ESP-IDF FreeRTOS SMP Changes¶
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 v10.2.0. This guide outlines the major differences between vanilla FreeRTOS and ESP-IDF FreeRTOS. The API reference for vanilla FreeRTOS can be found via https://www.freertos.org/a00106.html
For information regarding features that are exclusive to ESP-IDF FreeRTOS, see ESP-IDF FreeRTOS Additions.
Thread Local Storage Pointers & Deletion Callbacks: Deletion callbacks are called automatically during task deletion and are
used to free memory pointed to by TLSP. Call
vTaskSetThreadLocalStoragePointerAndDelCallback() to set TLSP and Deletion
Configuring ESP-IDF FreeRTOS: Several aspects of ESP-IDF FreeRTOS can be
set in the project configuration (
idf.py menuconfig) such as running ESP-IDF in
Unicore (single core) Mode, or configuring the number of Thread Local Storage Pointers
each task will have.
It is not necessary to manually start the FreeRTOS scheduler by calling
vTaskStartScheduler(). In ESP-IDF the
scheduler is started by the Application Startup Flow and is already running when the
app_main function is called (see Running the main task for details).
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
PinnedToCore to the names of the task creation functions in
vanilla FreeRTOS. The vanilla FreeRTOS functions of
xTaskCreateStatic() have led to the addition of
For more details see freertos/tasks.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.
0pins the task to PRO_CPU
1pins the task to APP_CPU
tskNO_AFFINITYallows the task to be run on both CPUs
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
xTaskCreateStatic() have been defined in ESP-IDF FreeRTOS as inline functions which call
tskNO_AFFINITY as the
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.
The vanilla FreeRTOS implements scheduling in the
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
Round Robin Scheduling¶
Given multiple tasks in the Ready state and of the same priority, vanilla FreeRTOS implements Round Robin scheduling between multiple ready state tasks of the same priority. This will result in running those tasks in turn each time the scheduler is called (e.g. when the tick interrupt occurs or when a task blocks/yields).
On the other hand, it is not possible for the ESP-IDF FreeRTOS scheduler to implement perfect Round Robin due to the fact that a particular task may not be able to run on a particular core due to the following reasons:
The task is pinned to the another core.
For unpinned tasks, the task is already being run by another core.
Therefore, when a core searches the ready state task list for a task to run, the core may need to skip over a few tasks in the same priority list or drop to a lower priority in order to find a ready state task that the core can run.
The ESP-IDF FreeRTOS scheduler implements a Best Effort Round Robin scheduling for ready state tasks of the same priority by ensuring that tasks that have been selected to run will be placed at the back of the list, thus giving unselected tasks a higher priority on the next scheduling iteration (i.e., the next tick interrupt or yield)
The following example demonstrates the Best Effort Round Robin Scheduling in action. Assume that:
There are four ready state tasks of the same priority
AX, B0, C1, D1where: - The priority is the current highest priority with ready state tasks - The first character represents the task’s names (i.e.,
A, B, C, D) - And the second character represents the tasks core pinning (and
The task list is always searched from the head
-------------------------------------------------------------------------------- 1. Starting state. None of the ready state tasks have been selected to run Head [ AX , B0 , C1 , D0 ] Tail -------------------------------------------------------------------------------- 2. Core 0 has tick interrupt and searches for a task to run. Task A is selected and is moved to the back of the list Core0--| Head [ AX , B0 , C1 , D0 ] Tail 0 Head [ B0 , C1 , D0 , AX ] Tail -------------------------------------------------------------------------------- 3. Core 1 has a tick interrupt and searches for a task to run. Task B cannot be run due to incompatible affinity, so core 1 skips to Task C. Task C is selected and is moved to the back of the list Core1-------| 0 Head [ B0 , C1 , D0 , AX ] Tail 0 1 Head [ B0 , D0 , AX , C1 ] Tail -------------------------------------------------------------------------------- 4. Core 0 has another tick interrupt and searches for a task to run. Task B is selected and moved to the back of the list Core0--| 1 Head [ B0 , D0 , AX , C1 ] Tail 1 0 Head [ D0 , AX , C1 , B0 ] Tail -------------------------------------------------------------------------------- 5. Core 1 has another tick and searches for a task to run. Task D cannot be run due to incompatible affinity, so core 1 skips to Task A Task A is selected and moved to the back of the list Core1-------| 0 Head [ D0 , AX , C1 , B0 ] Tail 0 1 Head [ D0 , C1 , B0 , AX ] Tail
The implications to users regarding the Best Effort Round Robin Scheduling:
Users cannot expect multiple ready state tasks of the same priority to run sequentially (as is the case in Vanilla FreeRTOS). As demonstrated in the example above, a core may need to skip over tasks.
