更改互斥体的优先级

Question

我对 RTOS 编程相当陌生，在使用互斥体时遇到了一些优先级问题。

我确定了以下优先事项。

#define T_HI_PRIORITY 10
#define T_ME_PRIORITY 50

我希望这段代码运行具有最高优先级的任务“tMePriorityTask”和具有中等优先级的“tHiPriorityTask”。 “tLoPriorityTask”已被注释，因此现在不应运行。

#include <stdio.h>
#include "main.h"
#include "vxWorks.h"
#include "semLib.h"
#include "taskLib.h"

SEM_ID semMutex;    // named semaphore object

char alphabet[27];  // memory resource to have exclusive access

void tHiPriorityTask (void)
{
   int i;

    // enter critical region - any other tasks wanting access to alphabet[] should
    // wait for available semaphore
    semTake (semMutex, WAIT_FOREVER);

   // write alphabet to global array
    for (i= 0; i < 26; i++)
        alphabet[i] = 'A' + i;

    alphabet[i] = '\0';

    printf("High priority.\n-Counting alphabet...\n");

    // leave critical region
    semGive (semMutex);
}

void tMePriorityTask (void)
{
    // enter critical region
    semTake (semMutex, WAIT_FOREVER);

    //medium priority task enters
    printf("Medium priority.\n-Just entering...\n");

    // leave critical region
    semGive (semMutex);
}

/*void tLoPriorityTask (void)
{
    // enter critical region
    semTake (semMutex, WAIT_FOREVER);

    // array members guaranteed stable while being read by this task
    printf("Low priority\n");
    printf ("-alphabet= %s ", alphabet);

    // leave critical region
    semGive (semMutex);
}*/

void main (void)
{
    //create binary semaphore which is initially full (available)
    semMutex = semBCreate (SEM_Q_PRIORITY, SEM_FULL);

    // spawn high priority task
    taskSpawn ("hi_priority_task", T_ME_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tHiPriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    // spawn medium priority task
    taskSpawn ("me_priority_task", T_HI_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tMePriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    // spawn low priority task
    //taskSpawn ("lo_priority_task", T_LO_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tLoPriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
}

我尝试在生成任务时更改优先级，但这似乎不起作用，至少它不会改变屏幕上的任何内容。我正在使用 VxWorks RTOS。

谢谢。

Answer 1

如果要保证两个线程按特定顺序运行，则必须使用正确的线程同步。仅仅设置优先级并希望它能工作是非常糟糕的设计，你可能会让它工作，但它最终会中断(例如，如果你进行阻塞调用，那么没有什么可以阻止任何其他线程运行。)

下面是修改代码的最简单方法，以保证 tMePriorityTask 线程在允许运行 tHiPriorityTask 之前运行完成，无论优先级或您在线程中执行的任何其他操作如何。

#include <stdio.h>
#include "main.h"
#include "vxWorks.h"
#include "semLib.h"
#include "taskLib.h"

SEM_ID semMutex;    // named semaphore object

char alphabet[27];  // memory resource to have exclusive access

void tHiPriorityTask (void)
{
   int i;

    // enter critical region - any other tasks wanting access to alphabet[] should
    // wait for available semaphore
    semTake (semMutex, WAIT_FOREVER);

   // write alphabet to global array
    for (i= 0; i < 26; i++)
        alphabet[i] = 'A' + i;

    alphabet[i] = '\0';

    printf("High priority.\n-Counting alphabet...\n");

    // leave critical region
    semGive (semMutex);
}

void tMePriorityTask (void)
{
    // enter critical region
    //semTake (semMutex, WAIT_FOREVER);

    //medium priority task enters
    printf("Medium priority.\n-Just entering...\n");

    // leave critical region
    semGive (semMutex);
}

/*void tLoPriorityTask (void)
{
    // enter critical region
    semTake (semMutex, WAIT_FOREVER);

    // array members guaranteed stable while being read by this task
    printf("Low priority\n");
    printf ("-alphabet= %s ", alphabet);

    // leave critical region
    semGive (semMutex);
}*/

void main (void)
{
    //create binary semaphore which is initially full (available)
    semMutex = semBCreate (SEM_Q_PRIORITY, SEM_EMPTY);

    // spawn high priority task
    taskSpawn ("hi_priority_task", T_ME_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tHiPriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    // spawn medium priority task
    taskSpawn ("me_priority_task", T_HI_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tMePriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
    // spawn low priority task
    //taskSpawn ("lo_priority_task", T_LO_PRIORITY, VX_FP_TASK, 10000, (FUNCPTR) tLoPriorityTask, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0);
}

我所做的就是使用 SEM_EMPTY 创建信号量，然后从 tMePriorityTask 中删除 semTake。

这样，tHiPriorityTask 可以以任何优先级运行，并且它将在 semTake 上阻塞，直到 tMePriorityTask 发出 semGive。此外，tMePriorityTask 线程可以执行其他阻塞调用(例如 I/O 调用)，并且 tHiPriorityTask 线程在 semGive 之前仍然无法运行。

查看 semBCreate 的 vxworks API 引用并阅读标题为“同步”的部分。