ESP-IDF FreeRTOS SMP Changes

Overview

The vanilla FreeRTOS is designed to run on a single core. However the ESP32 is dual core containing a Protocol CPU (known as CPU 0 or PRO_CPU) and an Application CPU (known as CPU 1 or APP_CPU). The two cores are identical in practice and share the same memory. This allows the two cores to run tasks interchangeably between them.

The ESP-IDF FreeRTOS is a modified version of vanilla FreeRTOS which supports symmetric multiprocessing (SMP). ESP-IDF FreeRTOS is based on the Xtensa port of FreeRTOS v8.2.0, however features such as static task creation and Thread Local Storage Pointers have been backported from later versions of FreeRTOS. This guide outlines the major differences between vanilla FreeRTOS and ESP-IDF FreeRTOS. The API reference for vanilla FreeRTOS can be found via http://www.freertos.org/a00106.html

Tasks and Task Creation: Use xTaskCreatePinnedToCore() or xTaskCreateStaticPinnedToCore() to create tasks in ESP-IDF FreeRTOS. The last parameter of the two functions is xCoreID. This parameter specifies which core the task is pinned to. Acceptable values are 0 for PRO_CPU, 1 for APP_CPU, or tskNO_AFFINITY which allows the task to run on both.

Round Robin Scheduling: The ESP-IDF FreeRTOS scheduler will skip tasks when implementing Round-Robin scheduling between multiple tasks in the Ready state that are of the same priority. To avoid this behavior, ensure that those tasks either enter a blocked state, or are distributed across a wider range of priorities.

Scheduler Suspension: Suspending the scheduler in ESP-IDF FreeRTOS will only affect the scheduler on the the calling core. In other words, calling vTaskSuspendAll() on PRO_CPU will not prevent APP_CPU from scheduling, and vice versa. Use critical sections or semaphores instead for simultaneous access protection.

Tick Interrupt Synchronicity: Tick interrupts of PRO_CPU and APP_CPU are not synchronized. Do not expect to use vTaskDelay or vTaskDelayUntil as an accurate method of synchronizing task execution between the two cores. Use a counting semaphore instead as their context switches are not tied to tick interrupts due to preemption.

Critical Sections & Disabling Interrupts: In ESP-IDF FreeRTOS, critical sections are implemented using mutexes. Entering critical sections involve taking a mutex, then disabling the scheduler and interrupts of the calling core. However the other core is left unaffected. If the other core attemps to take same mutex, it will spin until the calling core has released the mutex by exiting the critical section.

Thread Local Storage Pointers & Deletion Callbacks: ESP-IDF FreeRTOS has backported the Thread Local Storage Pointers feature. However they have the extra feature of deletion callbacks. Deletion callbacks are used to automatically free memory used by Thread Local Storage Pointers during the task deletion. Call vTaskSetThreadLocalStoragePointerAndDelCallback() to set Thread Local Storage Pointers and deletion callbacks.

FreeRTOS Hooks: Vanilla FreeRTOS Hooks were not designed for SMP. ESP-IDF provides its own Idle and Tick Hooks in addition to the Vanilla FreeRTOS hooks. For full details, see the ESP-IDF Hooks API Reference.

Configuring ESP-IDF FreeRTOS: Several aspects of ESP-IDF FreeRTOS can be configured using make meunconfig such as running ESP-IDF in Unicore Mode, or configuring the number of Thread Local Storage Pointers each task will have.

Tasks and Task Creation

Tasks in ESP-IDF FreeRTOS are designed to run on a particular core, therefore two new task creation functions have been added to ESP-IDF FreeRTOS by appending PinnedToCore to the names of the task creation functions in vanilla FreeRTOS. The vanilla FreeRTOS functions of xTaskCreate() and xTaskCreateStatic() have led to the addition of xTaskCreatePinnedToCore() and xTaskCreateStaticPinnedToCore() in ESP-IDF FreeRTOS.

For more details see freertos/task.c

The ESP-IDF FreeRTOS task creation functions are nearly identical to their vanilla counterparts with the exception of the extra parameter known as xCoreID. This parameter specifies the core on which the task should run on and can be one of the following values.

