c++ - Linux SCHED_FIFO 不考虑线程优先级

问题描述

设想

我创建了三个线程，固定到一个核心，具有以下优先级SCHED_FIFO：

主要：sched_priority = 99
线程_1：sched_priority = 97
线程_2：sched_priority = 98

工作线程 ( thread_1, thread_2) 计算 50,000,000 个素数的总和（约 10 秒）。它们直到结束（打印输出）才阻塞或执行系统调用。

主线程休眠一秒钟，然后检查工作线程的 Promise 以查看是否完成。

预期行为

主线程处于最高优先级。根据sched：

SCHED_FIFO 线程一直运行，直到它被 I/O 请求阻塞、被更高优先级的线程抢占或调用 sched_yield(2)。

因此，Main 应checking ...以秒为间隔打印 ( )。它是最高优先级，因此应该抢占任何正在运行的东西。当它休眠时，它是阻塞的，所以其他线程应该运行。

thread_1: 先完成，因为main不忙时优先。
thread_2：最后完成，只有在thread_1完全完成后才开始。

实际行为

线程以与预期相反的顺序完成：

Thread 1 summed 3001134 primes at priority level: 97
Thread 2 summed 3001134 primes at priority level: 98
Main: Checking ...
Main: Task 1 has finished!
Main: Task 2 has finished!
Main: Exiting at priority level: 99

颠倒优先级顺序以使main具有最低的结果会产生完全相同的结果。

复制

编译程序g++ -o <exec_name> <file_name>.cpp -pthread
运行：sudo taskset --cpu-list 1 ./<exec_name>

我的内核是5.4.0-42-generic，我的发行版（如果重要的话）：Ubuntu 18.04.5 LTS. 我没有安装preempt-rt补丁。

类似问题

我发现这个问题似乎描述了相同的问题，但没有给出答案。
我还在这个问题中读到我的高优先级线程可以被抢占，但我不关心这一点，只要它不能被同一进程产生的其他线程抢占。我没有足够的信息来说明这是否会发生。

示例代码

#include <thread>
#include <mutex>
#include <iostream>
#include <chrono>
#include <cstring>
#include <future>
#include <pthread.h>
#include <math.h>

// IO Access mutex
std::mutex g_mutex_io;

// Computation function (busy work)
static bool isPrime (unsigned int value)
{
    unsigned int i, root;
    if (value == 1)       return false;
    if (value == 2)       return true;
    if ((value % 2) == 0) return false;
    root = (int)(1.0 + sqrt(value));
    for (i = 3; (i < root) && (value % i != 0); i += 2);
    return (i < root ? false : true);
}

// Thread function
void foo (unsigned int id, unsigned int count)
{
    sched_param sch;
    int policy, sum = 0;

    // Get information about thread
    pthread_getschedparam(pthread_self(), &policy, &sch);

    // Compute primes
    for (unsigned int i = 1; i < count; ++i) {
        sum += (isPrime(i) ? 1 : 0);
    }

    // Print
    {
        std::lock_guard<std::mutex> lock(g_mutex_io);
        std::cout << "Thread " << id << " summed " << sum << " primes"
                  << " at priority level: " << sch.sched_priority << std::endl; 
    }

}

int main ()
{
    sched_param sch;
    int policy;

    // Declare and init task objects
    std::packaged_task<void(unsigned int, unsigned int)> task_1(foo);
    std::packaged_task<void(unsigned int, unsigned int)> task_2(foo);

    // Get the futures
    auto task_fut_1 = task_1.get_future();
    auto task_fut_2 = task_2.get_future();

    // Declare and init thread objects
    std::thread thread_1(std::move(task_1), 1, 50000000);
    std::thread thread_2(std::move(task_2), 2, 50000000);

    // Set first thread policy
    pthread_getschedparam(thread_1.native_handle(), &policy, &sch);
    sch.sched_priority = 97;
    if (pthread_setschedparam(thread_1.native_handle(), SCHED_FIFO, &sch)) {
        std::cerr << "pthread_setschedparam: " << std::strerror(errno) 
                  << std::endl;
        return -1;
    }

    // Set second thread policy
    pthread_getschedparam(thread_2.native_handle(), &policy, &sch);
    sch.sched_priority = 98;
    if (pthread_setschedparam(thread_2.native_handle(), SCHED_FIFO, &sch)) {
        std::cerr << "pthread_setschedparam: " << std::strerror(errno) 
                  << std::endl;
        return -1;
    }

    // Set main process thread priority
    pthread_getschedparam(pthread_self(), &policy, &sch);
    sch.sched_priority = 99;
    if (pthread_setschedparam(pthread_self(), SCHED_FIFO, &sch)) {
        std::cerr << "pthread_setschedparam: " << std::strerror(errno)
                  << std::endl;
        return -1;
    }

    // Detach these threads
    thread_1.detach(); thread_2.detach();

    // Check their status with a timeout
    for (int finished = 0; finished < 2; ) {
        std::this_thread::sleep_for(std::chrono::seconds(1));
        {
            std::lock_guard<std::mutex> lock(g_mutex_io);
            std::cout << "Main: Checking ..." << std::endl;
        }
        if (task_fut_1.wait_for(std::chrono::seconds(0)) == std::future_status::ready) {
            {
                std::lock_guard<std::mutex> lock(g_mutex_io);
                std::cout << "Main: Task 1 has finished!" << std::endl;
            }
            finished++;
        }
        if (task_fut_2.wait_for(std::chrono::seconds(0)) == std::future_status::ready) {
            {
                std::lock_guard<std::mutex> lock(g_mutex_io);
                std::cout << "Main: Task 2 has finished!" << std::endl;
            }
            finished++;
        }
    }
    pthread_getschedparam(pthread_self(), &policy, &sch);
    std::cout << "Main: Exiting at priority level: " << sch.sched_priority << std::endl;
    return 0;
}

实验

用两个内核运行这个程序sudo taskset --cpu-list 1,2会产生以下奇怪的输出：

Thread 2 computed 3001134 primes at priority level: 98
Thread 1 computed 3001134 primes at priority level: 0
Main: Checking ...
Main: Task 1 has finished!
Main: Task 2 has finished!
Main: Exiting at priority level: 99

的优先级thread_1为零。

如果我将其扩展为包括三个 cores sudo taskset --cpu-list 1,2,3，那么我会得到我期望的单核行为：

Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Main: Checking ...
Thread 2 computed 3001134 primes at priority level: 98
Thread 1 computed 3001134 primes at priority level: 0
Main: Checking ...
Main: Task 1 has finished!
Main: Task 2 has finished!
Main: Exiting at priority level: 99

重新排列配置优先级的顺序，以便主线程先完成，不会改变原始场景中的输出

标签： c++linuxmultithreadingstdthread