However, given enough ticks, a task will eventually be given some processing time.
If a core cannot find a task runnable task at the highest ready state priority, it will drop to a lower priority to search for tasks.
To achieve ideal round robin scheduling, users should ensure that all tasks of a particular priority are pinned to the same core.
In vanilla FreeRTOS, suspending the scheduler via
prevent calls of
vTaskSwitchContext from context switching until the
scheduler has been resumed with
xTaskResumeAll(). However servicing ISRs
are still permitted. Therefore any changes in task states as a result from the
current running task or ISRs 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,
xTaskSuspendAll() 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
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 with
portDISABLE_INTERRUPTS(). This prevents preemptive 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.
ESP-IDF contains some modifications to work with dual core concurrency, and the dual core API is used even on a single core only chip.
For this reason, ESP-IDF FreeRTOS implements critical sections using special
mutexes, referred by
portMUX_Type objects. These are implemented on top of a
specific spinlock component. Calls to
taskEXIT_CRITICAL each provide a spinlock object as an argument. The
spinlock is associated with a shared resource requiring access protection. When
entering a critical section in ESP-IDF FreeRTOS, the calling core will disable
interrupts similar to the vanilla FreeRTOS implementation, and will then take the
spinlock and enter the critical section. The other core is unaffected at this point,
unless it enters its own critical section and attempts to take the same spinlock.
In that case it will spin until the lock 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…
portENTER_CRITICAL_ISR(mux)are all macro defined to call internal function
portEXIT_CRITICAL_ISR(mux)are all macro defined to call internal function
portEXIT_CRITICAL_SAFE(mux)macro identifies the context of execution, i.e ISR or Non-ISR, and calls appropriate critical section functions (
port*_CRITICALin Non-ISR and
port*_CRITICAL_ISRin ISR) in order to be in compliance with Vanilla FreeRTOS.
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 spinlock is provided upon entering and exiting, the exact macro or function used for the call should not matter.
In FreeRTOS task deletion the freeing of task memory will occur
vTaskDelete()) if the task being deleted is not currently
running or is not pinned to the other core (with respect to the core
vTaskDelete() is called on). TLSP deletion callbacks will also run immediately
if the same conditions are met.
vTaskDelete() to delete a task that is either currently
running or pinned to the other core will still result in the freeing of memory
being delegated to the Idle Task.
Thread Local Storage Pointers & Deletion Callbacks¶
Thread Local Storage Pointers (TLSP) are pointers stored directly in the TCB. TLSP allow each task to have its own unique set of pointers to data structures. However task deletion behavior in vanilla FreeRTOS does not automatically free the memory pointed to by TLSP. Therefore if the memory pointed to by TLSP is not explicitly freed by the user before task deletion, memory leak will occur.
ESP-IDF FreeRTOS provides the added feature of Deletion Callbacks. Deletion Callbacks are called automatically during task deletion to free memory pointed to by TLSP. Each TLSP can have its own Deletion Callback. Note that due to the to Task Deletion behavior, there can be instances where Deletion Callbacks are called in the context of the Idle Tasks. Therefore Deletion Callbacks should never attempt to block and critical sections should be kept as short as possible to minimize priority inversion.
Deletion callbacks are of type
void (*TlsDeleteCallbackFunction_t)( int, void * ) where the first parameter
is the index number of the associated TLSP, and the second parameter is the
Deletion callbacks are set alongside TLSP by calling
vTaskSetThreadLocalStoragePointerAndDelCallback(). Calling the vanilla
vTaskSetThreadLocalStoragePointer() will simply set the
TLSP’s associated Deletion Callback to NULL meaning that no callback will be
called for that TLSP during task deletion. If a deletion callback is NULL,
users should manually free the memory pointed to by the associated TLSP before
task deletion in order to avoid memory leak.
For more details see FreeRTOS API reference.
Configuring ESP-IDF FreeRTOS¶
The ESP-IDF FreeRTOS can be configured in the project configuration menu
idf.py 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
As ESP32-S2 is a single core SoC, the config item CONFIG_FREERTOS_UNICORE is
always set. This means ESP-IDF only runs on the single CPU. Note that this is not
equivalent to running vanilla FreeRTOS. Behaviors of multiple components in ESP-IDF
will be modified. 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.
CONFIG_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.
CONFIG_FREERTOS_TASK_FUNCTION_WRAPPER will enclose all task functions
within a wrapper function. In the case that a task function mistakenly returns
(i.e. does not call
vTaskDelete()), the call flow will return to the
wrapper function. The wrapper function will then log an error and abort the
application, as illustrated below:
E (25) FreeRTOS: FreeRTOS task should not return. Aborting now! abort() was called at PC 0x40085c53 on core 0