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2.10. Generating the /etc/cgconfig.conf File

Configuration for the /etc/cgconfig.conf file can be generated from the current cgroup configuration using the cgsnapshot utility. This utility takes a snapshot of the current state of all subsystems and their cgroups and returns their configuration as it would appear in the /etc/cgconfig.conf file. Example 2.6, “Using the cgsnapshot utility” shows an example usage of the cgsnapshot utility.

Example 2.6. Using the cgsnapshot utility

Configure cgroups on the system using the following commands: 
~]# mkdir /cgroup/cpu
~]# mount -t cgroup -o cpu cpu /cgroup/cpu
~]# mkdir /cgroup/cpu/lab1
~]# mkdir /cgroup/cpu/lab2
~]# echo 2 > /cgroup/cpu/lab1/cpu.shares
~]# echo 3 > /cgroup/cpu/lab2/cpu.shares
~]# echo 5000000 > /cgroup/cpu/lab1/cpu.rt_period_us
~]# echo 4000000 > /cgroup/cpu/lab1/cpu.rt_runtime_us
~]# mkdir /cgroup/cpuacct
~]# mount -t cgroup -o cpuacct cpuacct /cgroup/cpuacct
The above commands mounted two subsystems and created two cgroups, for the cpu subsystem, with specific values for some of their parameters. Executing the cgsnapshot command (with the -s option and an empty /etc/cgsnapshot_blacklist.conf file[4]) then produces the following output: 
~]$ cgsnapshot -s
# Configuration file generated by cgsnapshot
mount {
        cpu = /cgroup/cpu;
        cpuacct = /cgroup/cpuacct;
}

group lab2 {
        cpu {
                cpu.rt_period_us="1000000";
                cpu.rt_runtime_us="0";
                cpu.shares="3";
        }
}

group lab1 {
        cpu {
                cpu.rt_period_us="5000000";
                cpu.rt_runtime_us="4000000";
                cpu.shares="2";
        }
}
The -s option used in the example above tells cgsnapshot to ignore all warnings in the output file caused by parameters not being defined in the blacklist or whitelist of the cgsnapshot utility. For more information on parameter blacklisting, refer to Section 2.10.1, “Blacklisting Parameters”. For more information on parameter whitelisting, refer to Section 2.10.2, “Whitelisting Parameters”.
By default, the output generated by cgsnapshot is returned on the standard output. Use the -f option to specify a file to which the output should be redirected. For example: 
~]$ cgsnapshot -f ~/test/cgconfig_test.conf

Warning

When using the -f option, note that it overwrites any content in the file you specify. Therefore, it is recommended not to direct the output straight to the /etc/cgconfig.conf file.
The cgsnapshot utility can also create configuration files per subsystem. By specifying the name of a subsystem, the output will consist of the corresponding configuration for that subsystem: 
~]$ cgsnapshot cpuacct
# Configuration file generated by cgsnapshot
mount {
        cpuacct = /cgroup/cpuacct;
}

2.10.1. Blacklisting Parameters

The cgsnapshot utility allows parameter blacklisting. If a parameter is blacklisted, it does not appear in the output generated by cgsnapshot. By default, the /etc/cgsnapshot_blacklist.conf file is checked for blacklisted parameters. If a parameter is not present in the blacklist, the whitelist is checked. To specify a different blacklist, use the -b option. For example: 
~]$ cgsnapshot -b ~/test/my_blacklist.conf

[4] The cpu.shares parameter is specified in the /etc/cgsnapshot_blacklist.conf file by default, which would cause it to be omitted in the generated output in Example 2.6, “Using the cgsnapshot utility”. Thus, for the purposes of the example, an empty /etc/cgsnapshot_blacklist.conf file is used.