【FreeRTOS】内核查找最高优先级就绪任务

查找最高优先级就绪任务

FreeRTOS\Source\tasks.c

#if ( configUSE_PORT_OPTIMISED_TASK_SELECTION == 0 )

/* If configUSE_PORT_OPTIMISED_TASK_SELECTION is 0 then task selection is
 * performed in a generic way that is not optimised to any particular
 * microcontroller architecture. */

/* 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 */

/*-----------------------------------------------------------*/

/* Define away taskRESET_READY_PRIORITY() and portRESET_READY_PRIORITY() as
 * they are only required when a port optimised method of task selection is
 * being used. */
    #define taskRESET_READY_PRIORITY( uxPriority )
    #define portRESET_READY_PRIORITY( uxPriority, uxTopReadyPriority )

#else /* configUSE_PORT_OPTIMISED_TASK_SELECTION */

/* If configUSE_PORT_OPTIMISED_TASK_SELECTION is 1 then task selection is
 * performed in a way that is tailored to the particular microcontroller
 * architecture being used. */

/* 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 ) );                        \
        }                                                                                              \
    }

#endif /* configUSE_PORT_OPTIMISED_TASK_SELECTION */

查找机制有两种：通用方法和基于特定处理器架构的优化方法

通用方法倒序遍历优先级链表，找出置位的最高优先级标志，进而拿到最高优先级就绪任务的控制块

优化方法使用的是处理器支持的计算前导零指令 clz

计算前导零

FreeRTOS\Source\portable\GCC\ARM_CM4F\portmacro.h

__attribute__( ( always_inline ) ) static inline uint8_t ucPortCountLeadingZeros( uint32_t ulBitmap )
{
    uint8_t ucReturn;
    __asm volatile ( "clz %0, %1" : "=r" ( ucReturn ) : "r" ( ulBitmap ) : "memory" );
    return ucReturn;
}

汇编代码说明：

__asm：c内嵌汇编

volatile：禁用编译器优化

"clz %0, %1"：汇编代码模板，0%对应第一个参数（ucReturn），1%对应第二个参数（ulBitmap）

"=r" ( ucReturn )：输出操作数，=r操作数使用通用寄存器

"r" ( ulBitmap )：输入操作数，r操作数使用通用寄存器

"memory"：寄存器

 

__asm volatile：用法详见 https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html