freertos源码解析-3任务和任务创建
任务
任务通常是一个类似以下代码的一个函数。
void vATaskFunction( void *pvParameters )
{
for( ;; )
{
/*-- 应用程序代码放在这里. --*/
}/*正常情况下任务函数是一个死循环，不会退出*/
vTaskDelete( NULL );/*如果任务完成，需要退出，需要删除该任务，vATaskFunction没有返回值*/
}
任务状态
- 运行：任务占用CPU，单核处理器任意时刻只有一个任务在运行。
- 就绪：任务处于准备随时可以运行的状态，等待调度器调用运行，因为有更高优先级或者相同优先级的任务已经在运行。
- 阻塞：任务需要等待特定的事件完成，因此被动停止运行。当等待事件（或者时间）到达，恢复就绪态。
- 挂起：区别于阻塞状态，挂起是一种主动行为，因此恢复就绪态也应该主动完成。没有等待时间的概念。任务的挂起和恢复必须分别通过vTaskSuspend()和xTaskResume()实现。
任务优先级
- 每个任务的优先级从0到（configMAX_PRIORITIES – 1），其中configMAX_PRIORITIES在FreeRTOSConfig.h中定义。
- 低优先级数字表示低优先级任务。该空闲任务的优先级为零（tskIDLE_PRIORITY=0）。
- 尽量设置满足应用情况下的最小优先级级数，以免嵌套过多层，消耗过多RAM。
- 多个任务可以共享一个优先级。
任务创建
任务创建过程分析，结合源码。就很清晰了。
BaseType_t xTaskCreate( TaskFunction_t pxTaskCode, /*任务函数指针，即任务函数*/
const char * const pcName, /*任务名称*/ /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
const configSTACK_DEPTH_TYPE usStackDepth,/*任务栈的长度*/
void * const pvParameters,/*任务函数的参数*/
UBaseType_t uxPriority,/*任务优先级*/
TaskHandle_t * const pxCreatedTask )/*任务句柄*/
{
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 *//*栈向下增长，先分配栈，再分配TCB，当栈溢出的时候，就不会损坏TCB*/
{
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;
}
/*-----------------------------------------------------------*/
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. */
/* 给分配的栈填充固定值， 可用于检测栈的溢出 */
( 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 ] );//找到栈顶的地址
//栈的地址对其，cortex-M3中是，8字节对齐，需要将pxTopOfStack的后八位清零。
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();
}
}
StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t pxCode, void *pvParameters )
{
/* Simulate the stack frame as it would be created by a context switch
interrupt. */
pxTopOfStack--; /* Offset added to account for the way the MCU uses the stack on entry/exit of interrupts. */
*pxTopOfStack = portINITIAL_XPSR; /* xPSR */
pxTopOfStack--;
*pxTopOfStack = ( ( StackType_t ) pxCode ) & portSTART_ADDRESS_MASK; /* PC */
pxTopOfStack--;
*pxTopOfStack = ( StackType_t ) prvTaskExitError; /* LR */
pxTopOfStack -= 5; /* R12, R3, R2 and R1. */
*pxTopOfStack = ( StackType_t ) pvParameters; /* R0 */
pxTopOfStack -= 8; /* R11, R10, R9, R8, R7, R6, R5 and R4. */
return pxTopOfStack;
}
参考资料
《Mastering the FreeRTOS™ Real Time Kernel》
《cortex M3 权威指南》
《freertos 开发手册》
freertos
公众号：嵌入式软件和硬件