  • 0 pins the task to PRO_CPU
  • 1 pins the task to APP_CPU
  • tskNO_AFFINITY allows the task to be run on both CPUs

For example xTaskCreatePinnedToCore(tsk_callback, “APP_CPU Task”, 1000, NULL, 10, NULL, 1) creates a task of priority 10 that is pinned to APP_CPU with a stack size of 1000 bytes. It should be noted that the uxStackDepth parameter in vanilla FreeRTOS specifies a task’s stack depth in terms of the number of words, whereas ESP-IDF FreeRTOS specifies the stack depth in terms of bytes.

Note that the vanilla FreeRTOS functions xTaskCreate and xTaskCreateStatic have been macro defined in ESP-IDF FreeRTOS to call xTaskCreatePinnedToCore() and xTaskCreateStaticPinnedToCore() respectively with tskNO_AFFINITY as the xCoreID value.

Each Task Control Block (TCB) in ESP-IDF stores the xCoreID as a member. Hence when each core calls the scheduler to select a task to run, the xCoreID member will allow the scheduler to determine if a given task is permitted to run on the core that called it.

Scheduling

The vanilla FreeRTOS implements scheduling in the vTaskSwitchContext() function. This function is responsible for selecting the highest priority task to run from a list of tasks in the Ready state known as the Ready Tasks List (described in the next section). In ESP-IDF FreeRTOS, each core will call vTaskSwitchContext() independently to select a task to run from the Ready Tasks List which is shared between both cores. There are several differences in scheduling behavior between vanilla and ESP-IDF FreeRTOS such as differences in Round Robin scheduling, scheduler suspension, and tick interrupt synchronicity.

Round Robin Scheduling

Given multiple tasks in the Ready state and of the same priority, vanilla FreeRTOS implements Round Robin scheduling between each task. This will result in running those tasks in turn each time the scheduler is called (e.g. every tick interrupt). On the other hand, the ESP-IDF FreeRTOS scheduler may skip tasks when Round Robin scheduling multiple Ready state tasks of the same priority.

The issue of skipping tasks during Round Robin scheduling arises from the way the Ready Tasks List is implemented in FreeRTOS. In vanilla FreeRTOS, pxReadyTasksList is used to store a list of tasks that are in the Ready state. The list is implemented as an array of length configMAX_PRIORITIES where each element of the array is a linked list. Each linked list is of type List_t and contains TCBs of tasks of the same priority that are in the Ready state. The following diagram illustrates the pxReadyTasksList structure.

Vanilla FreeRTOS Ready Task List Structure

Illustration of FreeRTOS Ready Task List Data Structure

Each linked list also contains a pxIndex which points to the last TCB returned when the list was queried. This index allows the vTaskSwitchContext() to start traversing the list at the TCB immediately after pxIndex hence implementing Round Robin Scheduling between tasks of the same priority.

In ESP-IDF FreeRTOS, the Ready Tasks List is shared between cores hence pxReadyTasksList will contain tasks pinned to different cores. When a core calls the scheduler, it is able to look at the xCoreID member of each TCB in the list to determine if a task is allowed to run on calling the core. The ESP-IDF FreeRTOS pxReadyTasksList is illustrated below.

ESP-IDF FreeRTOS Ready Task List Structure

Illustration of FreeRTOS Ready Task List Data Structure in ESP-IDF

Therefore when PRO_CPU calls the scheduler, it will only consider the tasks in blue or purple. Whereas when APP_CPU calls the scheduler, it will only consider the tasks in orange or purple.

Although each TCB has an xCoreID in ESP-IDF FreeRTOS, the linked list of each priority only has a single pxIndex. Therefore when the scheduler is called from a particular core and traverses the linked list, it will skip all TCBs pinned to the other core and point the pxIndex at the selected task. If the other core then calls the scheduler, it will traverse the linked list starting at the TCB immediately after pxIndex. Therefore, TCBs skipped on the previous scheduler call from the other core would not be considered on the current scheduler call. This issue is demonstrated in the following illustration.

ESP-IDF pxIndex Behavior

Illustration of pxIndex behavior in ESP-IDF FreeRTOS

Referring to the illustration above, assume that priority 9 is the highest priority, and none of the tasks in priority 9 will block hence will always be either in the running or Ready state.

