프로세스 및 스레드 컨텍스트(운영체제 책의 PCB) struct task_struct 들은 각각 CPU 마다 per cpu 변수로 할당되어있는  struct rq 자료구조(운영체제 책의 대기 큐)에 큐잉되고, struct rq 구조체를 보면 여러 종류 큐가 있는 것을 확인할 수 있습니다.

https://elixir.bootlin.com/linux/v6.2-rc7/source/kernel/sched/sched.h#L990

참고로, 커널 덤프를 확인할 때, 크래시 유틸리티에서 runq 커맨드로 해당 큐들의 내용물을 확인할 수 있습니다.

realtime thread 는 RT 큐에 할당되어 처리합니다. https://elixir.bootlin.com/linux/v6.2-rc7/source/kernel/sched/sched.h#L663

말씀하신 유저에서 사용한 시스템 콜은 정책 인자를 넣어줄 수 있습니다.

       3) If a call to sched_setscheduler(2), sched_setparam(2),
          sched_setattr(2), pthread_setschedparam(3), or
          pthread_setschedprio(3) changes the priority of the running or
          runnable SCHED_FIFO thread identified by pid the effect on the
          thread's position in the list depends on the direction of the
          change to threads priority:

- https://man7.org/linux/man-pages/man7/sched.7.html

- https://man7.org/linux/man-pages/man3/pthread_setschedparam.3.html

/nptl/pthread_setschedparam.c 어떻게 설정하는지 GLIBC 유저 구현을 볼까요? 

  /* Try to set the scheduler information.  */
  if (__builtin_expect (__sched_setscheduler (pd->tid, policy,
					      param) == -1, 0))

- https://elixir.bootlin.com/glibc/glibc-2.37.9000/source/nptl/pthread_setschedparam.c#L27

커널에서 해당 시스템 콜 구현을 볼까요?

/**
 * sys_sched_setscheduler - set/change the scheduler policy and RT priority
 * @pid: the pid in question.
 * @policy: new policy.
 * @param: structure containing the new RT priority.
 *
 * Return: 0 on success. An error code otherwise.
 */
SYSCALL_DEFINE3(sched_setscheduler, pid_t, pid, int, policy, struct sched_param __user *, param)
{
	if (policy < 0)
		return -EINVAL;

	return do_sched_setscheduler(pid, policy, param);
}

static int
do_sched_setscheduler(pid_t pid, int policy, struct sched_param __user *param)
{
	struct sched_param lparam;
	struct task_struct *p;
	int retval;

	if (!param || pid < 0)
		return -EINVAL;
	if (copy_from_user(&lparam, param, sizeof(struct sched_param)))
		return -EFAULT;

	rcu_read_lock();
	retval = -ESRCH;
	p = find_process_by_pid(pid);
	if (likely(p))
		get_task_struct(p);
	rcu_read_unlock();

	if (likely(p)) {
		retval = sched_setscheduler(p, policy, &lparam);
		put_task_struct(p);
	}

	return retval;
}

static int _sched_setscheduler(struct task_struct *p, int policy,
			       const struct sched_param *param, bool check)
{
	struct sched_attr attr = {
		.sched_policy   = policy,
		.sched_priority = param->sched_priority,
		.sched_nice	= PRIO_TO_NICE(p->static_prio),
	};

	/* Fixup the legacy SCHED_RESET_ON_FORK hack. */
	if ((policy != SETPARAM_POLICY) && (policy & SCHED_RESET_ON_FORK)) {
		attr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;
		policy &= ~SCHED_RESET_ON_FORK;
		attr.sched_policy = policy;
	}

	return __sched_setscheduler(p, &attr, check, true);
}
/**
 * sched_setscheduler - change the scheduling policy and/or RT priority of a thread.
 * @p: the task in question.
 * @policy: new policy.
 * @param: structure containing the new RT priority.
 *
 * Use sched_set_fifo(), read its comment.
 *
 * Return: 0 on success. An error code otherwise.
 *
 * NOTE that the task may be already dead.
 */
int sched_setscheduler(struct task_struct *p, int policy,
		       const struct sched_param *param)
{
	return _sched_setscheduler(p, policy, param, true);
}

static int __sched_setscheduler(struct task_struct *p,
				const struct sched_attr *attr,
				bool user, bool pi)

	/*
	 * Valid priorities for SCHED_FIFO and SCHED_RR are
	 * 1..MAX_RT_PRIO-1, valid priority for SCHED_NORMAL,
	 * SCHED_BATCH and SCHED_IDLE is 0.
	 */
	if (attr->sched_priority > MAX_RT_PRIO-1)
		return -EINVAL;
	if ((dl_policy(policy) && !__checkparam_dl(attr)) ||
	    (rt_policy(policy) != (attr->sched_priority != 0)))
		return -EINVAL;

https://elixir.bootlin.com/linux/latest/source/kernel/sched/core.c#L7892

동영상 재생이 좋은 예시일 것 같습니다.