[elastic-agent] support resource limitations on child processes about elastic-agent HOT 10 OPEN

import (
	"fmt"
	"os"
	"os/exec"

	"github.com/containerd/cgroups"
	specs "github.com/opencontainers/runtime-spec/specs-go"
)

// POC for working with v1 cgroups
// demonstrating how a process can be assigned to a cgroup and
// assign subprocesses to subcgroups with resource limitations.
//
// Example usage for runing on docker:
// docker build . -t go-cgroup
// time docker run -v /sys/fs/cgroup:/sys/fs/cgroup:rw go-cgroup
// The time command shows that the program needs significantly more
// time to finish when the subprocess resources are limited
func main() {
	// create a new cgroup and add the current process to it
	mainPid := os.Getpid()
	q := int64(9000)
	p := uint64(10000)
	resources := &specs.LinuxResources{CPU: &specs.LinuxCPU{Quota: &q, Period: &p}}
	mainCgroup, err := cgroups.New(cgroups.V1, cgroups.StaticPath(fmt.Sprintf("%v", mainPid)), resources)
	if err != nil {
		panic(err)
	}
	if err := mainCgroup.Add(cgroups.Process{Pid: mainPid}); err != nil {
		panic(err)
	}

	// create subprocess
	// run a script that creates some CPU load, e.g. fibonacci calculation
	cmd := exec.Cmd{Path: "./fibonacci.sh", Stdout: os.Stdout, Stderr: os.Stdout}
	cmd.Start()
	defer cmd.Wait()
	if cmd.Process == nil {
		panic("subprocess not successfully started")
	}
	childPid := cmd.Process.Pid

	// add subprocess to a new child cgroup
	// change the quota to period ratio for verifying that the CPU quotas applied to
	// the cgroup are limiting the processing of the script.
	// decreasing the quota -> increases run time
	q = int64(1000)
	p = uint64(10000)
	resources = &specs.LinuxResources{CPU: &specs.LinuxCPU{Quota: &q, Period: &p}}
	childCgroup, err := mainCgroup.New("childgroup", resources)
	if err != nil {
		panic(err)
	}
	if err := childCgroup.Add(cgroups.Process{Pid: childPid}); err != nil {
		panic(err)
	}
	listProcesses("main", mainCgroup)
	listProcesses("child", childCgroup)
	printStats(fmt.Sprint(mainPid), mainCgroup)
	printStats(fmt.Sprint(childPid), mainCgroup)
}

func listProcesses(name string, cg cgroups.Cgroup) {
	processes, err := cg.Processes(cgroups.Pids, true)
	if err != nil {
		panic(err)
	}
	fmt.Println(fmt.Sprintf("Processes in %s cgroup", name))
	for _, p := range processes {
		fmt.Println(fmt.Sprintf("Pid: %v", p.Pid))
	}
}

func printStats(name string, cg cgroups.Cgroup) {
	stats, err := cg.Stat()
	if err == nil {
		fmt.Println(fmt.Sprintf("CPU usage for %s: %v: %v", name, stats.CPU.Usage.Total, stats.CPU.Usage.User))
	} else {
		panic(err)
	}
}

systemctl set-property elastic-agent.service MemoryLimit=50G

systemctl status elastic-agent.service
● elastic-agent.service - Elastic Agent is a unified agent to observe, monitor and protect your system.
     Loaded: loaded (/etc/systemd/system/elastic-agent.service; enabled; vendor preset: enabled)
    Drop-In: /etc/systemd/system.control/elastic-agent.service.d
             └─50-MemoryLimit.conf
     Active: active (running) since Tue 2022-05-24 21:00:20 CEST; 18h ago
   Main PID: 2756710 (elastic-agent)
      Tasks: 587 (limit: 618877)
     Memory: 724.9M (limit: 50.0G)
     CGroup: /system.slice/elastic-agent.service
             ├─2756710 /opt/Elastic/Agent/elastic-agent
             ├─2842320 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/filebeat-8.2.0-linux-x86_64/filebeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E gc_percent=${FILEBEAT_GOG>
             ├─2842351 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/metricbeat-8.2.0-linux-x86_64/metricbeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E gc_percent=${METRICBE>
             ├─2842651 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/osquerybeat-8.2.0-linux-x86_64/osquerybeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E logging.level=error>
             ├─2842677 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/filebeat-8.2.0-linux-x86_64/filebeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E gc_percent=${FILEBEAT_GOG>
             ├─2842706 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/metricbeat-8.2.0-linux-x86_64/metricbeat -E setup.ilm.enabled=false -E setup.template.enabled=false -E management.enabled=true -E logging.level=debug -E gc_percent=${METRICBE>
             ├─2842840 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/osquerybeat-8.2.0-linux-x86_64/osqueryd --flagfile=osquery/osquery.flags --pack_delimiter=_ --extensions_socket=/var/run/120651786/osquery.sock --database_path=osquery/osquer>
             └─2842901 /opt/Elastic/Agent/data/elastic-agent-b9a28a/install/osquerybeat-8.2.0-linux-x86_64/osquery-extension.ext --socket /var/run/120651786/osquery.sock --timeout 10 --interval 3