FreeRTOS系列学习笔记四---task
一、任务队列
/* Lists for ready and blocked tasks. --------------------*/
PRIVILEGED_DATA static List_t pxReadyTasksLists[ configMAX_PRIORITIES ];/*< Prioritised ready tasks. */
PRIVILEGED_DATA static List_t xDelayedTaskList1; /*< Delayed tasks. */
PRIVILEGED_DATA static List_t xDelayedTaskList2; /*< Delayed tasks (two lists are used - one for delays that have overflowed the current tick count. */
PRIVILEGED_DATA static List_t * volatile pxDelayedTaskList; /*< Points to the delayed task list currently being used. */
PRIVILEGED_DATA static List_t * volatile pxOverflowDelayedTaskList; /*< Points to the delayed task list currently being used to hold tasks that have overflowed the current tick count. */
PRIVILEGED_DATA static List_t xPendingReadyList; /*< Tasks that have been readied while the scheduler was suspended. They will be moved to the ready list when the scheduler is resumed. */
1、从它 pxReadyTasksLists[ configMAX_PRIORITIES ] 可以看出每个任务优先级都有一个队列，这样方便实现同优先级任务分时间片调度。
2、有两个延时队列xDelayedTaskList1和xDelayedTaskList2，为什么要两个呢？FreeRTOS里面的延时是以系统节拍为时基，并且用的步进链表，举例：
systick = 10; task1要延时250，那么xItemValue=260；如果xItemValue是8位的，这个xItemValue就溢出了，因此xItemValue=260%255；并且放到延时 队列里面；xDelayedTaskList1和xDelayedTaskList2每次systick 溢出时候，交换使用；
3、xPendingReadyList，根据源码解释，当调度器被挂起时候，就绪任务放到了这个里面，当调度器恢复时候，再把他们移到就绪任务列表；我感觉这么处 理的原因是，当调度器挂起时候，就绪队列是禁止操作，这时如果激活了其它任务或者添加了其它任务，就放到了这个“临时”缓存里面；（自己猜想，回头 看看源码）；
二、任务控制块
/*
* Task control block. A task control block (TCB) is allocated for each task,
* and stores task state information, including a pointer to the task's context
* (the task's run time environment, including register values)
*/
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;三、FreeRTOS的最高优先级任务选择
configUSE_PORT_OPTIMISED_TASK_SELECTION这个宏为0，则没有进行优化，与特定平台无关，优先级也没有限制。
/* uxTopReadyPriority holds the priority of the highest priority ready
state task. */
#define taskRECORD_READY_PRIORITY( uxPriority ) \
{ \
if( ( uxPriority ) > uxTopReadyPriority ) \
{ \
uxTopReadyPriority = ( uxPriority ); \
} \
} /* taskRECORD_READY_PRIORITY */
/*-----------------------------------------------------------*/
#define taskSELECT_HIGHEST_PRIORITY_TASK() \
{ \
UBaseType_t uxTopPriority = uxTopReadyPriority; \
\
/* Find the highest priority queue that contains ready tasks. */ \
while( listLIST_IS_EMPTY( &( pxReadyTasksLists[ uxTopPriority ] ) ) ) \
{ \
configASSERT( uxTopPriority ); \
--uxTopPriority; \
} \
\
/* listGET_OWNER_OF_NEXT_ENTRY indexes through the list, so the tasks of \
the same priority get an equal share of the processor time. */ \
listGET_OWNER_OF_NEXT_ENTRY( pxCurrentTCB, &( pxReadyTasksLists[ uxTopPriority ] ) ); \
uxTopReadyPriority = uxTopPriority; \
} /* taskSELECT_HIGHEST_PRIORITY_TASK */
/*-----------------------------------------------------------*/说明：
1、宏taskRECORD_READY_PRIORITY( uxPriority )找到优先级最大的任务就绪队列；
2、taskSELECT_HIGHEST_PRIORITY_TASK()找到优先级最最高的任务，条用listGET_OWNER_OF_NEXT_ENTRY（）返回任务控制块；
configUSE_PORT_OPTIMISED_TASK_SELECTION为1时候，需要硬件支持CLZ指令，同时优先级最大31级。
零计数指令CLZ
ARMv5及其以上版本提供了一条新的指令——零计数指令CLZ（Count Leading Zeros）。
该指令用于计算最高符号位与第一个1之间的0的个数。当一些操作数需要规范化
（使其最高位为1）时，该指令用于计算操作数需要左移的位数。
/* A port optimised version is provided. Call the port defined macros. */
#define taskRECORD_READY_PRIORITY( uxPriority ) portRECORD_READY_PRIORITY( uxPriority, uxTopReadyPriority )
/*-----------------------------------------------------------*/
#define taskSELECT_HIGHEST_PRIORITY_TASK() \
{ \
UBaseType_t uxTopPriority; \
\
/* Find the highest priority list that contains ready tasks. */ \
portGET_HIGHEST_PRIORITY( uxTopPriority, uxTopReadyPriority ); \
configASSERT( listCURRENT_LIST_LENGTH( &( pxReadyTasksLists[ uxTopPriority ] ) ) > 0 ); \
listGET_OWNER_OF_NEXT_ENTRY( pxCurrentTCB, &( pxReadyTasksLists[ uxTopPriority ] ) ); \
} /* taskSELECT_HIGHEST_PRIORITY_TASK() */
/*-----------------------------------------------------------*/
/* A port optimised version is provided, call it only if the TCB being reset
is being referenced from a ready list. If it is referenced from a delayed
or suspended list then it won't be in a ready list. */
#define taskRESET_READY_PRIORITY( uxPriority ) \
{ \
if( listCURRENT_LIST_LENGTH( &( pxReadyTasksLists[ ( uxPriority ) ] ) ) == ( UBaseType_t ) 0 ) \
{ \
portRESET_READY_PRIORITY( ( uxPriority ), ( uxTopReadyPriority ) ); \
} \
}把就绪任务放入位图：
#define taskRECORD_READY_PRIORITY( uxPriority ) portRECORD_READY_PRIORITY( uxPriority, uxTopReadyPriority )
#define portRECORD_READY_PRIORITY( uxPriority, uxReadyPriorities ) ( uxReadyPriorities ) |= ( 1UL << ( uxPriority ) )
查找优先级任务最高的任务队列：
portGET_HIGHEST_PRIORITY( uxTopPriority, uxTopReadyPriority );
#define portGET_HIGHEST_PRIORITY( uxTopPriority, uxReadyPriorities ) uxTopPriority = ( 31UL - ( uint32_t ) __clz( ( uxReadyPriorities ) ) )
查找最高优先级任务：
listGET_OWNER_OF_NEXT_ENTRY（）
未完，待续。。。。。。