FreeRTOS任务的简易分析
- 前言：毫无疑问，任务整个FreeRTOS的核心，所以分析起来很困难，我只能去各个地方看不同人的文档或笔记，把别人的东西变为自己的东西，最后记录这篇内容。我无法向介绍List那样把全部代码看一遍，只能把大概的流程写下来。
1.任务结构体tskTaskControlBlock
typedef struct tskTaskControlBlock
{
volatile StackType_t *pxTopOfStack; /*< Points to the location of the last item placed on the tasks stack. THIS MUST BE THE FIRST MEMBER OF THE TCB STRUCT. */
#if ( portUSING_MPU_WRAPPERS == 1 )
xMPU_SETTINGS xMPUSettings; /*< The MPU settings are defined as part of the port layer. THIS MUST BE THE SECOND MEMBER OF THE TCB STRUCT. */
#endif
ListItem_t xStateListItem; /*< The list that the state list item of a task is reference from denotes the state of that task (Ready, Blocked, Suspended ). */
ListItem_t xEventListItem; /*< Used to reference a task from an event list. */
UBaseType_t uxPriority; /*< The priority of the task. 0 is the lowest priority. */
StackType_t *pxStack; /*< Points to the start of the stack. */
char pcTaskName[ configMAX_TASK_NAME_LEN ];/*< Descriptive name given to the task when created. Facilitates debugging only. */ /*lint !e971 Unqualified char types are allowed for strings and single characters only. */
#if ( portSTACK_GROWTH > 0 )
StackType_t *pxEndOfStack; /*< Points to the end of the stack on architectures where the stack grows up from low memory. */
#endif
#if ( portCRITICAL_NESTING_IN_TCB == 1 )
UBaseType_t uxCriticalNesting; /*< Holds the critical section nesting depth for ports that do not maintain their own count in the port layer. */
#endif
#if ( configUSE_TRACE_FACILITY == 1 )
UBaseType_t uxTCBNumber; /*< Stores a number that increments each time a TCB is created. It allows debuggers to determine when a task has been deleted and then recreated. */
UBaseType_t uxTaskNumber; /*< Stores a number specifically for use by third party trace code. */
#endif
#if ( configUSE_MUTEXES == 1 )
UBaseType_t uxBasePriority; /*< The priority last assigned to the task - used by the priority inheritance mechanism. */
UBaseType_t uxMutexesHeld;
#endif
#if ( configUSE_APPLICATION_TASK_TAG == 1 )
TaskHookFunction_t pxTaskTag;
#endif
#if( configNUM_THREAD_LOCAL_STORAGE_POINTERS > 0 )
void *pvThreadLocalStoragePointers[ configNUM_THREAD_LOCAL_STORAGE_POINTERS ];
#endif
#if( configGENERATE_RUN_TIME_STATS == 1 )
uint32_t ulRunTimeCounter; /*< Stores the amount of time the task has spent in the Running state. */
#endif
#if ( configUSE_NEWLIB_REENTRANT == 1 )
/* Allocate a Newlib reent structure that is specific to this task.
