liunx 下线程优先级反转 的使用示例

在Linux系统中，线程优先级反转（Thread Priority Inversion）是指一个高优先级线程被阻塞，导致其后续运行的低优先级线程有机会临时提升其优先级并获得CPU资源的情况。这种现象通常是因为某些锁竞争或同步机制导致的。

一个简单的示例如下：

#include <pthread.h>
#include <semaphore.h>

#define MAX_THREADS 5
#define HIGH_PRIORITY 10
#define LOW_PRIORITY 5

typedef struct {
    pthread_t thread;
    int priority;
} ThreadData;

void *high_priority_func(void *arg) {
    ThreadData *data = (ThreadData *) arg;
    sem_wait(&lock); // 模拟获取某个需要同步的资源
    data->priority = HIGH_PRIORITY;
    pthread_setschedparam(pthread_self(), SCHED_FIFO, &data->priority);
    printf("High priority thread started.\n");
    sem_post(&lock); // 释放资源
    return NULL;
}

void *low_priority_func(void *arg) {
    ThreadData *data = (ThreadData *) arg;
    while (1) {
        sem_wait(&lock); // 阻塞等待
        if (data->priority == LOW_PRIORITY) { // 如果变为低优先级，说明发生了优先级反转
            printf("Priority inversion occurred, high priority thread is blocked.\n");
            break;
        }
        sem_post(&lock); // 立即释放锁并继续循环
    }
    printf("Low priority thread exiting.\n");
    return NULL;
}

int main() {
    sem_t lock;
    init_sem(&lock, 1); // 初始化互斥量

    ThreadData threads[MAX_THREADS];
    for (int i = 0; i < MAX_THREADS; i++) {
        threads[i].priority = LOW_PRIORITY;
        pthread_create(&threads[i].thread, NULL, low_priority_func, &threads[i]);
    }

    // 创建一个高优先级线程
    threads[0].priority = HIGH_PRIORITY;
    pthread_create(&threads[0].thread, NULL, high_priority_func, &threads[0]);

    // 主函数不再阻塞
    pthread_join(threads[0].thread, NULL);

    return 0;
}

在这个示例中，`low_priority_func`会一直尝试获取锁，而`high_priority_func`在获取到锁后会将自身设置为高优先级。当`high_priority_func`因等待锁而被阻塞时，`low_priority_func`就会占据CPU时间，看似低优先级，但实际上由于高优先级线程被阻塞，就形成了优先级反转。