ESP-IDF FreeRTOS SMP Changes¶
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. 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.
Tasks and Task Creation: Use
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
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
Tick Interrupt Synchronicity: Tick interrupts of PRO_CPU and APP_CPU
are not synchronized. Do not expect to use
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.
Floating Point Arithmetic: The ESP32 supports hardware acceleration of single
precision floating point arithmetic (
float). However the use of hardware
acceleration leads to some behavioral restrictions in ESP-IDF FreeRTOS.
Therefore, tasks that utilize
float will automatically be pinned to a core if
not done so already. Furthermore,
float cannot be used in interrupt service
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.
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 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
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
Each linked list also contains a
pxIndex which points to the last TCB
returned when the list was queried. This index allows the
to start traversing the list at the TCB immediately after
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
pxReadyTasksList is illustrated below.
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
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.
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 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,
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 in
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 special mutexes, referred by portMUX_Type objects on top of specific spinlock component and calls to enter or exit a critical must provide a spinlock object 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 locks whilst the other core is left unaffected during the critical section. If the other core attempts to take the spinlock, 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
portEXIT_CRITICAL_ISR(mux)are all macro defined to call
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 type of call should not matter.
Floating Point Arithmetic¶
ESP-IDF FreeRTOS implements Lazy Context Switching for FPUs. In other words,
the state of a core’s FPU registers are not immediately saved when a context
switch occurs. Therefore, tasks that utilize
float must be pinned to a
particular core upon creation. If not, ESP-IDF FreeRTOS will automatically pin
the task in question to whichever core the task was running on upon the task’s
first use of
float. Likewise due to Lazy Context Switching, only interrupt
service routines of lowest priority (that is it the Level 1) can use
higher priority interrupts do not support FPU usage.
ESP32 does not support hardware acceleration for double precision floating point
double is implemented via software hence the
behavioral restrictions with regards to
float do not apply to
that due to the lack of hardware acceleration,
double operations may consume
significantly larger amount of CPU time in comparison to
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
CONFIG_FREERTOS_UNICORE will run ESP-IDF FreeRTOS only
on PRO_CPU. Note that this is not equivalent to running vanilla
FreeRTOS. Note that this option may affect behavior of components other than
freertos. For more details regarding the
effects of running ESP-IDF FreeRTOS on a single core, search for
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