1) PRO_CPU calls the scheduler and selects Task A to run, hence moves pxIndex to point to Task A

2) APP_CPU calls the scheduler and starts traversing from the task after pxIndex which is Task B. However Task B is not selected to run as it is not pinned to APP_CPU hence it is skipped and Task C is selected instead. pxIndex now points to Task C

3) PRO_CPU calls the scheduler and starts traversing from Task D. It skips Task D and selects Task E to run and points pxIndex to Task E. Notice that Task B isn’t traversed because it was skipped the last time APP_CPU called the scheduler to traverse the list.

4) The same situation with Task D will occur if APP_CPU calls the scheduler again as pxIndex now points to Task E

One solution to the issue of task skipping is to ensure that every task will enter a blocked state so that they are removed from the Ready Task List. Another solution is to distribute tasks across multiple priorities such that a given priority will not be assigned multiple tasks that are pinned to different cores.

Scheduler Suspension

In vanilla FreeRTOS, suspending the scheduler via vTaskSuspendAll() will prevent calls of vTaskSwitchContext() from context switching until the scheduler has been resumed with vTaskResumeAll(). However servicing ISRs are still permitted. Therefore any changes in task states as a result from the current running task or ISRSs will not be executed until the scheduler is resumed. Scheduler suspension in vanilla FreeRTOS is a common protection method against simultaneous access of data shared between tasks, whilst still allowing ISRs to be serviced.

In ESP-IDF FreeRTOS, vTaskSuspendAll() will only prevent calls of vTaskSwitchContext() from switching contexts on the core that called for the suspension. Hence if PRO_CPU calls vTaskSuspendAll(), APP_CPU will still be able to switch contexts. If data is shared between tasks that are pinned to different cores, scheduler suspension is NOT a valid method of protection against simultaneous access. Consider using critical sections (disables interrupts) or semaphores (does not disable interrupts) instead when protecting shared resources in ESP-IDF FreeRTOS.

In general, it’s better to use other RTOS primitives like mutex semaphores to protect against data shared between tasks, rather than vTaskSuspendAll().

Tick Interrupt Synchronicity

In ESP-IDF FreeRTOS, tasks on different cores that unblock on the same tick count might not run at exactly the same time due to the scheduler calls from each core being independent, and the tick interrupts to each core being unsynchronized.

In vanilla FreeRTOS the tick interrupt triggers a call to xTaskIncrementTick() which is responsible for incrementing the tick counter, checking if tasks which have called vTaskDelay() have fulfilled their delay period, and moving those tasks from the Delayed Task List to the Ready Task List. The tick interrupt will then call the scheduler if a context switch is necessary.

In ESP-IDF FreeRTOS, delayed tasks are unblocked with reference to the tick interrupt on PRO_CPU as PRO_CPU is responsible for incrementing the shared tick count. However tick interrupts to each core might not be synchronized (same frequency but out of phase) hence when PRO_CPU receives a tick interrupt, APP_CPU might not have received it yet. Therefore if multiple tasks of the same priority are unblocked on the same tick count, the task pinned to PRO_CPU will run immediately whereas the task pinned to APP_CPU must wait until APP_CPU receives its out of sync tick interrupt. Upon receiving the tick interrupt, APP_CPU will then call for a context switch and finally switches contexts to the newly unblocked task.

Therefore, task delays should NOT be used as a method of synchronization between tasks in ESP-IDF FreeRTOS. Instead, consider using a counting semaphore to unblock multiple tasks at the same time.

Critical Sections & Disabling Interrupts

Vanilla FreeRTOS implements critical sections in vTaskEnterCritical which disables the scheduler and calls portDISABLE_INTERRUPTS. This prevents context switches and servicing of ISRs during a critical section. Therefore, critical sections are used as a valid protection method against simultaneous access in vanilla FreeRTOS.

On the other hand, the ESP32 has no hardware method for cores to disable each other’s interrupts. Calling portDISABLE_INTERRUPTS() will have no effect on the interrupts of the other core. Therefore, disabling interrupts is NOT a valid protection method against simultaneous access to shared data as it leaves the other core free to access the data even if the current core has disabled its own interrupts.