Note Newlib support has been included by popular demand, but is not
used by the FreeRTOS maintainers themselves. FreeRTOS is not
responsible for resulting newlib operation. User must be familiar with
newlib and must provide system-wide implementations of the necessary
stubs. Be warned that (at the time of writing) the current newlib design
implements a system-wide malloc() that must be provided with locks. */
struct _reent xNewLib_reent;
#endif
#if( configUSE_TASK_NOTIFICATIONS == 1 )
volatile uint32_t ulNotifiedValue;
volatile uint8_t ucNotifyState;
#endif
/* See the comments above the definition of
tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE. */
#if( tskSTATIC_AND_DYNAMIC_ALLOCATION_POSSIBLE != 0 )
uint8_t ucStaticallyAllocated; /*< Set to pdTRUE if the task is a statically allocated to ensure no attempt is made to free the memory. */
#endif
#if( INCLUDE_xTaskAbortDelay == 1 )
uint8_t ucDelayAborted;
#endif
} tskTCB;
这个成员非常多，只能看几个。
| 成员 | 作用 |
|---|---|
| pxTopOfStack | 它指向任务堆栈的栈顶。当任务切换时，保存现场或者恢复现场时，会从这里保存或取出 |
| xStateListItem | 状态链表里的链表项 |
| xEventListItem | 事件链表里的链表项 |
| uxPriority | 任务的优先级，调用创建函数时传入 |
| pxStack | 任务堆栈开始的地方 |
| pcTaskName | 任务的名字，创建任务时传入 |
| uxTCBNumber | 每个任务唯一的值 |
首先单独拿pxTopOfStack和pxStack出来说，pxTopOfStack它保存的东西是关于任务切换的现场(xPSR、PC、LR、R12、R3R0、R11R4)。当PendSV（Cortex-M3 SVC、PendSV和SysTick）切换上下文的时候，就是用当前任务的pxTopOfStack来操作的。
1.1创建任务函数xTaskCreate仿真
1.1.1pxStack、pxNewTCB、pxNewTCB->pxTopOfStack的跟踪
我仿真程序后，打了两个断点在xTaskCreate里面：
{
StackType_t *pxStack;
/* Allocate space for the stack used by the task being created. */
pxStack = ( StackType_t * ) pvPortMalloc( ( ( ( size_t ) usStackDepth ) * sizeof( StackType_t ) ) ); /*lint !e961 MISRA exception as the casts are only redundant for some ports. */
if( pxStack != NULL )
{
/* Allocate space for the TCB. */
pxNewTCB = ( TCB_t * ) pvPortMalloc( sizeof( TCB_t ) ); /*lint !e961 MISRA exception as the casts are only redundant for some paths. */
断点1：----->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;
}
}
...
...
prvInitialiseNewTask( pxTaskCode, pcName, ( uint32_t ) usStackDepth, pvParameters, uxPriority, pxCreatedTask, pxNewTCB, NULL );
断点2------> prvAddNewTaskToReadyList( pxNewTCB );
xReturn = pdPASS;
在断点1时，pxStack=0x20000520,pxNewTCB = 0x20000728，此时的pxNewTCB->pxTopOfStack为0。然后进入函数prvInitialiseNewTask里面。首先给pxTopOfStack赋值，具体如下:
#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 MISRA exception. Avoiding casts between pointers and integers is not practical. Size differences accounted for using portPOINTER_SIZE_TYPE type. */
/* Check the alignment of the calculated top of stack is correct. */
configASSERT( ( ( ( portPOINTER_SIZE_TYPE ) pxTopOfStack & ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK ) == 0UL ) );
}
赋值后pxTopOfStack为0x20000718，接着执行到pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
函数pxPortInitialiseStack具体如下：
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;
}
没错，这一段很明显就是保存现场（Cortex-M3 入栈顺序：xPSR、PC、LR、R12、R3R0、R11R4），在创建任务时，内核把新建的任务当成一个刚刚运行的任务。还有一点注意的，pxTopOfStack是向下增长的。执行完这段代码后，pxTopOfStack的值是从0x20000718变为0x200006D8。当然这些前提是ARM架构下，栈是向下增长的，FreeRTOS中有具体的宏定义portSTACK_GROWTH为-1时，就是向下增长。
得出以下结果：
以上就是创建任务函数时，pxStack、pxNewTCB、pxNewTCB->pxTopOfStack的分析。
1.1.2 TCB结构体成员的初始化
1.1.2.1 pcTaskName的初始化
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 ] == 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';
比较简单，就是把传入的赋值即可，注意一下最后一个是‘\0’即可。
1.1.2.2 uxPriority的初始化
/* 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;
把传入的优先级赋值。
1.1.2.3 xStateListItem、xEventListItem的初始化
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 );
结合前面的List分析：
