esp-idf/components/freertos/FreeRTOS-Kernel-SMP/porting_notes.md

Overview

This document outlines some useful notes about

• The porting of SMP FreeRTOS to ESP-IDF
• And the difference between IDF FreeRTOS and SMP FreeRTOS

Terminology

The following terms will be used in this document to avoid confusion between the different FreeRTOS versions currently in ESP-IDF

• SMP FreeRTOS: The SMP branch of the FreeRTOS kernel found here
• IDF FreeRTOS: The version of FreeRTOS used in mainline ESP-IDF that contained custom modifications to support SMP features specific to the ESP chips.

Organization

This directory contains a copy of SMP FreeRTOS based off of upstream commit 2eff03708

• IDF FreeRTOS remains in components/freertos/FreeRTOS-Kernel
• SMP FreeRTOS is entirely contained in components/freertos/FreeRTOS-Kernel-SMP
  • port.c and portmacro.h were mostly re-written from scratch
  • Some changes were made to SMP FreeRTOS tasks.c in order to be compatible with IDF.
  • All of these changes are wrapped in #ifdef ESP_PLATFORM
  • These additions will be removed after the compatibility issues are resolved in IDF.
• SMP FreeRTOS is only compiled if CONFIG_FREERTOS_SMP is set in menuconfig
• All changes made to the rest of ESP-IDF to support SMP FreeRTOS are wrapped in #ifdef CONFIG_FREERTOS_SMP. Thus SMP FreeRTOS changes should have no effect on mainline ESP-IDF if CONFIG_FREERTOS_SMP is disabled.

Behavioral Differences and Porting

This section covers

• The major feature/behavior differences between SMP FreeRTOS vs IDF FreeRTOS
• The changes made to the port or SMP FreeRTOS sources to remedy these differences
• Future Todos

Task Core Affinity

IDF FreeRTOS:

• Tasks can either be pinned to a core or completely unpinned

SMP FreeRTOS:

• Tasks in SMP FreeRTOS use an affinity mask thus can run on any combination of cores

Changes Made:

• Upstreamed xTaskCreate...AffinitySet() to allow creation of tasks with affinity (instead of requiring a separate call to set the affinity).
• Wrapped the IDF xTaskCreate...PinnedToCore() functions to call xTaskCreate...AffinitySet()

Scheduler Suspension

IDF FreeRTOS:

• Each core had its own suspendable scheduler (i.e., calling vTaskSuspendAll() on one core has no effect on the other)
• A core that calls vTaskSuspendAll() will only disable task switching on that core.

SMP FreeRTOS:

• vTaskSuspendAll() is global (no cores can switch tasks once the scheduler is disabled)
• While the scheduler is suspend, any other core that attempts to block, call FreeRTOS API, or also suspend the scheduler will result that the other core spinning with interrupts disabled (due to task lock contention)

Changes Made:

• Replaced all calls of vTaskSuspendAll()/xTaskResumeAll() in ESP-IDF with vTaskPreemptionDisable()/vTaskPreemptionEnable() when using SMP FreeRTOS

Critical Sections

IDF FreeRTOS:

• IDF FreeRTOS uses granular spin locks for critical sections
  • Callers need to instantiate and provide their own spinlocks
  • Allows different cores to simultaneously enter separate critical sections using different locks without contention
  • Some FreeRTOS objects (such as queues and timers) have their own spinlocks
• portENTER_CRITICAL()/portEXIT_CRITICAL() modified to accept a spinlock argument
• Added variant portTRY_ENTER_CRITICAL() to allow for timeout to be specified when entering critical sections

SMP FreeRTOS:

• Uses a giant lock (task and ISR locks to be specific). Cores simultaneously entering a critical section will contest with each other.
• portENTER_CRITICAL()/portEXIT_CRITICAL() does not accept arguments. Maps directly to vTaskEnterCritical()/vTaskExitCritical()
• The critical sections now checks for state change. This means if a task has just entered a critical section and another higher priority task can run, the critical section will exit to yield and then retry entering.

Changes Made:

• In order to be compatible with IDF style critical sections, portENTER_CRITICAL(...)/portEXIT_CRITICAL(...) now accepts VA_ARGS
  • If no argument exists, it calls SMP FreeRTOS critical sections vTaskEnterCritical()
  • If an argument exists, it calls IDF FreeRTOS critical sections (where the argument is the spinlock)

Todo:

• Assess how using a giant lock (i.e., SMP FreeRTOS critical sections) will affect performance on time sensitive components of ESP-IDF (e.g., BT, WiFi)
  • If performance is acceptable, replace all critical sections in ESP-IDF with SMP FreeRTOS critical sections
  • Else, refactor IDF FreeRTOS critical sections to a separate API, and have IDF components call those instead.

Idle Tasks

IDF FreeRTOS:

• Only uses a single idle task function prvIdleTask()
• An idle task is created for each core and pinned to that core
• prvIdleTask() calls a custom esp_vApplicationIdleHook(), mostly used to feed the task watchdog
• IDF adds a Thread Local Storage Pointer (TLSP) Deletion Callback that is called on task deletion (i.e., can be called from the idle task).

