freeRtos源码解析(一)–任务创建与任务调度
一、简介
??1.1 代码来源及版本
??代码来源于STM32CubeIDE工具一键生成功能，对应freeRtos版本为 FreeRTOS Kernel V10.2.1。对应MCU型号为STM32MP157D cortex-M4。
?? 生成的中间层代码以及源码目录如下：
??1.2 学习参考
??[野火?]《FreeRTOS 内核实现与应用开发实战—基于STM32》、
?《FreeRTOS高级篇》https://blog.csdn.net/zhzht19861011/article/details/51418383
??freeRtos学习的人非常广泛，资料众多，每个版本间有细微的差别，可以相互参考。
二、任务与freeRtos
??freeRtos的核心，一句话就是：在裸机系统上实现多任务的实时调度以及提供任务间的通信同步机制。 归结为一个词，就是多任务 （任务创建，任务调度，任务通信）。
??有些人把任务理解为linux中的线程，不过在freeRtos中没有进程线程的概念，统一的叫法是任务task而不是thread。进程、线程的概念要到了linux内核中才存在。
三、任务创建
?? 3.1 xTaskCreate 任务创建接口
??任务创建对应的过程非常重要，需要仔细阅读。接口代码如下：
#if( configSUPPORT_DYNAMIC_ALLOCATION == 1 )
BaseType_t xTaskCreate( TaskFunction_t pxTaskCode,
const char * const pcName, /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
const configSTACK_DEPTH_TYPE usStackDepth,
void * const pvParameters,
UBaseType_t uxPriority,
TaskHandle_t * const pxCreatedTask )
{
TCB_t *pxNewTCB;
BaseType_t xReturn;
/* If the stack grows down then allocate the stack then the TCB so the stack
does not grow into the TCB. Likewise if the stack grows up then allocate
the TCB then the stack. */
#if( portSTACK_GROWTH > 0 )
{
/* Allocate space for the TCB. Where the memory comes from depends on
the implementation of the port malloc function and whether or not static
allocation is being used. */
pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
if( pxNewTCB != NULL )
{
/* Allocate space for the stack used by the task being created.
The base of the stack memory stored in the TCB so the task can
be deleted later if required. */
pxNewTCB->pxStack = ( StackType_t * ) pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
if( pxNewTCB->pxStack == NULL )
{
/* Could not allocate the stack. Delete the allocated TCB. */
vPortFree( pxNewTCB );
pxNewTCB = NULL;
}
}
}
#else /* portSTACK_GROWTH */
{
StackType_t *pxStack;
/* Allocate space for the stack used by the task being created. */
pxStack = pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) ); /*lint !e9079 All values returned by pvPortMalloc() have at least the alignment required by the MCU's stack and this allocation is the stack. */
if( pxStack != NULL )
{
/* Allocate space for the TCB. */
pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) ); /*lint !e9087 !e9079 All values returned by pvPortMalloc() have at least the alignment required by the MCU's stack, and the first member of TCB_t is always a pointer to the task's stack. */
if( pxNewTCB != NULL )
{
/* Store the stack location in the TCB. */
pxNewTCB->pxStack = pxStack;
}
else
{
/* The stack cannot be used as the TCB was not created. Free
it again. */
vPortFree( pxStack );
}
}
else
{
pxNewTCB = NULL;
}
}
#endif /* portSTACK_GROWTH */
if( pxNewTCB != NULL )
{
#if( tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE != 0 ) /*lint !e9029 !e731 Macro has been consolidated for readability reasons. */
{
/* Tasks can be created statically or dynamically, so note this
task was created dynamically in case it is later deleted. */
pxNewTCB->ucStaticallyAllocated = tskDYNAMICALLY_ALLOCATED_STACK_AND_TCB;
}
#endif /* tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE */
prvInitialiseNewTask( pxTaskCode, pcName, ( uint32_t ) usStackDepth, pvParameters, uxPriority, pxCreatedTask, pxNewTCB, NULL );
prvAddNewTaskToReadyList( pxNewTCB );
xReturn = pdPASS;
}
else
{
xReturn = errCOULD_NOT_ALLOCATE_REQUIRED_MEMORY;
}
return xReturn;
}
#endif /* configSUPPORT_DYNAMIC_ALLOCATION */
??相关宏定义：
#define configSUPPORT_DYNAMIC_ALLOCATION 1 // 表示支持动态分配TCB块的空间和TCB对应栈的空间
#define portSTACK_GROWTH ( -1 ) // 表示栈地址是从低到高增长的
tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE // 未定义
??因此从函数38行开始，申请栈大小*4字节的堆空间，作为任务栈的大小。
pxStack = pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) );
??申请TCB任务控制块的空间，并且为其成员pxStack 赋值，作为任务栈的起始地址。
pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) );
pxNewTCB->pxStack = pxStack;
