Cannot set thread priority to real time despite using cap_sys_nice

Question

I have an application that checks on POSIX environment whether thread priorities can be set to real time by calling

struct sched_param param;
param.sched_priority = 1;
int canSetRealTimeThreadPriority = (pthread_setschedparam(pthread_self(), SCHED_FIFO, &param) == 0);

On one system system A this works, but on another system B the check fails and I would like to find out why.

On both systems:

• the application is started as a systemd service via a service startup script.

• calling getcap on the binary returns among others cap_sys_nice+eip.

• the service script defines that the application is run by a non root user via User=[non root user]

• the service scripts sets LimitRTPRIO=20

• calling sysctl -n kernel.sched_rt_runtime_us returns 950000, which should be the default

• calling sysctl -n kernel.sched_rt_period_usreturns 1000000, which should be the default

• systemctl show [serviceName] returns among others LimitRTPRIO=20

• calling the limits on the running process of the application (prlimit --pid [application_pid]) will show among others:

RESOURCE   DESCRIPTION                             SOFT      HARD UNITS
NICE       max nice prio allowed to raise             0         0
RTPRIO     max real-time priority                    20        20
RTTIME     timeout for real-time tasks        unlimited unlimited microsecs

On system B where it doesn't allow real time thread priorities:

• etc/security/limits.conf contains line

[non root user]    -    rtprio    20

• kernel version is 3.10.0-862.el7.x86_64 and OS version Red Hat Enterprise Linux Server release 7.4 (Maipo)

On system A where realtime thread priorities can be set:

• kernel version is 3.10.0-957.56.1.el7.x86_64 and OS version Red Hat Enterprise Linux Server release 7.6 (Maipo)

When I test on system A and remove cap_sys_nice+eip from the binary via setcap '' [binary] I also cannot set real time thread priorities. I assume some setting on system B overrides the cap_sys_nice setting because it has a higher priority, so I wonder what that can be.

Answer 1

It turned out the reason why realtime thread priorities could not be set system wide was another service running, which had following settings in service script defined:

CPUShares=20
CPUQuota=500%

After removing these settings from the script and restarting the service, thread priorities could be set system wide again.