On the 2.6.28-11 Linux kernel, I am using setpriority to bias the amount of cpu time different user processes receive. User 1 is to receive double the CPU power of user 2. Using Linux's setpriority, I have assigned user 2 a lower priority (higher in terms of nice values). When I run the exact same program via the shell with the computer under load, user 2's execution takes twice as long as user 1's. However, if I run the program as root, and then use setuid, seteuid, setgid, setegid, and setgroups to become user 2 (once again the computer is under load with the exact same program being run by user 1 in the same manner), the programs take the exact same amount of time to execute; User 2's process was not being scheduled any less than user 1's. How can I get around this? Do I need to set anything else for the scheduler to realize that the process is now part of a different user?
The program being run is in Python.
Right, this is the designed behavior, even if it's not what you want. You can update your own priority to match what you should have inherited, if you were started normally:
/* C */
#include <sys/resource.h>
int proc_prio = getpriority(PRIO_PROCESS, getpid()),
pgrp_prio = getpriority(PRIO_PGRP, getpgrp()),
user_prio = getpriority(PRIO_USER, getuid());
setpriority(PRIO_PROCESS, getpid(),
proc_prio < pgrp_prio ? pgrp_prio < user_prio ? user_prio
: pgrp_prio
: proc_prio < user_prio ? user_prio
: proc_prio);
# Python
import ctypes
import os
PRIO_PROCESS, PRIO_PGRP, PRIO_USER = 0, 1, 2
libc = ctypes.CDLL('libc.so.6')
libc.setpriority(PRIO_PROCESS, os.getpid(),
max(libc.getpriority(PRIO_PROCESS, os.getpid()),
libc.getpriority(PRIO_PGRP, os.getpgrp()),
libc.getpriority(PRIO_USER, os.getuid())))
Or of course you could fix another process's priority, with appropriate privileges.