Niceness level for systemd

[akhmarov]

Hello

We have VM with CentOS 8 based distro and FRRouting 10.4.1 installed
Found strange behaviour with Nice=-5 under FRR systemd unit file

Default systemd unit file for FRR services that was installed from RedHat 8 / Rocky 8 repository on https://rpm.frrouting.org do have niceness option:

$ cat /usr/lib/systemd/system/frr.service | grep Nice
Nice=-5

If we try to enable and start that service we will receive such an error:

Aug 19 09:17:05 centos systemd[563]: frr.service: Failed to set up process scheduling priority (nice level): Permission denied
Aug 19 09:17:05 centos systemd[563]: frr.service: Failed at step NICE spawning /usr/lib/frr/frrinit.sh: Permission denied

To overcome this issue we have to specify limit for frr user:

$ cat /etc/security/limits.d/90-frr-limits.conf
frr             hard    nice           -5

This action seems like a quick solution but if we check processes that were forked we could mention that niceness level is not applied to those daemons:

$ ps -flu frr
F S UID        PID  PPID  C PRI  NI ADDR SZ WCHAN  STIME TTY          TIME CMD
1 S frr       1532     1  0  80   0 - 127884 -     10:36 ?        00:00:08 /usr/lib/frr/zebra -d -F traditional -A 127.0.0.1 -s 90000000
1 S frr       1537     1  0  80   0 - 14204 -      10:36 ?        00:00:01 /usr/lib/frr/mgmtd -d -F traditional -A 127.0.0.1
5 S frr       1539     1  0  80   0 - 54114 -      10:36 ?        00:00:00 /usr/lib/frr/bgpd -d -F traditional -A 127.0.0.1
1 S frr       1545     1  0  80   0 - 12792 -      10:36 ?        00:00:00 /usr/lib/frr/staticd -d -F traditional -A 127.0.0.1
1 S frr       1548     1  0  80   0 - 12910 -      10:36 ?        00:00:01 /usr/lib/frr/bfdd -d -F traditional -A 127.0.0.1

What was the option to use "Nice=-5" under systemd unit file?
To assign high priority to all frr daemons?
To assign high priority to /usr/lib/frr/frrinit.sh script?
Something else?

Hope someone could help me to better understand that

Thanks,
Vladimir