3.FreeRTOS任務的簡易分析
阿新 • • 發佈：2020-12-13
FreeRTOS任務的簡易分析
- 前言：毫無疑問，任務整個FreeRTOS的核心，所以分析起來很困難，我只能去各個地方看不同人的文件或筆記，把別人的東西變為自己的東西，最後記錄這篇內容。我無法向介紹List那樣把全部程式碼看一遍，只能把大概的流程寫下來。
1.任務結構體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;
這個成員非常多，只能看幾個。
| 成員 | 作用 |
|---|---|
| pxTopOfStack | 它指向任務堆疊的棧頂。當任務切換時，儲存現場或者恢復現場時，會從這裡儲存或取出 |
| xStateListItem | 狀態連結串列裡的連結串列項 |
| xEventListItem | 事件連結串列裡的連結串列項 |
| uxPriority | 任務的優先順序，呼叫建立函式時傳入 |
| pxStack | 任務堆疊開始的地方 |
| pcTaskName | 任務的名字，建立任務時傳入 |
| uxTCBNumber | 每個任務唯一的值 |
首先單獨拿pxTopOfStack和pxStack出來說，pxTopOfStack它儲存的東西是關於任務切換的現場(xPSR、PC、LR、R12、R3R0、R11
pxTopOfStack來操作的。
1.1建立任務函式xTaskCreate模擬
1.1.1pxStack、pxNewTCB、pxNewTCB->pxTopOfStack的跟蹤
我模擬程式後，打了兩個斷點在xTaskCreate裡面：
{ 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. */ 斷點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 ); 斷點2------> prvAddNewTaskToReadyList( pxNewTCB ); xReturn = pdPASS;
在斷點1時，pxStack=0x20000520,pxNewTCB = 0x20000728，此時的pxNewTCB->pxTopOfStack為0。然後進入函式prvInitialiseNewTask裡面。首先給pxTopOfStack賦值，具體如下:
#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 ) );
}
賦值後pxTopOfStack為0x20000718，接著執行到pxNewTCB->pxTopOfStack = pxPortInitialiseStack( pxTopOfStack, pxTaskCode, pvParameters );
函式pxPortInitialiseStack具體如下：
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;
}
沒錯，這一段很明顯就是儲存現場（Cortex-M3 入棧順序：xPSR、PC、LR、R12、R3R0、R11R4），在建立任務時，核心把新建的任務當成一個剛剛執行的任務。還有一點注意的，pxTopOfStack是向下增長的。執行完這段程式碼後，pxTopOfStack的值是從0x20000718變為0x200006D8。當然這些前提是ARM架構下，棧是向下增長的，FreeRTOS中有具體的巨集定義portSTACK_GROWTH為-1時，就是向下增長。
得出以下結果：
以上就是建立任務函式時，pxStack、pxNewTCB、pxNewTCB->pxTopOfStack的分析。
1.1.2 TCB結構體成員的初始化
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';
比較簡單，就是把傳入的賦值即可，注意一下最後一個是‘\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;
把傳入的優先順序賦值。
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 );
結合前面的List分析：
1.首先把兩個連結串列項都初始化
2.然後把xStateListItem裡的pxOwner成員指向TCB結構體
3.優先順序作為引數，賦值到xEventListItem的成員xItemValue裡面
4.然後把xEventListItem裡的pxOwner成員指向TCB結構體
1.1.3 TCB新增到ReadyList中
新增的具體函式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這是一個靜態私有變數，表示任務的數量。我們重點看prvInitialiseTaskLists();這是一個初始化連結串列的函式，只有當第一個任務建立時才會執行。
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;
}
裡面初始化了xDelayedTaskList1、xDelayedTaskList2、xPendingReadyList、xTasksWaitingTermination、xSuspendedTaskList以及各個優先順序的連結串列 pxReadyTasksLists[ configMAX_PRIORITIES ]，這裡有一個注意，如果configMAX_PRIORITIES 定義得越大，那麼需要的RAM就越多。
往下到prvAddTaskToReadyList()，這個實際上是一個巨集：
/*
* 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 )
有部分函式看不了，都是核心函式，主要是看taskRECORD_READY_PRIORITY()和vListInsertEnd()。
1.taskRECORD_READY_PRIORITY()是用於記錄處於就緒態的最高任務優先順序
2.vListInsertEnd() 就是把辛辛苦苦建立的xStateListItem塊用尾插的方式，插進pxReadyTasksLists[ ( pxTCB )->uxPriority ] 這個優先順序的就緒連結串列中
插入完成後，如果排程器已經啟動了，如果NewTCB的優先順序大於CurrentTCB的優先順序，就該呼叫taskYIELD_IF_USING_PREEMPTION();進行任務上下文切換了。