For this reason, ESP-IDF FreeRTOS implements critical sections using mutexes, and calls to enter or exit a critical must provide a mutex that is associated with a shared resource requiring access protection. When entering a critical section in ESP-IDF FreeRTOS, the calling core will disable its scheduler and interrupts similar to the vanilla FreeRTOS implementation. However, the calling core will also take the mutex whilst the other core is left unaffected during the critical section. If the other core attempts to take the same mutex, it will spin until the mutex is released. Therefore, the ESP-IDF FreeRTOS implementation of critical sections allows a core to have protected access to a shared resource without disabling the other core. The other core will only be affected if it tries to concurrently access the same resource.

The ESP-IDF FreeRTOS critical section functions have been modified as follows…

  • taskENTER_CRITICAL(mux), taskENTER_CRITICAL_ISR(mux), portENTER_CRITICAL(mux), portENTER_CRITICAL_ISR(mux) are all macro defined to call vTaskEnterCritical()
  • taskEXIT_CRITICAL(mux), taskEXIT_CRITICAL_ISR(mux), portEXIT_CRITICAL(mux), portEXIT_CRITICAL_ISR(mux) are all macro defined to call vTaskExitCritical()

For more details see freertos/include/freertos/portmacro.h and freertos/task.c

It should be noted that when modifying vanilla FreeRTOS code to be ESP-IDF FreeRTOS compatible, it is trivial to modify the type of critical section called as they are all defined to call the same function. As long as the same mutex is provided upon entering and exiting, the type of call should not matter.

Thread Local Storage Pointers & Deletion Callbacks

Thread Local Storage Pointers are pointers stored directly in the TCB which allows each task to have a pointer to a data structure containing that is specific to that task. However vanilla FreeRTOS provides no functionality to free the memory pointed to by the Thread Local Storage Pointers. Therefore if the memory pointed to by the Thread Local Storage Pointers is not explicitly freed by the user before a task is deleted, memory leak will occur.

ESP-IDF FreeRTOS provides the added feature of deletion callbacks. These deletion callbacks are used to automatically free the memory pointed to by the Thread Local Storage Pointers when a task is deleted. Each Thread Local Storage Pointer can have its own call back, and these call backs are called when the Idle tasks cleans up a deleted tasks.

Vanilla FreeRTOS sets a Thread Local Storage Pointers using vTaskSetThreadLocalStoragePointer() whereas ESP-IDF FreeRTOS sets a Thread Local Storage Pointers and Deletion Callbacks using vTaskSetThreadLocalStoragePointerAndDelCallback() which accepts a pointer to the deletion call back as an extra parameter of type `TlsDeleteCallbackFunction_t. Calling the vanilla FreeRTOS API vTaskSetThreadLocalStoragePointer() is still valid however it is internally defined to call vTaskSetThreadLocalStoragePointerAndDelCallback() with a NULL pointer as the deletion call back. This results in the selected Thread Local Storage Pointer to have no deletion call back.

In IDF the FreeRTOS thread local storage at index 0 is reserved and is used to implement the pthreads API thread local storage (pthread_getspecific() & pthread_setspecific()). Other indexes can be used for any purpose, provided FREERTOS_THREAD_LOCAL_STORAGE_POINTERS is set to a high enough value.

For more details see freertos/include/freertos/task.h

Configuring ESP-IDF FreeRTOS

The ESP-IDF FreeRTOS can be configured using make menuconfig under Component_Config/FreeRTOS. The following section highlights some of the ESP-IDF FreeRTOS configuration options. For a full list of ESP-IDF FreeRTOS configurations, see FreeRTOS

FREERTOS_UNICORE will run ESP-IDF FreeRTOS only on PRO_CPU. Note that this is not equivalent to running vanilla FreeRTOS. Behaviors of multiple components in ESP-IDF will be modified such as esp32/cpu_start.c. For more details regarding the effects of running ESP-IDF FreeRTOS on a single core, search for occurences of CONFIG_FREERTOS_UNICORE in the ESP-IDF components.

FREERTOS_THREAD_LOCAL_STORAGE_POINTERS will define the number of Thread Local Storage Pointers each task will have in ESP-IDF FreeRTOS.

SUPPORT_STATIC_ALLOCATION will enable the backported functionality of xTaskCreateStaticPinnedToCore() in ESP-IDF FreeRTOS

FREERTOS_ASSERT_ON_UNTESTED_FUNCTION will trigger a halt in particular functions in ESP-IDF FreeRTOS which have not been fully tested in an SMP context.