What is RLIMIT_RTPRIO?
RLIMIT_RTPRIO is a Linux-specific limit (since kernel 2.6.12) that sets a ceiling on the realtime scheduling priority that a process may set for itself using sched_setscheduler() and sched_setparam(). Realtime priorities on Linux range from 1 (lowest) to 99 (highest).
Without RLIMIT_RTPRIO, unprivileged processes would need CAP_SYS_NICE to use realtime scheduling at all. With RLIMIT_RTPRIO set to a nonzero value, an unprivileged process can use realtime scheduling up to the ceiling set by the limit.
How It Works
The soft RLIMIT_RTPRIO value directly sets the ceiling: a process may set its realtime priority to any value from 0 up to rlim_cur. Attempting to set a priority above the ceiling fails with EPERM.
| RLIMIT_RTPRIO soft | Allowed RT priorities |
|---|---|
| 0 | None (cannot use RT scheduling without CAP_SYS_NICE) |
| 10 | SCHED_FIFO or SCHED_RR with priority 1–10 |
| 99 | SCHED_FIFO or SCHED_RR with priority 1–99 (full range) |
Code Example — RLIMIT_RTPRIO
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sched.h>
#include <sys/resource.h>
int main(void)
{
struct rlimit rl;
struct sched_param sp;
getrlimit(RLIMIT_RTPRIO, &rl);
printf("RLIMIT_RTPRIO: soft=%lld hard=%lld\n",
(long long)rl.rlim_cur, (long long)rl.rlim_max);
/* Allow RT priority up to 10 */
rl.rlim_cur = 10;
rl.rlim_max = 10;
setrlimit(RLIMIT_RTPRIO, &rl);
/* Try to set SCHED_FIFO priority 5 (within limit) */
sp.sched_priority = 5;
if (sched_setscheduler(0, SCHED_FIFO, &sp) == -1) {
if (errno == EPERM)
printf("EPERM: cannot set RT (need root or higher limit)\n");
else
perror("sched_setscheduler");
} else {
printf("RT priority 5 set (SCHED_FIFO)\n");
/* Restore normal scheduling */
sp.sched_priority = 0;
sched_setscheduler(0, SCHED_OTHER, &sp);
}
/* Try priority 11 — above ceiling — should fail */
sp.sched_priority = 11;
if (sched_setscheduler(0, SCHED_FIFO, &sp) == -1 && errno == EPERM)
printf("EPERM: priority 11 exceeds RLIMIT_RTPRIO ceiling of 10\n");
return 0;
}
/* Compile: gcc -o rtprio_demo rtprio_demo.c
Note: on most systems, this still requires root unless
RLIMIT_RTPRIO was set by a privileged parent. */Interview Questions
It sets a ceiling on the realtime scheduling priority a process can set for itself via sched_setscheduler() and sched_setparam(). Available since Linux 2.6.12. Linux-specific, not in SUSv3.
No. With rlim_cur=0, the process cannot use any realtime scheduling policy (SCHED_FIFO, SCHED_RR) without CAP_SYS_NICE. It can only use non-realtime policies (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH).