FreeRTOS Producer Consumer Tasks

Objective

  • Learn how Tasks and Queues work
  • Assess how task priorities affect the RTOS Queue cooperative scheduling

Queues and Task Priorities

Tasks of equal priority that are both ready to run are scheduled by the RTOS in a round-robin fashion.  This type of context switch is called Preemptive Context Switch.

Queues' API can also perform context switches, but this is a type of Cooperative Context Switch.  What this means is that if xQueueSend() API is sending an item to a higher priority task that was waiting on the same queue using the xQueueReceive() API, then the sending task will switch context inside of the xQueueSend() function over to the other task.  Therefore, task priorities matter when using the queue API.

Also note that when the cooperative context switch occurs, it does not wait for the next tick of preemptive scheduling to switch context. Typical RTOSes support both cooperative and preemptive scheduling, and in fact, you can turn off preemptive scheduling in FreeRTOSConfig.h

static QueueHandle_t switch_queue;

// TODO: Create this task at PRIORITY_LOW
void producer(void *p) {
  int switch_value = 0;
  while (1) {
    // This xQueueSend() will internally switch context to "consumer" task because it is higher priority than this "producer" task
    // Then, when the consumer task sleeps, we will resume out of xQueueSend()and go over to the next line
    
    // TODO:
    switch_value = get_switch_input_from_switch0();
 
    // TODO: Print a message before xQueueSend()
    // Note: Use printf() and not fprintf(stderr, ...) because stderr is a polling printf
    xQueueSend(switch_queue, &switch_value, 0);
    // TODO: Print a message after xQueueSend()
    
    vTaskDelay(1000);
  }
}

// TODO: Create this task at PRIORITY_HIGH
void consumer(void *p) {
  int x;
  while (1) {
    // TODO: Print a message before xQueueReceive()
    xQueueReceive(switch_queue, &x, portMAX_DELAY);
    // TODO: Print a message after xQueueReceive()
  }
}

void main(void) {
  // TODO: Create your tasks
  // TODO: Configure your switch pin
  
  // Queue handle is not valid until you create it
  switch_queue = xQueueCreate(...);
  
  vTaskStartScheduler();
}

Assignment

  • Finish producer task that reads a switch value and sends it to the queue
    • Create an enumeration such as typedef enum { switch__off, switch__on} switch_e;
  • Create a queue, and have the producer task send switch values every second to the queue
  • Finish consumer task that is waiting on the enumeration sent by the producer task

After ensuring that the producer task is sending values to the consumer task, do the following:

  • Ensure that the following is already setup:
    • Print a message before and after sending the switch value to the queue
    • Print a message after the consumer task receives an item from the queue
  • Note down the Observations by doing the following:
    • Use higher priority for producer task, and note down the order of the print-outs
    • Use higher priority for consumer task, and note down the order of the print-outs
    • Use same priority level for both tasks, and note down the order of the print-outs

Answer Additional Questions:

  • What is the purpose of the block time during xQueueReceive()?
  • What if you use ZERO block time during xQueueReceive()?
What to turn in
  
  • Submit all relevant source code
  • Relevant screenshots of serial terminal output
  • Submit explanation to the questions as comments in your code at the top of your source code file
    • Explanation of the Observations
    • Explanation for the Additional Questions

Extra Credit

This extra credit will help you in future labs, so it is highly recommended that you achieve this. You will add a CLI handler to be able to:

  • Suspend a task by name
  • Resume a task by name

Please follow this article to add your CLI command. Here is reference code for your CLI:

app_cli_status_e cli__task_control(app_cli__argument_t argument, sl_string_t user_input_minus_command_name,
                                   app_cli__print_string_function cli_output) {
  sl_string_t s = user_input_minus_command_name;

  // If the user types 'taskcontrol suspend led0' then we need to suspend a task with the name of 'led0'
  // In this case, the user_input_minus_command_name will be set to 'suspend led0' with the command-name removed
  if (sl_string__begins_with_ignore_case(s, "suspend")) {
    // TODO: Use sl_string API to remove the first word, such that variable 's' will equal to 'led0'
    // TODO: Or you can do this: char name[16]; sl_string__scanf("%*s %16s", name);
    
    // Now try to query the tasks with the name 'led0'
    TaskHandle_t task_handle = xTaskGetHandle(s);
    if (NULL == task_handle) {
      // note: we cannot use 'sl_string__printf("Failed to find %s", s);' because that would print existing string onto itself
      sl_string__insert_at(s, "Could not find a task with name:");
      cli_output(NULL, s);
    } else {
      // TODO: Use vTaskSuspend()
    }
    
  } else if (sl_string__begins_with_ignore_case(s, "resume")) {
    // TODO
  } else {
    cli_output(NULL, "Did you mean to say suspend or resume?\n");
  }

  return APP_CLI_STATUS__SUCCESS;
}

 


 

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