Real Time Operating Systems

Introduction toReal-Time Operating

Systems(Part II)

Course originally developed by Maj Ron Smith

Apr 20, 2023

Outline

Multitasking tasks, kernels, context switching

Scheduling types, priorities, interrupts, the “tick”

Resource Sharing (Mutual Exclusion) reentrancy, techniques (semaphores), deadlock

Intertask Communication queues and mailboxes

2Dr Alain Beaulieu

Apr 20, 2023

Multitasking

multitasking the process of scheduling the CPU between

several sequential tasks maximizes CPU usage provides for modular construction of programs facilitates real-time application development the tough part: selecting and organizing the

tasks

3Dr Alain Beaulieu

Apr 20, 2023

Multitasking

task (thread of control) a process which thinks it has the CPU all to itself has priority, its own set of CPU registers and its

own stack typically, an infinite loop in one of 5 states:

dormant, ready, running, waiting or interrupted types of tasks:

“fast service” operation suspended awaiting event (keyboard) periodic service background, after everything else (display update)

4Dr Alain Beaulieu

Apr 20, 2023

Multitasking

kernel (RTOS) responsible for the management of tasks and

communication between tasks its fundamental service is to facilitate task

switching (context switching) adds overhead to the system

ROM & RAM RAM per task 2-5% CPU usage

saves the developer from having to write code for services such as mutual exclusion, mailboxes, queues, time delays, etc..

5Dr Alain Beaulieu

Apr 20, 2023

Multitasking

context switch when a task switch is decided upon:

the context (CPU registers) of the executing task is saved in its context storage area (the task control block) on its stack

the new task’s context is restored from its storage area on its stack

execution of the new task is initiated (based upon its context)

when a task switch occurs, depends upon the scheduling policy of the kernel (as we’ll see later)

6Dr Alain Beaulieu

Apr 20, 2023

Multitasking - context switch

Task 1 Task 2 Task n

CPU registers context

stack stack stack

...

tcbtcb

...priority

task IDstack pointer

tcb

...priority

task IDstack pointer

...priority

task IDstack pointer

Memory

CPU

7Dr Alain Beaulieu

Apr 20, 2023

Multitasking - context switch

next task.priority

taskIDstack pointer

task control block

running task

ready queue

head ...priority

task IDstack pointer

...priority

task IDstack pointer

...priority

task IDstack pointer

Idle task..

8Dr Alain Beaulieu

Apr 20, 2023

Scheduling - types

non-preemptive (cooperative multitasking) tasks decide for themselves when to give up the

CPU; either when completed or explicitly asynchronous events are handled using interrupts tasks still have an assigned priority

advantages:- interrupt latency time is low- less need to “guard” shared resources

disadvantages:- responsiveness is non-optimal & non-deterministic- one task may lock-up system

9Dr Alain Beaulieu

Apr 20, 2023

Scheduling - types (non-preemptive)

Time

low priority task

high priority task

ISR

ISR makes high priority task ready(1) (2)

(3)(4)

(5)

(6)

(7)low priority taskrelinquishes CPU

Ref [2]

10Dr Alain Beaulieu

Apr 20, 2023

Scheduling - types

preemptive the highest priority task (ready) is given control

of the CPU current running task is suspended after an Interrupt Service Routine (ISR), the new

highest priority task is run

advantages:- responsiveness is optimal & deterministic

disadvantages:- shared resources must be properly guarded

11Dr Alain Beaulieu

Apr 20, 2023

high priority taskis switched in

Scheduling - types (preemptive)

Time

low priority task

high priority task

ISR

ISR makes high priority task ready(1) (2)

(3) (4)

(7)

(6)

(5)

Ref [2]

12Dr Alain Beaulieu

Apr 20, 2023

Scheduling - types

round-robin (time-slicing) special case of preemptive tasks with the same priority are only allowed to

run for a predetermined time period, “a quantum”

task switching then occurs giving each same priority task equal opportunity at the CPU

not necessarily supported by all preemptive kernels microC/OS-II does not support round-robin

