C++ Embedded Task Monitor

• Download TaskMonitor-1.3.zip - 1.1 MB

Introduction

When working in multi-threaded environments, having a task monitor is one of the most useful and critical components for robustness, safety, and stability. Essentially, the task monitor makes sure that all the other tasks in the system are working properly and as intended. If a task stops checking in with the task monitor, the task monitor can identify the violation and do various actions. A very basic action is to allow the watchdog to timeout and reset the device.

Nearly every product I've worked on has included some type of task monitor and having improved the implementation over various projects and years of work, I felt this would be useful for the community.

Background

Many operating systems have built in functionality for task monitoring (e.g. linux). However, many smaller real time operating systems do not have this functionality (e.g., FreeRTOS). This implementation is intended for use in smaller embedded systems that may not have built in functionality.

Features

• Monitor any number of tasks
• Each task can have a unique timeout period
• Task Monitor will send out a message for a task to respond
• OR a task can send a message to the Task Monitor at anytime without have to respond to a specific message
• Task Monitor automatically determines which task is checking in (No need to track or send Task IDs)
• Watchdog only needs to be managed in a single place
• Ideal for event driven and message driven tasks
• Small footprint
• Can be used for very small embedded systems (e.g., FreeRTOS on 8-bit micro if desired)
• RTOS agnostic (can be used with any RTOS with minor updates)
• Messages will only go out once per period for that task timeout (e.g., task timeout is 100ms, then messages sent out will happen at most once every 100ms, regardless of how quickly the response is received)
• Unsolicited messages received for task checkin will automatically reset the timing, so a message won't go out to a task unnecessarily

Using the Code

There are only two basic functions needed to utilize the functionality within existing tasks/threads.

First, the task/thread needs to register itself with the Task monitor. This should be done as part of the initialization of the task/thread, or at the start of task/thread.

• The first argument is the maximum time the task monitor will wait for a response from a task, from a checkin request.
• The second argument is the function pointer to the checkin call the task monitor uses to send a message to the task when a checkin request is issued.

TaskMonitor::Register(TIMEOUT, &TaskCheckin);

The callback in the Register function above should be something that can be used to notify the task registering with the task monitor.  For example:

void Task1::TaskCheckin()
{
    Message msg(Message::TASK_CHECKIN);
    ASSERT(PASS == QSend(msgQHandle, &msg));
} 

The TaskCheckin will be called by the task monitor task when it requests a checkin from Task1. This implementation will create a message to put in the message queue that Task1 listens on and take appropriate action on receiving that message (e.g. call TaskCheckin() as shown below).

For another solution without relying on message queue in Task1, see OS Porting below.

The other main function is to checkin when desired with the task monitor.

TaskMonitor::TaskCheckin();

The following sequence diagram shows how this process operates.

The code linked with this article shows example usage with four different tasks utilizing the task monitor in different ways. A task can respond to a checkin request, or preemptively checkin as desired.

The task monitor will send a request to the registered task once every timeout period (TIMEOUT time provided during registration). If the registered task posts a checkin to the task monitor before a request has been sent by the task monitor, then the timer will reset and the task monitor will not send a request until another full timeout period has elapsed.

Task Monitor Task

The task monitor functionality relies on the operating system in use and is required to have its own task/thread to operate in. Therefore, the developer needs to create and start the task monitor task as part of the rest of the system task initializations. For example, in FreeRTOS:

void StartTaskMonitor(void *argument)
{
    TaskMonitor taskMon;
    taskMon.Initialize();
    taskMon.Run();
}

void app_main(void)
{
    TaskHandle_t taskHandle = nullptr;

    // Task monitor
    xTaskCreate(&StartTaskMonitor, "TaskMonitor", TASK_MONITOR_STACK_SIZE, 
                NULL, TASK_MONITOR_PRIORITY, &taskHandle);
}

OS Porting

The task monitor also uses some of the operating system calls to operate. Therefore, TargetPort.hpp and TargetPort.cpp were created to easily port for different operating systems being utilitized. The main items for porting purposes are message queue operations, task identification operations and watchdog operations.

Alternative TaskCheckin Callback Solution

If a task in the system does not utilize a message queue, you can use a static boolean to accomplish the same outcome. The callback function could look like something below:

static bool checkinRequested = false;

void Task1::TaskCheckin() 
{ 
   checkinRequested = true;
} 

In the task execution loop, you can simply check if this flag is set, send the checkin message to the TaskMonitor, and continue the task execution.  An example of this is below:

void Task1::Run()
{
   // Task loop - never exits
   while (true)
   {
      if (true == checkinRequested)
      {
         TaskMonitor::TaskCheckin();
         checkinRequested = false;
      }

      // Rest of Task execution
      // ...
   }
}

Violation Actions

The task monitor takes an action when it determines that a task has violated the time limit to checkin. The default action is to do the following:

// check for expired task checkins... if there was a violation, 
// we can print the name of the violator and then wait until the watchdog bites
if (monInfo.elapsedTimeWaitingForResponse > monInfo.timeout)
{
   LOG_ERROR("TaskMonitor", "Task: %s - Exceeded Checkin Time. 
              Elapsed time %u ms, Timeout %u ms",
              GetTaskName(taskId), monInfo.elapsedTimeWaitingForResponse, 
              monInfo.timeout);

   while(1) {}
}

The while(1) {} allows the task to spin forever until the watchdog resets the device. This code can be updated to do whatever the user desires:

// Designers can also replace the above with their own logic that can reset tasks, 
// end tasks, or take other actions based on violations. Example below:
// if (monInfo.elapsedTimeWaitingForResponse > monInfo.timeout)
// {
//    // Do action, such as restart the task that violated the timeout
// }

The attached code and Github link provide more thorough examples and details.

History

• 16^th November, 2022: Updated License to MIT
• 29^th October, 2022: Added sequence diagram, callback implementation and alternative solutions without message queues.
• 27^th October, 2022: Initial revision.