UbuntuのQtCreatorのプログラムのPriorityの設定について

UbuntuのQtCreatorのWidgetアプリケーションのプログラム環境です。

下記の『set_realtime_priority()』を実行しますが、
コード『ret = pthread_setschedparam(this_thread, SCHED_FIFO, &params);』の実行が成功しません。
つまりret=!0です。

Q1)ここプログラムの環境では、priorityの設定は、出来ないでしょうか？

void MainWindow::set_realtime_priority(){
// struct sched_param is used to store the scheduling priority
struct sched_param params;
int ret;
// We'll operate on the currently running thread.
pthread_t this_thread = pthread_self();

// We'll set the priority to the maximum.
params.sched_priority = sched_get_priority_max(SCHED_FIFO);

qDebug("Trying to set thread realtime prio=%d ",params.sched_priority);

// Attempt to set thread real-time priority to the SCHED_FIFO policy
ret = pthread_setschedparam(this_thread, SCHED_FIFO, &params);
//ret = pthread_setschedparam(pthread_self(), SCHED_FIFO, &params);
if(ret!=0){
    // Print the error
    qDebug("%s","Unsuccessful in setting thread realtime prio");
    return;
}
// Now verify the change in thread priority
int policy = 0;
ret = pthread_getschedparam(this_thread, &policy, &params);
if (ret != 0) {
    qDebug("%s","Couldn't retrieve real-time scheduling paramers");
    return;
}

// Check the correct policy was applied
if(policy != SCHED_FIFO) {
    qDebug("%s","Scheduling is NOT SCHED_FIFO!");
} else {
    qDebug("%s","SCHED_FIFO OK");
}

// Print thread scheduling priority
qDebug("Thread priority=%d ",params.sched_priority);