?? 3.2 prvInitialiseNewTask 初始化TCB任务控制块
??初始化TCB任务控制块的成员，代码如下：
static void prvInitialiseNewTask( TaskFunction_t pxTaskCode,
const char * const pcName, /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
const uint32_t ulStackDepth,
void * const pvParameters,
UBaseType_t uxPriority,
TaskHandle_t * const pxCreatedTask,
TCB_t *pxNewTCB,
const MemoryRegion_t * const xRegions )
{
StackType_t *pxTopOfStack;
UBaseType_t x;
#if( portUSING_MPU_WRAPPERS == 1 )
/* Should the task be created in privileged mode? */
BaseType_t xRunPrivileged;
if( ( uxPriority & portPRIVILEGE_BIT ) != 0U )
{
xRunPrivileged = pdTRUE;
}
else
{
xRunPrivileged = pdFALSE;
}
uxPriority &= ~portPRIVILEGE_BIT;
#endif /* portUSING_MPU_WRAPPERS == 1 */
/* Avoid dependency on memset() if it is not required. */
#if( tskSET_NEW_STACKS_TO_KNOWN_VALUE == 1 )
{
/* Fill the stack with a known value to assist debugging. */
( void ) memset( pxNewTCB->pxStack, ( int ) tskSTACK_FILL_BYTE, ( size_t ) ulStackDepth * sizeof( StackType_t ) );
}
#endif /* tskSET_NEW_STACKS_TO_KNOWN_VALUE */
/* Calculate the top of stack address. This depends on whether the stack
grows from high memory to low (as per the 80x86) or vice versa.
portSTACK_GROWTH is used to make the result positive or negative as required
by the port. */
#if( portSTACK_GROWTH < 0 )
{
pxTopOfStack = &( pxNewTCB->pxStack[ ulStackDepth - ( uint32_t ) 1 ] );
pxTopOfStack = ( StackType_t * ) ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack ) & ( ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) ) ); /*lint !e923 !e9033 !e9078 MISRA exception. Avoiding casts between pointers and integers is not practical. Size differences accounted for using portPOINTER_SIZE_TYPE type. Checked by assert(). */
/* Check the alignment of the calculated top of stack is correct. */
configASSERT( ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack & ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) == 0UL ) );
#if( configRECORD_STACK_HIGH_ADDRESS == 1 )
{
/* Also record the stack's high address, which may assist
debugging. */
pxNewTCB->pxEndOfStack = pxTopOfStack;
}
#endif /* configRECORD_STACK_HIGH_ADDRESS */
}
#else /* portSTACK_GROWTH */
{
pxTopOfStack = pxNewTCB->pxStack;
/* Check the alignment of the stack buffer is correct. */
configASSERT( ( ( ( portPOINTER_SIZE_TYPE ) pxNewTCB->pxStack & ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) == 0UL ) );
/* The other extreme of the stack space is required if stack checking is
performed. */
pxNewTCB->pxEndOfStack = pxNewTCB->pxStack + ( ulStackDepth - ( uint32_t ) 1 );
}
#endif /* portSTACK_GROWTH */
/* Store the task name in the TCB. */
if( pcName != NULL )
{
for( x = ( UBaseType_t ) 0; x < ( UBaseType_t ) configMAX_TASK_NAME_LEN; x++ )
{
pxNewTCB->pcTaskName[ x ] = pcName[ x ];
/* Don't copy all configMAX_TASK_NAME_LEN if the string is shorter than
configMAX_TASK_NAME_LEN characters just in case the memory after the
string is not accessible (extremely unlikely). */
if( pcName[ x ] == ( char ) 0x00 )
{
break;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
/* Ensure the name string is terminated in the case that the string length
was greater or equal to configMAX_TASK_NAME_LEN. */
pxNewTCB->pcTaskName[ configMAX_TASK_NAME_LEN - 1 ] = '\0';
}
else
{
/* The task has not been given a name, so just ensure there is a NULL
terminator when it is read out. */
pxNewTCB->pcTaskName[ 0 ] = 0x00;
}
/* This is used as an array index so must ensure it's not too large. First
remove the privilege bit if one is present. */
if( uxPriority >= ( UBaseType_t ) configMAX_PRIORITIES )
{
uxPriority = ( UBaseType_t ) configMAX_PRIORITIES - ( UBaseType_t ) 1U;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
pxNewTCB->uxPriority = uxPriority;
#if ( configUSE_MUTEXES == 1 )
{
pxNewTCB->uxBasePriority = uxPriority;
pxNewTCB->uxMutexesHeld = 0;
}
#endif /* configUSE_MUTEXES */
