Real time threads don't work in libvirt containers under CentOS 7.3

[Peter Steele <pwsteele@xxxxxxxxx>]

We've been using libvirt based containers under CentOS 7 and everything 
has been working fine. One application we run in our containers is ctdb, 
which uses SCHED_FIFO (real time) threads. This has been working without 
problems until our recent upgrade to CentOS 7.3. For some reason, ctdb 
is no longer able to create real time threads, and I've tried a simple 
program myself that confirms this. The same program works fine on the 
hypervisor so I know the kernel supports real time. Does anyone know 
what may have changed in CentOS 7.3 that breaks real time threads in 
libvirt containers?

This is the simple test program I created to verify the real time 
threads are failing. This same program works in a libvirt container in 
CentOS 7.2.

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

pthread_t test_thread;

void *test(void *arg)
{
    printf("Starting thread\n");
    sleep(1);
    printf("Thread complete\n");
    return 0;
}


int main(int argc, char *argv[])
{
    int rc;
    printf("Starting main\n");
    struct sched_param tsparam;
    pthread_attr_t tattr;
    memset(&tsparam, 0, sizeof(tsparam));
    pthread_attr_init(&tattr);
    pthread_attr_setinheritsched(&tattr, PTHREAD_EXPLICIT_SCHED);
    pthread_attr_setschedpolicy(&tattr, SCHED_FIFO);
    tsparam.sched_priority = sched_get_priority_max(SCHED_FIFO) - 7;
    pthread_attr_setschedparam(&tattr, &tsparam);
    if ((rc = pthread_create(&test_thread, &tattr, test, NULL)) != 0) {
        printf("Unable to start rt thread\n");
    }
    return 0;
}

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