Resource manager task pattern in FreeRTOS

Introduction

The resource manager task pattern helps control access to shared resources safely in FreeRTOS. It avoids conflicts by letting one task manage the resource requests.

When multiple tasks need to use a shared hardware device like a sensor or communication port.

When you want to serialize access to a resource to prevent data corruption.

When you want to centralize resource control for easier debugging and maintenance.

When tasks have different priorities but must share a resource fairly.

When you want to avoid blocking tasks directly on resource access.

Syntax

FreeRTOS

/* Pseudocode for resource manager task pattern in FreeRTOS */

// Resource manager task function
void ResourceManagerTask(void *params) {
    for (;;) {
        // Wait for resource request message
        ResourceRequest req;
        ReceiveRequest(&req);

        // Process request (e.g., lock resource)
        AccessResource(req);

        // Notify requester when done
        NotifyRequester(req);
    }
}

// Other tasks send requests
void ClientTask(void *params) {
    ResourceRequest req = CreateRequest();
    SendRequestToManager(&req);
    WaitForResponse(&req);
    UseResource();
}

The resource manager task runs in a loop, handling requests one by one.

Communication between tasks usually uses queues or message buffers.

Examples

A client task sends a request struct to the resource manager via a queue.

FreeRTOS

// Example: Sending a request to resource manager
ResourceRequest req = { .taskHandle = xTaskGetCurrentTaskHandle(), .action = READ };
xQueueSend(resourceQueue, &req, portMAX_DELAY);

The resource manager waits for requests, processes them, then notifies the requesting task.

FreeRTOS

// Resource manager receives and processes request
ResourceRequest req;
xQueueReceive(resourceQueue, &req, portMAX_DELAY);
// Perform resource action
ProcessRequest(req);
// Notify client task
xTaskNotifyGive(req.taskHandle);

Sample Program

This program creates a resource manager task and two client tasks. Clients send requests to the manager via a queue. The manager processes requests one by one and notifies clients when done.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "queue.h"
#include <stdio.h>

// Define a simple request struct
typedef struct {
    TaskHandle_t requester;
    int action; // 0 = read, 1 = write
} ResourceRequest;

QueueHandle_t resourceQueue;

void ResourceManagerTask(void *params) {
    ResourceRequest req;
    for (;;) {
        if (xQueueReceive(resourceQueue, &req, portMAX_DELAY) == pdPASS) {
            // Simulate resource access
            if (req.action == 0) {
                printf("ResourceManager: Reading resource for task %p\n", req.requester);
            } else {
                printf("ResourceManager: Writing resource for task %p\n", req.requester);
            }
            // Notify requester task
            xTaskNotifyGive(req.requester);
        }
    }
}

void ClientTask(void *params) {
    ResourceRequest req = { .requester = xTaskGetCurrentTaskHandle(), .action = (int)params };
    printf("ClientTask %p: Sending request action %d\n", req.requester, req.action);
    xQueueSend(resourceQueue, &req, portMAX_DELAY);
    // Wait for resource manager to finish
    ulTaskNotifyTake(pdTRUE, portMAX_DELAY);
    printf("ClientTask %p: Resource access done\n", req.requester);
    vTaskDelete(NULL);
}

int main(void) {
    resourceQueue = xQueueCreate(5, sizeof(ResourceRequest));
    xTaskCreate(ResourceManagerTask, "ResourceManager", 1000, NULL, 2, NULL);
    xTaskCreate(ClientTask, "Client1", 1000, (void *)0, 1, NULL); // read
    xTaskCreate(ClientTask, "Client2", 1000, (void *)1, 1, NULL); // write
    vTaskStartScheduler();
    for (;;) {}
    return 0;
}

Important Notes

Use queues or message buffers for communication between tasks.

Always notify the requesting task after resource access to avoid deadlocks.

Keep the resource manager task priority higher than clients to ensure timely handling.

Summary

The resource manager task pattern centralizes control of shared resources.

Tasks send requests to the manager instead of accessing resources directly.

This pattern prevents conflicts and makes resource sharing safe and organized.

Practice

1. What is the main purpose of the Resource Manager Task Pattern in FreeRTOS?

easy

A. To centralize control of shared resources and prevent conflicts

B. To create multiple tasks accessing resources directly

C. To increase CPU usage by running more tasks simultaneously

D. To eliminate the need for task synchronization mechanisms

Solution

1. Step 1: Understand resource sharing challenges

   When multiple tasks access shared resources directly, conflicts and data corruption can occur.

2. Step 2: Role of Resource Manager Task

   The Resource Manager Task centralizes access, so tasks send requests to it instead of accessing resources directly, preventing conflicts.