vListInitialiseItem( &( pxNewTCB->xStateListItem ) );
vListInitialiseItem( &( pxNewTCB->xEventListItem ) );
/* Set the pxNewTCB as a link back from the ListItem_t. This is so we can get
back to the containing TCB from a generic item in a list. */
listSET_LIST_ITEM_OWNER( &( pxNewTCB->xStateListItem ), pxNewTCB );
/* Event lists are always in priority order. */
listSET_LIST_ITEM_VALUE( &( pxNewTCB->xEventListItem ), ( TickType_t ) configMAX_PRIORITIES - ( TickType_t ) uxPriority ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
listSET_LIST_ITEM_OWNER( &( pxNewTCB->xEventListItem ), pxNewTCB );
#if ( portCRITICAL_NESTING_IN_TCB == 1 )
{
pxNewTCB->uxCriticalNesting = ( UBaseType_t ) 0U;
}
#endif /* portCRITICAL_NESTING_IN_TCB */
#if ( configUSE_APPLICATION_TASK_TAG == 1 )
{
pxNewTCB->pxTaskTag = NULL;
}
#endif /* configUSE_APPLICATION_TASK_TAG */
#if ( configGENERATE_RUN_TIME_STATS == 1 )
{
pxNewTCB->ulRunTimeCounter = 0UL;
}
#endif /* configGENERATE_RUN_TIME_STATS */
#if ( portUSING_MPU_WRAPPERS == 1 )
{
vPortStoreTaskMPUSettings( &( pxNewTCB->xMPUSettings ), xRegions, pxNewTCB->pxStack, ulStackDepth );
}
#else
{
/* Avoid compiler warning about unreferenced parameter. */
( void ) xRegions;
}
#endif
#if( configNUM_THREAD_LOCAL_STORAGE_POINTERS != 0 )
{
for( x = 0; x < ( UBaseType_t ) configNUM_THREAD_LOCAL_STORAGE_POINTERS; x++ )
{
pxNewTCB->pvThreadLocalStoragePointers[ x ] = NULL;
}
}
#endif
#if ( configUSE_TASK_NOTIFICATIONS == 1 )
{
pxNewTCB->ulNotifiedValue = 0;
pxNewTCB->ucNotifyState = taskNOT_WAITING_NOTIFICATION;
}
#endif
#if ( configUSE_NEWLIB_REENTRANT == 1 )
{
/* Initialise this task's Newlib reent structure. */
_REENT_INIT_PTR( ( &( pxNewTCB->xNewLib_reent ) ) );
}
#endif
#if( INCLUDE_xTaskAbortDelay == 1 )
{
pxNewTCB->ucDelayAborted = pdFALSE;
}
#endif
/* Initialize the TCB stack to look as if the task was already running,
but had been interrupted by the scheduler. The return address is set
to the start of the task function. Once the stack has been initialised
the top of stack variable is updated. */
#if( portUSING_MPU_WRAPPERS == 1 )
{
/* If the port has capability to detect stack overflow,
pass the stack end address to the stack initialization
function as well. */
#if( portHAS_STACK_OVERFLOW_CHECKING == 1 )
{
#if( portSTACK_GROWTH < 0 )
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxNewTCB->pxStack, pxTaskCode, pvParameters, xRunPrivileged );
}
#else /* portSTACK_GROWTH */
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxNewTCB->pxEndOfStack, pxTaskCode, pvParameters, xRunPrivileged );
}
#endif /* portSTACK_GROWTH */
}
#else /* portHAS_STACK_OVERFLOW_CHECKING */
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters, xRunPrivileged );
}
#endif /* portHAS_STACK_OVERFLOW_CHECKING */
}
#else /* portUSING_MPU_WRAPPERS */
{
/* If the port has capability to detect stack overflow,
pass the stack end address to the stack initialization
function as well. */
#if( portHAS_STACK_OVERFLOW_CHECKING == 1 )
{
#if( portSTACK_GROWTH < 0 )
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxNewTCB->pxStack, pxTaskCode, pvParameters );
}
#else /* portSTACK_GROWTH */
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxNewTCB->pxEndOfStack, pxTaskCode, pvParameters );
}
#endif /* portSTACK_GROWTH */
}
#else /* portHAS_STACK_OVERFLOW_CHECKING */
{
pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
}
#endif /* portHAS_STACK_OVERFLOW_CHECKING */
}
#endif /* portUSING_MPU_WRAPPERS */
if( pxCreatedTask != NULL )
{
/* Pass the handle out in an anonymous way. The handle can be used to
change the created task's priority, delete the created task, etc.*/
*pxCreatedTask = ( TaskHandle_t ) pxNewTCB;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
?? 3.3 pxPortInitialiseStack 初始化任务栈指针（根据MCU内核通用寄存器偏移）
?? 3.4 prvAddNewTaskToReadyList 将TCB控制块加入到ready列表中
??至此，完成了任务的创建和初始化。
四、启动任务调度器
??启动任务调度器之后，CPU正式开始按照任务的方式调度各任务。