SMP FreeRTOS:

• There are now two types of idle task functions. The prvIdleTask() and prvMinimalIdleTask()
  • prvMinimalIdleTask() simply calls the vApplicationMinimalIdleHook()
  • prvIdleTask() calls prvCheckTasksWaitingTermination(), vApplicationIdleHook(), vApplicationMinimalIdleHook(), and handles tickless idle.
  • On an N core build, one prvIdleTask() task is created and N-1 prvMinimalIdleTask() tasks are created.
• The created idle tasks are all unpinned. The idle tasks are run on a "first come first serve" basis meaning when a core goes idle, it selects whatever available idle task it can run.

Changes Made:

• The esp_vApplicationIdleHook() is now called from vApplicationMinimalIdleHook() since every idle task calls the vApplicationMinimalIdleHook().
• Since the idle tasks are unpinned, the task WDT has been updated to use the "User" feature. Thus, feeding the task watchdog now tracks which "core" has fed the task WDT instead of which specific idle task has fed.
• Since prvIdleTask() is solely responsible for calling prvCheckTasksWaitingTermination() but can run on any core, multiple IDF cleanup routines are now routed through portCLEAN_UP_TCB()
  • FPU registers of a task are now cleaned up via portCLEAN_UP_TCB() -> vPortCleanUpCoprocArea() and can clean FPU save areas across cores.
  • TLSP Deletion callbacks are now run via portCLEAN_UP_TCB() -> vPortTLSPointersDelCb()

Todo:

• Add support for configIDLE_SHOULD_YIELD

Tick Interrupt

IDF FreeRTOS:

• All cores have their own tick interrupts (same frequency but can be out of phase)
• Each core calls xTaskIncrementTick() independently. xTaskIncrementTick() has been modified as such:
  • Core 0 is responsible for time keeping (incrementing tick count and unblocking timed out tasks)
  • Each core independently checks for time slicing, and calls vApplicationTickHook()
  • There's also an additional esp_vApplicationTickHook() used to feed the interrupt watchdog.

SMP FreeRTOS:

• SMP FreeRTOS requires that xTaskIncrementTick() only be called from one core (core 0 in our case). That core's tick interrupt is solely responsible for the following via its call to xTaskIncrementTick():
  • Time keeping (increment ticks and unblocking timed out tasks)
  • Time slicing (calls prvYieldCore() on each core that needs to yield)
  • Calls vApplicationTickHook()
  • Increments pended ticks when the scheduler is suspended

Changes Made:

• Each core still has a HW tick interrupt that occurs (where the ISR xPortSysTickHandler()) and will:
  • Call esp_vApplicationTickHook() in order to feed the interrupt WDT on behalf of the current core.
  • Call xTaskIncrementTick() only on core 0.

Todo:

• See if we can find another way to implement the interrupt watchdog. Requiring each core to interrupt just to feed the watchdog loses the benefit of having core 0 handling the time slicing of all cores.

IDF Additional Features and API

IDF FreeRTOS added multiple features/APIs that are specific to IDF. For SMP FreeRTOS, these features/APIs have either been:

• Upstreamed to SMP FreeRTOS officially
• Moved their definition/declaration to freertos_tasks_c_additions.h/idf_additions.h respectively. Users should #include "freertos/idf_additions.h manually so that we can keep the SMP FreeRTOS API clean.
• Deprecated/Removed from IDF entirely

xTaskCreatePinnedToCore()/xTaskCreateStaticPinnedToCore()

• The xTaskCreate...AffinitySet() API has been added in order to apply a core affinity on task creation.
• xTaskCreate...PinnedToCore() now just wrap the xTaskCreate...AffinitySet() equivalent functions and handle the conversion of a core ID into a core affinity mask.
• xTaskCreate...PinnedToCore() is now provided via idf_additions.h

vTaskSetThreadLocalStoragePointerAndDelCallback()

• If TLSP deletion callbacks are used, configNUM_THREAD_LOCAL_STORAGE_POINTERS will be doubled (in order to store the callback pointers in the same array as the TLSPs themselves)
• vTaskSetThreadLocalStoragePointerAndDelCallback() moved to freertos_tasks_c_additions.h/idf_additions.h
• Deletion callbacks invoked from the main idle task via portCLEAN_UP_TCB()

xTaskGetCurrentTaskHandleForCPU()

• Convenience function to the get current task of a particular CPU
• Moved to freertos_tasks_c_additions.h/idf_additions.h for now

Todo: Check if this can be upstreamed

xTaskGetIdleTaskHandleForCPU()

• Currently moved to freertos_tasks_c_additions.h/idf_additions.h

Todo: This needs to be deprecated as there is no longer the concept of idle tasks pinned to a particular CPU

xTaskGetAffinity()

• Returns what core a task is pinned to, and not the task's affinity mask.
• Moved to freertos_tasks_c_additions.h/idf_additions.h and simple wraps vTaskCoreAffinityGet()
• If the task's affinity mask has more than one permissible core, we simply return tskNO_AFFINITY even if the task is not completely unpinned.

Todo: This needs to be deprecated and users should call vTaskCoreAffinityGet() instead