Designing the Scheduler Structure
- Understand the real-time requirements of your system, including task deadlines and periods.
- Create a task control block structure to store essential information for each task, such as task ID, task state, stack pointer, and priority level.
- Determine the appropriate real-time scheduling algorithm for your tasks. Popular options include Rate Monotonic Scheduling (RMS) and Earliest Deadline First (EDF).
Implementing a Task Control Block
Implementing a task control block (TCB) is a key step in setting up your scheduler. The TCB usually contains all the necessary information to manage your tasks effectively.
typedef struct {
uint32_t task_id;
void (*task_function)(void *);
uint8_t priority;
uint32_t *stack_pointer;
uint32_t period;
uint32_t deadline;
// Additional properties may include task states, execution time, etc.
} TaskControlBlock;
Implementing the Scheduling Algorithm
- For a Rate Monotonic Scheduler, tasks are assigned fixed priority levels based on their periodic intervals. A task with a shorter period receives a higher priority.
- In Earliest Deadline First scheduling, priorities are dynamic, and the scheduler always selects the task with the nearest deadline.
Here's an example of a simple scheduler function:
void scheduler(TaskControlBlock tasks[], uint8_t num_tasks) {
for (;;) { // Infinite loop for the scheduler
for (uint8_t i = 0; i < num_tasks; i++) {
if (task_ready_to_run(&tasks[i])) { // Assuming a function to check task readiness
run_task(&tasks[i]); // Function to switch context and run the task
}
}
// Other scheduler operations
}
}
Managing Context Switching
- Design context switching mechanisms to save and restore the state of tasks, allowing preemption and multitasking without data corruption.
- Implement functions to update the stack pointer, save CPU registers, and manage task states during context switches.
Clock and Timer Management
Set up a hardware timer or clock source to generate interrupts. These interrupts are crucial for scheduling tasks at the correct intervals. Utilize an ISR (Interrupt Service Routine) to update system tick counts and manage time-related tasks.
void SysTick_Handler(void) {
increment_system_ticks();
update_task_timers();
}
Handling Task Preemption
- Incorporate preemption within your scheduler to ensure high-priority tasks can interrupt lower-priority ones.
- Handle potential race conditions by disabling interrupts briefly during critical sections of the scheduler code.
Testing and Validation
- Test the scheduler under various load conditions to ensure it meets real-time constraints. Utilize debugging techniques to analyze task execution and system performance.
- Simulate potential worst-case scenarios and perform stress testing to validate deterministic behavior in your firmware.
Optimization and Fine-Tuning
- Optimize task code and scheduling functions to reduce overhead and improve execution speed.
- Continuously monitor system performance, adjust task priorities, and refine the scheduling algorithm as necessary for optimal operation.
