System-wide scheduling modification

Question

Is there a way to change the process scheduling policy of the whole system? For example, I want to change the default SCHED_OTHER to SCHED_RR for all processes. The command chrt operates on a process. I would like to change the settings system-wide. Any idea about that?

UPDATE:

To be more specific, I want to launch some commands, e.g. tar with different policies and the problem is that prior to launching the program, I don't know the process ID.

Answer 1

chrt can not only change the scheduling priority of a running process (you'll actually need the pid) but also launch a new process following a particular scheduling policy :

   chrt [options] priority command argument ...

   chrt [options] -p [priority] PID

DESCRIPTION

   chrt sets or retrieves the real-time scheduling attributes of an
   existing PID, or runs command with the given attributes.

So if you want to launch tar running SCHED_RR rtprio 10 straight from the beginning, simply

chrt -r 10 tar blahblahraroptions

If, as written in comments, it would make no sense to have all running processes running the same scheduling policy at the same priority level, there is just no way to achieve that.
It is however possible to launch all your users' processes automatically to your desired scheduling policy / priority since this is (per default) inherited by child processes.

1/ fork a bash process scheduled SCHED_FIFO / priority 10 :

$ chrt -f 10 bash

2/ Whatever process launched from now on will be scheduled identically without the need to use chrt.

/proc/2074 $ ps -eHo pid,ppid,tid,class,rtprio,ni,pri,comm
...
4168  2074  4168 FF      10   -  50         bash
4192  4168  4192 FF      10   -  50           ps
...

Use with caution and only for your test purposes, and of course do avoid launching fork bombs as well as assigning the highest possible priority !