3. Final Answer:

   To centralize control of shared resources and prevent conflicts -> Option A

4. Quick Check:

   Centralized control = Prevent conflicts [OK]

Hint: Resource manager centralizes resource access to avoid conflicts [OK]

Common Mistakes:

• Thinking tasks access resources directly
• Assuming it increases CPU usage
• Believing it removes synchronization needs

2. Which of the following is the correct way to send a request to a Resource Manager Task in FreeRTOS?

easy

A. Using a queue to send a request message to the manager task

B. Calling the resource manager function without synchronization

C. Directly accessing the shared resource variable

D. Using global variables without any task communication

Solution

1. Step 1: Identify communication method

   Resource Manager Tasks typically receive requests via queues to serialize access.

2. Step 2: Evaluate options

   Only Using a queue to send a request message to the manager task uses a queue to send requests, which is the correct and safe method.

3. Final Answer:

   Using a queue to send a request message to the manager task -> Option A

4. Quick Check:

   Queue communication = Correct request method [OK]

Hint: Use queues to send requests to resource manager task [OK]

Common Mistakes:

• Accessing shared variables directly
• Ignoring synchronization
• Using global variables without communication

3. Given the following FreeRTOS code snippet, what will be the output?

typedef struct {
  int command;
  int data;
} RequestMessage;

QueueHandle_t resourceQueue;

void ResourceManagerTask(void *params) {
  RequestMessage req;
  while (1) {
    if (xQueueReceive(resourceQueue, &req, portMAX_DELAY) == pdPASS) {
      if (req.command == 1) {
        printf("Processed data: %d\n", req.data * 2);
      }
    }
  }
}

void ClientTask(void *params) {
  RequestMessage msg = {1, 5};
  xQueueSend(resourceQueue, &msg, 0);
  vTaskDelete(NULL);
}

medium

A. Processed data: 5

B. Processed data: 10

C. No output, queue is empty

D. Runtime error due to invalid queue

Solution

1. Step 1: Analyze ClientTask sending message

   ClientTask sends a message with command=1 and data=5 to the resourceQueue.

2. Step 2: Analyze ResourceManagerTask processing

   ResourceManagerTask receives the message, checks command==1, then prints data*2 = 5*2 = 10.

3. Final Answer:

   Processed data: 10 -> Option B

4. Quick Check:

   Data 5 doubled = 10 output [OK]

Hint: Multiply data by 2 when command is 1 [OK]

Common Mistakes:

• Confusing data value with output
• Assuming no message received
• Ignoring the multiplication by 2

4. Identify the bug in this Resource Manager Task code snippet:

void ResourceManagerTask(void *params) {
  RequestMessage req;
  while (1) {
    if (xQueueReceive(resourceQueue, &req, 0) == pdPASS) {
      if (req.command == 1) {
        // Process command
      }
    }
  }
}

What is the problem?

medium

A. xQueueReceive should not be used in Resource Manager Task

B. RequestMessage struct is not defined

C. ResourceManagerTask should not use a while loop

D. Using zero block time causes missed messages if queue is empty

Solution

1. Step 1: Understand xQueueReceive block time

   Using 0 as block time means the call returns immediately if queue is empty, possibly missing messages.

2. Step 2: Effect on Resource Manager Task

   The task may spin without waiting, causing high CPU usage and missed processing.

3. Final Answer:

   Using zero block time causes missed messages if queue is empty -> Option D

4. Quick Check:

   Zero block time = possible missed messages [OK]

Hint: Use portMAX_DELAY to wait for messages in resource manager [OK]

Common Mistakes:

• Ignoring block time effect
• Thinking struct is undefined
• Believing while loop is wrong

5. You want to design a Resource Manager Task that handles multiple resource types (e.g., UART, SPI) with different commands. Which approach best applies the Resource Manager Task Pattern to handle this efficiently?

hard

A. Create separate manager tasks for each resource type and let clients send requests directly to each

B. Let clients access resources directly with mutexes protecting each resource

C. Use a single queue with a message struct containing resource type and command fields, then switch-case inside the manager task

D. Use global variables to store resource states and update them without task communication

Solution

1. Step 1: Consider centralized control for multiple resources

   A single Resource Manager Task can handle multiple resource types by differentiating requests.

2. Step 2: Use message struct with resource type and command

   This allows the manager task to use a switch-case to process requests efficiently and safely.

3. Final Answer:

   Use a single queue with a message struct containing resource type and command fields, then switch-case inside the manager task -> Option C

4. Quick Check:

   Single queue + message struct = efficient multi-resource management [OK]

Hint: Use message struct with resource type and switch-case in manager [OK]

Common Mistakes:

• Creating multiple manager tasks unnecessarily
• Allowing direct resource access with mutexes
• Using global variables without communication