, more efficient kernels (lower-power applications) Easier unit testing and software tracing (V&V) Higher level of abstraction suitable for modeling and code generation Welcome to the 21st century! 24

25 Demo: Blinky on Arduino 25

26 Demo: PELICAN on Arduino 26

27 Demo: Dining Philosophers with Q-SPY QF Philo[n] thinking (2) (1) Philo[m] Table thinking serving (3) TIMEOUT hungry eating TIMEOUT HUNGRY(m) EAT(m) (5) (4) (6) hungry (7) HUNGRY(n) (8) TIMEOUT (10) eating thinking DONE(m) (9) EAT(n) 27

28 Demo: Fly 'n' Shoot game on Windows 28

Modern Embedded Systems Programming: Beyond the RTOS

Modern Embedded Systems Programming: Beyond the RTOS Miro Samek Quantum Leaps, LLC 1 Presentation Outline A quick introduction to RTOS and the perils of blocking Active objects State machines ~40 min Active