How to get OS thread id (with uint64_t) in userspace with Linux OS

Question

I am working on a project where I make some operation in any multithread application. I want to specify that I don't have a possibility to see the source code of that multithread app, it can be any multithread app. My objective in that project is when the multithread app is running, I retrieve the threads of multithreads app on CFS scheduler and put them on another schedulers. In order to achieve this I just retrieve their values in /proc/parent_pid/task/ where parent_pid is pid of multithread app. But my problem is that the value got on /proc/parent_pid/task/ is not sufficient for me, I need the OS thread identifier of each threads. Since it is not possible for me to have access on source code of multithread app, my question is how can I get the OS thread identifier of those threads when their are running ? I think that it is not possible for me to make some syscall such as syscall(__NR_gettid), or gettid(), or pthread_self() because those functions are call generally in the thread source code. It is possible to get that value in some files locate at /proc/parent_pid/? I use Ubuntu 20.04.