Advanced Embedded Systems Design

Pre-emptive scheduler

BAE 5030 Fall 2004

Roshani JayasekaraBiosystems and Agricultural Engineering

Oklahoma State University

Pre-emption

• In practice sequential operation is rare– eg: Simple infinite loop in the main() of a C program for an embedded

system • Even though it may appear to be sequential, this code is subject to

interruption at any time by the hardware • When peripheral’s interrupt occurs, the corresponding ISR starts to run

instead of main() • This process is called pre-emption

• As a result of pre-emption:– main() will execute slowly– Change of system state

• Nested interrupts– An ISR that interrupts the program could itself be interrupted by the

ISR for a higher-priority interrupt

• As each pre-emption occurs:– The processor’s flags, pointer and other key registers must be saved

(typically in RAM)

ISR

• While majority of embedded systems are required to run one program, they do need to run multiple tasks– eg: Read the input from ADC every 1ms, read one or more

switches every 200ms and update an LCD display every 3ms

• For above example you can use three independent timers to perform three tasks [Listing 13.5, pond]

• Difficulties with hardware interrupts– If we have 100 tasks we need 100 timers– Hard to maintenance and update– Difficult to extend

• What is the solution?• Scheduler

Schedulers

• What is a scheduler?– Simple operating system that allows tasks to be called periodically

or one-shot basis• Periodic tasks: to be performed periodically• One-shot tasks: to be performed one after delay

– Single timer interrupt service routing that is shared between many different tasks

• Advantages– Only one timer needs to be initialized– Any changes for the timing generally required only one function to

be altered– Can use the same scheduler for many different tasks

• A similar technique can make a processor as responsive to software events as it is to hardware event– The goal is more or less to divide the software up into a set of

individual event handlers called “tasks”– A pre-emptive schedulers makes this possible

Schedulers

• Different types of schedulers– The co-operative scheduler: single-tasking system

– The pre-emptive scheduler: multitasking system

– The hybrid scheduler: limited multitasking capabilities

Scheduler type Task execution Device I/OEndless loop No tasks Polled only

Basic cyclic executive As often as possible Polled only

Time-driven cyclic executive Single frequency Polled only

Multi-rate cyclic executive Multiple frequency Polled only

Deadline Periodic/non-periodic Polled and interrupt driven

Pre-emptive Schedulers

• A pre-emptive schedulers provide a multitasking system architecture

• The system can immediately suspend processing of the lower priority event and commence with the new event

• Operation:– Tasks are scheduled to run at specific time– When a task is scheduled to run it is added to the waiting list– The task is not a Run To Completion (RTC) model

Pre-emptive Scheduler

• Implementation:– Comparatively complicated

• eg: semaphores must be implement to avoid conflict when “concurrent” tasks attempts to access shared resources

– The scheduler must allocate memory to hold all the intermediate states of pre-empted tasks

– The scheduler will generally be written in Assembly language

– The scheduler is created as a separate application

Pre-emptive schedulers

• Software task – Sequential function– Often ending with an infinite loop– Each task has specific job

• Reading sensor

• Updating a display

• Logging data

– Each task has its own dedicated stack area in RAM

Task Control

• Task control must include information about each task:– Stating address– Relative priority– Amount of stack space

• Task body include other system calls related to software event or timing

• Task priority– Random– Rate Monotonic Algorithm (RMA)

Tradeoffs

• Less predictable and reliable– Multiple tasks are active at any point in time

• Memory cost– Extra ROM for the system calls plus RAM for task-specific

stacks

• Tasks share resources– Race conditions– Priority inversion– Deadlock

• Expensive – Larger code framework– Less portable

Questions ?