【FreeRTOS学习03】小白都能懂的Task Management 任务管理基本概念介绍
2024-09-24 04:58:48
在FreeRTOS中,线程的术语又可以被称之为任务,或许这样更加合适,本文将介绍任务的创建/删除,任务参数的使用,以及任务优先级；
1 软实时和硬实时
硬实时系统的任务运行正确性与响应时限是紧密相关的，一旦超过时限将导致严重的后果，比如导弹控制系统、高铁自动驾驶系统等，都是需要严格的响应时限的。
软实时系统中，虽然也存在时限指标，但是如果输出响应超过时限，一般不会造成严重后果，比如Windows桌面任务，DVD播放机的视频播放。
大多数嵌入式系统不仅能满足硬实时要求，也能满足软实时要求。
软实时：
Windows；Linux系统通常为软实时，当然有补丁可以将内核做成硬实时的系统，不过商用没有这么做的。
硬实时：
VxWorks，uCOS，FreeRTOS，WinCE，RT-thread等实时系统；
2 任务概述
2.1 基本写法
FreeRTOS是多任务的实时系统，其最基本的运行单元为任务，其表示形式为由C语言函数实现的，该函数原型要求必须返回 void，并且带一个 void 类型指针的参数；具体如下所示；
void ATaskFunc(void *args);
每个任务都是在自己权限范围内的一个小程序。其具有程序入口，通常会运行在一个死循环中，也不会退出，具体如下；
void ATaskFunc(void *args){
while(1){
//TODO
}
}
FreeRTOS 任务不允许以任何方式从实现函数中返回——它们绝不能有一条return语句，也不能执行到函数末尾，如果不再需要，则在任务中调用删除任务的API，具体如下所示；
```c
void ATaskFunc(void *args){
vTaskDelete( NULL );
}
2.2 TCB
TCB为任务控制块，或者是线程控制块，另外操作系统中还有PCB为进程控制块，主要封装了一个任务在系统调度中所需要的所有资源，FreeRTOS中TCB的成员，具体如下所示；
任务状态如下所示；
typedef struct xTASK_STATUS
{
/* The handle of the task to which the rest of the information in the structure relates. */
TaskHandle_t xHandle;
/* A pointer to the task's name.*/
const char *pcTaskName;
/* A number unique to the task. */
UBaseType_t xTaskNumber;
/* The state in which the task existed when the structure was populated. */
eTaskState eCurrentState;
/* The priority at which the task was running (may be inherited) when the structure was populated. */
UBaseType_t uxCurrentPriority;
UBaseType_t uxBasePriority;
uint32_t ulRunTimeCounter;
StackType_t *pxStackBase;
uint16_t usStackHighWaterMark;
} TaskStatus_t;
每一个任务都会拥有一个自己的TCB，具体如下图所示；
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 ) || ( configRECORD_STACK_HIGH_ADDRESS == 1 ) )
StackType_t *pxEndOfStack; /*< Points to the highest valid address for the stack. */
#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 )
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 ) /*lint !e731 Macro has been consolidated for readability reasons. */
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;
3 任务状态
任务顶层存在两种状态，运行态和非运行态；
但是非运行的任务状态又可以分为：堵塞状态 / Blocked，挂起状态 / Suspend，就绪状态 / Ready，下面简单做一下介绍；
运行状态 /
Running
运行态的任务完全占用CPU的使用权，如果当前CPU只有一个内核，那么在某个时刻只能运行一个任务，就是所谓的单核单线程；堵塞状态 /
Blocked；
用户可以主动调用vTaskDelay(T)将任务进入堵塞状态，直到任务堵塞时间已经达到T；或者该任务在等待队列，信号量，事件组，通知或信号量事件时，也将处于堵塞状态；处于堵塞状态的任务不再占用CPU，同样也不能直接进入运行状态，而是先进入就绪状态；挂起状态 /
Suspend；
任何状态的下的任务都可以通过调用vTaskSuspend函数进入挂起状态，并且无法直接进入运行态，只能通过调用xTaskResume函数进入就绪状态；就绪状态 /
Ready；
被抢占的任务将处于就绪状态，挂起的任务被回复的会处于就绪状态，堵塞的任务收到相应事件也会处于就绪状态，如果当前没有更高优先级的任务处于运行，则当前就绪状态的任务进入运行状态；
其主要的状态转换关系如下图所示；
FreeRTOS使用一个枚举封装了任务的状态，具体如下所示；
typedef enum
{
eRunning = 0,/* A task is querying the state of itself, so must be running. */
eReady, /* The task being queried is in a read or pending ready list. */
eBlocked, /* The task being queried is in the Blocked state. */
eSuspended, /* The task being queried is in the Suspended state*/
eDeleted, /* The task being queried has been deleted, but its TCB has not yet been freed. */
eInvalid /* Used as an 'invalid state' value. */
} eTaskState;
4 任务优先级
FreeRTOS的最低优先级是0，分配给了空闲任务，空闲任务的优先级使用宏定义tskIDLE_PRIORITY进行表示，最大的优先级为需要用户进行配置；在FreeRTOS.h中可以看到预编译指令，具体如下；
#ifndef configMAX_PRIORITIES
#error Missing definition: configMAX_PRIORITIES must be defined in FreeRTOSConfig.h. \
See the Configuration section of the FreeRTOS API documentation for details.
#endif
#if configMAX_PRIORITIES < 1
#error configMAX_PRIORITIES must be defined to be greater than or equal to 1.
#endif
configMAX_PRIORITIES需要用户在FreeRTOSConfig.h进行定义，则任务最大的优先级为configMAX_PRIORITIES - 1。
5 相关函数
任务管理的函数声明和一些基本类型都封装在源码tasks.h中；
5.1 创建任务
使用xTaskCreate创建一个任务，具体如下所示；
// Task to be created.
void vTaskCode( void * pvParameters )
{
for( ;; )
{
// Task code goes here.
}
}
// Function that creates a task.
void vOtherFunction( void )
{
static uint8_t ucParameterToPass;
TaskHandle_t xHandle = NULL;
// Create the task, storing the handle. Note that the passed parameter ucParameterToPass
// must exist for the lifetime of the task, so in this case is declared static. If it was just an
// an automatic stack variable it might no longer exist, or at least have been corrupted, by the time
// the new task attempts to access it.
xTaskCreate( vTaskCode, "NAME", STACK_SIZE, &ucParameterToPass, tskIDLE_PRIORITY, &xHandle );
configASSERT( xHandle );
// Use the handle to delete the task.
if( xHandle != NULL )
{
vTaskDelete( xHandle );
}
}
5.2 函数删除
使用函数vTaskDelete对函数进行删除；
void vTaskDelete( TaskHandle_t xTaskToDelete ) PRIVILEGED_FUNCTION;
void vOtherFunction( void )
{
TaskHandle_t xHandle;
// Create the task, storing the handle.
xTaskCreate( vTaskCode, "NAME", STACK_SIZE, NULL, tskIDLE_PRIORITY, &xHandle );
// Use the handle to delete the task.
vTaskDelete( xHandle );
}
5.3 堵塞任务
使用vTaskDelay函数可以将任务堵塞一定时间；
void vTaskDelay( const TickType_t xTicksToDelay ) PRIVILEGED_FUNCTION;
void vTaskFunction( void * pvParameters )
{
// Block for 500ms.
const TickType_t xDelay = 500 / portTICK_PERIOD_MS;
for( ;; )
{
// Simply toggle the LED every 500ms, blocking between each toggle.
vToggleLED();
vTaskDelay( xDelay );
}
}
5.4 挂起和恢复
使用函数vTaskSuspend可以将函数挂起，通过vTaskResume(xHandle)函数可以将挂起的函数恢复到就绪状态；
void vTaskSuspend( TaskHandle_t xTaskToSuspend ) PRIVILEGED_FUNCTION;
void vAFunction( void )
{
TaskHandle_t xHandle;
// Create a task, storing the handle.
xTaskCreate( vTaskCode, "NAME", STACK_SIZE, NULL, tskIDLE_PRIORITY, &xHandle );
// ...
// Use the handle to suspend the created task.
vTaskSuspend( xHandle );
// ...
// The created task will not run during this period, unless
// another task calls vTaskResume( xHandle ).
//...
// Suspend ourselves.
vTaskSuspend( NULL );
// We cannot get here unless another task calls vTaskResume
// with our handle as the parameter.
}
6 总结
先了解任务之前的状态，先学会使用FreeRTOS的常用接口，后续对于其调度算法和内存管理的算法可以分析和学习一下。
文中难免有错误和纰漏之处，请大佬们不吝赐教
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