Getting EPERM when calling pthread_create() for SCHED_FIFO thread as root on Linux

Question

I am trying to spawn threads with SCHED_FIFO or SCHED_RR policies as root on a Linux system but my calls to pthread_create() are returning 1 (EPERM). The man page for pthread_create() says that EPERM indicates that "[t]he caller does not have appropriate permission to set the required scheduling parameters or scheduling policy." Shouldn't root be able to specify SCHED_FIFO or SCHED_RR?

I've stripped out the code that creates a thread into a small program that does only that. It looks right to me but still gets the error. What am I doing wrong?

The program:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

static void *_Thread(void *arg)
{
    (void)arg;
    printf("Thread running!\n");
    return NULL;
}

int main(void)
{
    int retVal;
    pthread_attr_t attr;
    struct sched_param schedParam;
    pthread_t thread;

    retVal = pthread_attr_init(&attr);
    if (retVal)
    {
        fprintf(stderr, "pthread_attr_init error %d\n", retVal);
        exit(1);
    }

    retVal = pthread_attr_setinheritsched(&attr, PTHREAD_EXPLICIT_SCHED);
    if (retVal)
    {
        fprintf(stderr, "pthread_attr_setinheritsched error %d\n", retVal);
        exit(1);
    }

    retVal = pthread_attr_setschedpolicy(&attr, SCHED_FIFO);
    if (retVal)
    {
        fprintf(stderr, "pthread_attr_setschedpolicy error %d\n", retVal);
        exit(1);
    }

    schedParam.sched_priority = 1;
    retVal = pthread_attr_setschedparam(&attr, &schedParam);
    if (retVal)
    {
        fprintf(stderr, "pthread_attr_setschedparam error %d\n", retVal);
        exit(1);
    }

    retVal = pthread_create(&thread,
                            &attr,
                            _Thread,
                            NULL);
    if (retVal)
    {
        fprintf(stderr, "pthread_create error %d\n", retVal);
        exit(1);
    }

    retVal = pthread_join(thread, NULL);
    if (retVal)
    {
        fprintf(stderr, "pthread_join error %d\n", retVal);
        exit(1);
    }

    printf("main run successfully\n");
    return 0;
}

This program has been compiled and run as root. When run, it program fails at the call to pthread_create, returning EPERM.

Changing the thread to SCHED_RR scheduling has no effect - EPERM still returned by pthread_create.

Changing the thread to SCHED_OTHER scheduling and its priority to 0 allows the program to run without error.

Answer 1

It must be that your real-time priority soft limit is too restrictive.

Either call ulimit -r unlimited in shell before running your code. Or call setrlimit(RLIMIT_RTPRIO, ...) directly in your code.

System-wide limits are set in /etc/security/limits.conf.