Considerations for RTOS Task Stack Allocation

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Author | strongerHuang

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Some friends asked this question: Do I need to allocate a large stack for a task with a lot of code?

Actually, it’s not that the more code there is, the more stack space needs to be allocated. It mainly depends on the number of “temporary variables” included in your task.

Considerations for RTOS Task Stack Allocation

(The South China Electronics Show in Munich will be held at the end of the month, and everyone can register in advance)

1RTOS Task Stack Allocation

Many RTOS tasks on the market require stack size to be allocated in advance, which means allocating the stack size when creating the task.

For example, to create a check task in uCOS:

// Task priority
#define TASK_CHECK_PRIO                        6
// Task stack size
#define TASK_CHECK_STK_SIZE                    128
// Stack
OS_STK TaskCheckStk[TASK_CHECK_STK_SIZE];
// Create task - signal detection
OSTaskCreateExt((void (*)(void *)) AppTaskCheck,                (void           *) 0,                (OS_STK         *)&amp;TaskCheckStk[TASK_CHECK_STK_SIZE-1],                (INT8U           ) TASK_CHECK_PRIO,                (INT16U          ) TASK_CHECK_PRIO,                (OS_STK         *)&amp;TaskCheckStk[0],                (INT32U          ) TASK_CHECK_STK_SIZE,                (void           *) 0,                (INT16U          )(OS_TASK_OPT_STK_CHK | OS_TASK_OPT_STK_CLR));
// Task implementation
void AppTaskCheck(void *p_arg){  // Code···
  (void)p_arg;
  for(;;)  {    // Code···  }
}

Creating a task in FreeRTOS is similar; stack size is also allocated at creation:

#define TASK_CHECK_PRIO                        6
#define TASK_CHECK_STK_SIZE                    128
BaseType_t xReturn;
xReturn = xTaskCreate(AppTaskCheck, "AppTaskCheck", TASK_CHECK_STK_SIZE, NULL, TASK_CHECK_PRIO, NULL);

In addition to the stack, message queues and message mailboxes also need to have their stack allocated in advance.

For example, creating a CLI message queue in FreeRTOS:

#define CLI_QUEUE_NUM             256                      // Number of CLI receive queues
#define CLI_PACKAGE_LEN           2                        // Length of CLI data packet
QueueHandle_t xCLIRcvQueue = NULL;
/* Create queue */
if(xCLIRcvQueue == NULL){  xCLIRcvQueue = xQueueCreate(CLI_QUEUE_NUM, CLI_PACKAGE_LEN);}

This is the allocation of stack when creating tasks (or queues). As for how much to allocate, it depends on your actual situation, which I will describe in the following sections.

2Task Code Size

The code size of a task is the amount of code called in your task.

For example, the code in the above example:

// Task implementation
void AppTaskCheck(void *p_arg){  // Code···
  (void)p_arg;
  for(;;)  {    // Code···  }
}

There could be thousands of lines of code here, or it could call hundreds of functions, each with a lot of code inside.

Thus, the code size of this task becomes very large.

3Is There a Relationship Between Task Code Size and Stack Size?

Many people have a doubt: If a task is suspended, it needs to temporarily save the task in the stack. If this task has a lot of code, doesn’t it need a large stack space?

Answer:Not necessarily. The task code size and stack do not have a direct relationship.

Many beginners may have a misconception: saving a task means saving all the code of that task (in the stack).

The stack mainly saves the task’s own variables (control blocks), as well as temporary variables and other key variable information, rather than saving all the code.

4How Much Stack Should Be Allocated?

The size of the task stack mainly depends on the number of temporary variables in your task.

Note: Temporary variables include all temporary variables in nested functions in your code.

For processors with relatively large RAM resources, you can allocate a bit more stack resources.

However, many times, our RAM resources are relatively tight. At this point, you need to balance comprehensively.

For example, static local variables:

void AppTaskCheck(void *p_arg){  static uint8_t aaa;  // Static local variable
  (void)p_arg;
  for(;;)  {    // Code···  }
}

The variable aaa here will not occupy the stack space of the task, but it will occupy global variable (RAM) space.

Whether to use static local variables or temporary variables depends on your project’s specific situation, such as: RAM resources, code running efficiency, etc. (Temporary variables will also have a data copy process).

So, how to allocate the stack, whether to use static or temporary variables, needs to be considered comprehensively based on your project’s situation.

(The South China Electronics Show in Munich will be held at the end of the month, and everyone can register in advance)

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1RTOS Task Stack Allocation

2Task Code Size

3Is There a Relationship Between Task Code Size and Stack Size?

4How Much Stack Should Be Allocated?

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