1.首先把两个链表项都初始化
2.然后把xStateListItem里的pxOwner成员指向TCB结构体
3.优先级作为参数，赋值到xEventListItem的成员xItemValue里面
4.然后把xEventListItem里的pxOwner成员指向TCB结构体
1.1.3 TCB添加到ReadyList中
添加的具体函数prvAddNewTaskToReadyList：
static void prvAddNewTaskToReadyList( TCB_t *pxNewTCB )
{
/* Ensure interrupts don't access the task lists while the lists are being
updated. */
taskENTER_CRITICAL();
{
uxCurrentNumberOfTasks++;
if( pxCurrentTCB == NULL )
{
/* There are no other tasks, or all the other tasks are in
the suspended state - make this the current task. */
pxCurrentTCB = pxNewTCB;
if( uxCurrentNumberOfTasks == ( UBaseType_t ) 1 )
{
/* This is the first task to be created so do the preliminary
initialisation required. We will not recover if this call
fails, but we will report the failure. */
prvInitialiseTaskLists();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
/* If the scheduler is not already running, make this task the
current task if it is the highest priority task to be created
so far. */
if( xSchedulerRunning == pdFALSE )
{
if( pxCurrentTCB->uxPriority <= pxNewTCB->uxPriority )
{
pxCurrentTCB = pxNewTCB;
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
uxTaskNumber++;
#if ( configUSE_TRACE_FACILITY == 1 )
{
/* Add a counter into the TCB for tracing only. */
pxNewTCB->uxTCBNumber = uxTaskNumber;
}
#endif /* configUSE_TRACE_FACILITY */
traceTASK_CREATE( pxNewTCB );
prvAddTaskToReadyList( pxNewTCB );
portSETUP_TCB( pxNewTCB );
}
taskEXIT_CRITICAL();
if( xSchedulerRunning != pdFALSE )
{
/* If the created task is of a higher priority than the current task
then it should run now. */
if( pxCurrentTCB->uxPriority < pxNewTCB->uxPriority )
{
taskYIELD_IF_USING_PREEMPTION();
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
else
{
mtCOVERAGE_TEST_MARKER();
}
}
uxCurrentNumberOfTasks这是一个静态私有变量，表示任务的数量。我们重点看prvInitialiseTaskLists();这是一个初始化链表的函数，只有当第一个任务创建时才会执行。
static void prvInitialiseTaskLists( void )
{
UBaseType_t uxPriority;
for( uxPriority = ( UBaseType_t ) 0U; uxPriority < ( UBaseType_t ) configMAX_PRIORITIES; uxPriority++ )
{
vListInitialise( &( pxReadyTasksLists[ uxPriority ] ) );
}
vListInitialise( &xDelayedTaskList1 );
vListInitialise( &xDelayedTaskList2 );
vListInitialise( &xPendingReadyList );
#if ( INCLUDE_vTaskDelete == 1 )
{
vListInitialise( &xTasksWaitingTermination );
}
#endif /* INCLUDE_vTaskDelete */
#if ( INCLUDE_vTaskSuspend == 1 )
{
vListInitialise( &xSuspendedTaskList );
}
#endif /* INCLUDE_vTaskSuspend */
/* Start with pxDelayedTaskList using list1 and the pxOverflowDelayedTaskList
using list2. */
pxDelayedTaskList = &xDelayedTaskList1;
pxOverflowDelayedTaskList = &xDelayedTaskList2;
}
里面初始化了xDelayedTaskList1、xDelayedTaskList2、xPendingReadyList、xTasksWaitingTermination、xSuspendedTaskList以及各个优先级的链表 pxReadyTasksLists[ configMAX_PRIORITIES ]，这里有一个注意，如果configMAX_PRIORITIES 定义得越大，那么需要的RAM就越多。
往下到prvAddTaskToReadyList()，这个实际上是一个宏：
/*
* Place the task represented by pxTCB into the appropriate ready list for
* the task. It is inserted at the end of the list.
*/
#define prvAddTaskToReadyList( pxTCB ) \
traceMOVED_TASK_TO_READY_STATE( pxTCB ); \
taskRECORD_READY_PRIORITY( ( pxTCB )->uxPriority ); \
vListInsertEnd( &( pxReadyTasksLists[ ( pxTCB )->uxPriority ] ), &( ( pxTCB )->xStateListItem ) ); \
tracePOST_MOVED_TASK_TO_READY_STATE( pxTCB )
有部分函数看不了，都是核心函数，主要是看taskRECORD_READY_PRIORITY()和vListInsertEnd()。
1.taskRECORD_READY_PRIORITY()是用于记录处于就绪态的最高任务优先级
2.vListInsertEnd() 就是把辛辛苦苦创建的xStateListItem块用尾插的方式，插进pxReadyTasksLists[ ( pxTCB )->uxPriority ] 这个优先级的就绪链表中
插入完成后，如果调度器已经启动了，如果NewTCB的优先级大于CurrentTCB的优先级，就该调用taskYIELD_IF_USING_PREEMPTION();进行任务上下文切换了。