13Dr Alain Beaulieu

Apr 20, 2023

Scheduling - priorities

static the priority of a task is set at compile time and does not

change during application executionadvantage:

all timing constraints can be known at compile timedisadvantage:

may get priority inversion

dynamic each task can change its priority during executionadvantage:

useful in avoiding priority inversiondisadvantage:

not easy to guarantee schedulability

14Dr Alain Beaulieu

Apr 20, 2023

Scheduling - priorities

assigning task priorities non-trivial based upon the soft and hard real-time

requirements based upon the type of scheduling policy interrelated to kernel selection requires detailed knowledge of kernel and

processor latency and response times involves schedulability* analysis and techniques

for example rate monotonic scheduling

*covered later

15Dr Alain Beaulieu

Apr 20, 2023

Scheduling - interrupts

Interrupts and ISRs a hardware mechanism to inform the CPU that an

asynchronous event has occurred the CPU partially saves its context and jumps or vectors

to an interrupt service routine (ISR) return from interrupt is to:

the interrupted task for non-preemptive kernels the highest priority task for preemptive kernels

interrupts may be nested interrupts may be disabled / enabled in software

Nonmaskable Interrupt can not be disabled via software

16Dr Alain Beaulieu

Apr 20, 2023

Scheduling - the “tick” or quantum

the clock tick a special interrupt that occurs periodically

polling versus pushing (keyboard example) the time between interrupts is application

specific generally between 10 and 200 milliseconds

allows the kernel to delay tasks for integral numbers of ticks or provide timeouts awaiting events resolution of delayed tasks is one clock tick the accuracy is not one clock tick

the faster the tick rate, the greater the accuracy but the higher the system overhead

17Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - reentrancy

a function is defined as being reentrant if more than one task can use it without fear of data corruption

in other words it can be interrupted, followed by a context switch, and subsequently resumed without loss of data

a function can be made reentrant with local variables or through the use of protected global variables

18Dr Alain Beaulieu

Apr 20, 2023

Reentrant Function Example

non-reentrant

int temp; // global

function swap(int *x, int *y) {

temp = *x;*x = *y;*y = temp;

}

reentrant version

function swap (int *x, int *y) {int temp; // local

temp = *x;*x = *y;*y = temp;

}

19Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - techniques

besides shared functions, we must also deal with other shared resources such as global data stores, buffers, I/O devices, etc

techniques to address mutual exclusion include: disabling/enabling interrupts disabling scheduling performing test-and-set operations (indivisibly) using semaphores

20Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - techniques

disabling/enabling interrupts used to access a few variables or small data

structures used when the process time is minimal affects interrupt latency

you should not disable interrupts for any longer than the kernel does

in C/OS-II:

OS_ENTER_CRITICAL( );… // you can access shared data hereOS_EXIT_CRITICAL( ):

21Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - techniques

disabling scheduling instead of disabling all maskable interrupts, you

may just want to disable the scheduler an interrupted task is returned to, not necessarily the

highest priority task implies that data is not shared with the ISRs

in C/OS-II:

OSSchedLock( );… // you can access shared data hereOSSchedUnlock( );

22Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - techniques

perform test-and-set operations (indivisibly) two tasks may agree that to use a common

resource, they must first check a global variable while it is being checked, and before it is set, by

one task, it must be made inaccessible by the other

this can be achieved by: disabling interrupts (above) or some processors now support a single TAS instruction

this common computing problem lead to a 1965 software invention -> the ?

23Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - semaphores

invented by Dijkstra in 1965 a protocol mechanism to control access to

shared resources, signal the occurrence of an event or allow two tasks to synchronize

one analogy is that in order for a task to use a resource it must first draw a key for it if the key(s) is in use, the task suspends

wait on (pend) & signal (post) semaphore two types of semaphores:

binary and counting (general)

24Dr Alain Beaulieu

Apr 20, 2023

Visualizing a Semaphore

task 2

task 1 1. pend

4. post

Printer

2. pend

6. post 5. “my name is task 2”

3. “my name is task 1”

25Dr Alain Beaulieu

Apr 20, 2023

Semaphore Example

accessing shared data in C/OS-II

OS_EVENT *SharedDataSem;

SharedDataSem = OSSemCreate(1); ...void Function (void) {INT8U err; //read as “char *err” (string)

...

OSSemPend(SharedDataSem, 0, &err);

… //access shared data

OSSemPost(SharedDataSem);

}

binary semaphore

timeout value{wait forever in this case}

error message code

26Dr Alain Beaulieu

Apr 20, 2023

Mutual Exclusion - deadlock

a situation in which two tasks are (unknowingly) waiting for resources held by the other

avoid this situation by having tasks: acquire all resources before proceeding acquire all resources in the same order (across

tasks) release resources in reverse order

most kernels allow you to specify a timeout when acquiring a semaphore (with error code)

27Dr Alain Beaulieu

Apr 20, 2023

Intertask Communication

Synchronization unilateral rendezvous - one task can be

synchronized with another (no data transferred) by using a semaphore

the semaphore is analogous to a flag signaling the occurrence of an event (vice a key for mutex)

in this case, the semaphore is initialized to 0

Task 2 Task 12. Post 1. Pend

28Dr Alain Beaulieu

Apr 20, 2023

Intertask Communication

Synchronization bilateral rendezvous - two tasks can be

synchronized with each other by using two separate semaphores

Task 2 Task 1

1. Signal Task 2

2. wait for signal from task 2

4. wait for signal from task 1

3. Signal Task 1

29Dr Alain Beaulieu

Apr 20, 2023

Intertask Communication

data transfer there are two general ways in which tasks

exchange data: through global data stores, or by sending messages

if global data is used, each task or ISR must ensure it has exclusive rights if an ISR is involved, the only way to ensure this is through

the disabling of interrupts if two tasks are involved you can disable interrupts or use

semaphores more commonly, messages are sent between

tasks usually achieved via a message mailbox or message

queue

30Dr Alain Beaulieu

Apr 20, 2023

Intertask Communication

message mailbox messages are typically pointer-sized variables a task or an ISR can deposit a message

(pointer) into a mailbox. similarly, one or more tasks can retrieve the

message from the mailbox the sending and receiving tasks must agree on

what type of data the message points to

Task 1 Task 21. Post 2. PendI 100msec

31Dr Alain Beaulieu

Apr 20, 2023

message mailbox (cont’d) C/OS-II provides the following type of typical

mailbox services:

initialization OSMboxCreate(...);deposit (post) OSMboxPost(…);wait (pend) OSMboxPend(…); //includes

timeout

get (accept) OSMboxAccept(…);query OSMboxQuery(…);

Intertask Communication

*pend will wait on message (for some timeout period, while accept will get the message if it is there but not wait for it)

32Dr Alain Beaulieu

Apr 20, 2023

Intertask Communication

message queues used to send one or more messages to tasks it is basically an array of mailboxes messages can be retrieved either FIFO or LIFO a kernel will normally provide the same

mechanisms for a message queue as for a message mailbox

Task 1 Task 21. Post 2. Pend

100msecII5

33Dr Alain Beaulieu

Apr 20, 2023

References

[1] Cooling, J.E., “Software Design for Real-time Systems”, Chapters 1 & 9.

[2] Labrosse, J.J., “MicroC/OS-II”, Chapter 2.[3] Furht et al, “Real-Time UNIX Systems”,

Chapter 2.[4] Greenfiled J.D., “The 68HC11

Microcontroller” Chapter 3.[5] http://www.realtime-info.be/encyc/

market/rtos/rtos.htm

34Dr Alain Beaulieu