freertos源碼解析-3任務和任務創建和任務切換

任務

任務通常是一個類似以下代碼的一個函數。

void vATaskFunction( void *pvParameters )
{
    for( ;; )
    {
        /*-- 應用程序代碼放在這裏. --*/
    }/*正常情況下任務函數是一個死循環，不會退出*/
    vTaskDelete( NULL );/*如果任務完成，需要退出，需要刪除該任務，vATaskFunction沒有返回值*/
}

任務狀態

• 運行：任務佔用CPU，單核處理器任意時刻只有一個任務在運行。
• 就緒：任務處於準備隨時可以運行的狀態，等待調度器調用運行，因爲有更高優先級或者相同優先級的任務已經在運行。
• 阻塞：任務需要等待特定的事件完成，因此被動停止運行。當等待事件（或者時間）到達，恢復就緒態。
• 掛起：區別於阻塞狀態，掛起是一種主動行爲，因此恢復就緒態也應該主動完成。沒有等待時間的概念。任務的掛起和恢復必須分別通過vTaskSuspend()和xTaskResume()實現。
  

任務優先級

• 每個任務的優先級從0到（configMAX_PRIORITIES – 1），其中configMAX_PRIORITIES在FreeRTOSConfig.h中定義。
• 低優先級數字表示低優先級任務。該空閒任務的優先級爲零（tskIDLE_PRIORITY=0）。

• 儘量設置滿足應用情況下的最小優先級級數，以免嵌套過多層，消耗過多RAM。

• 多個任務可以共享一個優先級。

任務創建

任務創建過程分析，結合源碼。就很清晰了。

	BaseType_t xTaskCreate(	TaskFunction_t pxTaskCode,  /*任務函數指針，即任務函數*/
							const char * const pcName,	/*任務名稱*/	/*lint !e971 Unqualified char types are allowed for strings and single characters only. */
							const configSTACK_DEPTH_TYPE usStackDepth,/*任務棧的長度*/
							void * const pvParameters,/*任務函數的參數*/
							UBaseType_t uxPriority,/*任務優先級*/
							TaskHandle_t * const pxCreatedTask )/*任務句柄*/
	{
	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 *//*棧向下增長，先分配棧，再分配TCB，當棧溢出的時候，就不會損壞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. */
		/* 給分配的棧填充固定值， 可用於檢測棧的溢出 */
		( 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 ] );//找到棧頂的地址
		//棧的地址對其，cortex-M3中是，8字節對齊，需要將pxTopOfStack的後八位清零。
		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;
}

參考資料

《Mastering the FreeRTOS™ Real Time Kernel》
《cortex M3 權威指南》
《freertos 開發手冊》
freertos

公衆號：嵌入式軟件和硬件