Tasks

Task: a long-living execution block.

  • The application is made of several tasks.
  • These tasks may have to exchange data and to synchronize.
  • Each task has its own execution context
  • Each task can be assigned a priority
  • Each task is assigned a "fair" proportion of processing time by the scheduler

A task may be in one of several states. Usually:

  • Ready: the task is ready to run, but another one is using the processor
  • Running: the task is being executed
  • Blocked (or Waiting): the task is waiting for an event (timer timeout, reception of a byte on a serial link, etc.)
  • Suspended: the task won't use the processor anymore

For FreeRTOS:

Source: FreeRTOS

Note: for our practice sessions, we will use FreeRTOS

For Zephyr OS:

Source: Zephyr Project

On a microcontroller with one core: only one task can be executed at a given time.

⇒ Difference between concurrency and parallelism:

  • Concurrency: several tasks appear to be running at the same time
  • Parallelism: several tasks are really running at the same time

The scheduler is the magic behind concurrency.

Scheduling method depends on the RTOS.

For FreeRTOS:

  • Fixed priority - task priority is not changed (excepted for priority inheritance - see farther)
  • Preemptive - if a higher priority task enters ready state (due to some event), and a lower priority task is being executed, the scheduler stops it and starts the higher priority one
  • Round-robin time slicing - every task in the set of tasks with same priority is guaranteed to be executed after some time

When a task requests to wait for some event:

  • The task enters the blocked (waiting) state
  • The scheduler schedules the task with the highest priority, having waited for the longest time period

When a task with a given priority loops, for instance polling for an event without waiting for it:

  • Tasks with lower priority will never be executed.

API example

  • task_handle_t task_create(...)
  • status_t task_delete(task_handle_t th)
  • status_t task_priority_set(task_handle_t th)
  • status_t task_suspend(task_handle_t th)
  • status_t task_resume(task_handle_t th)
  • ...

A good overview of task scheduling is provided by the FreeRTOS website.

Practice session 11

Creating and starting one task