support cgroup2

* only shim v2 runc v2 ("io.containerd.runc.v2") is supported
* only PID metrics is implemented. Others should be implemented in separate PRs.
* lots of code duplication in v1 metrics and v2 metrics. Dedupe should be separate PR.

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

vendor/github.com/cilium/ebpf/internal/cpu.go

@@ -0,0 +1,64 @@
+package internal
+
+import (
+	"fmt"
+	"os"
+	"sync"
+
+	"github.com/pkg/errors"
+)
+
+var sysCPU struct {
+	once sync.Once
+	err  error
+	num  int
+}
+
+// PossibleCPUs returns the max number of CPUs a system may possibly have
+// Logical CPU numbers must be of the form 0-n
+func PossibleCPUs() (int, error) {
+	sysCPU.once.Do(func() {
+		sysCPU.num, sysCPU.err = parseCPUs("/sys/devices/system/cpu/possible")
+	})
+
+	return sysCPU.num, sysCPU.err
+}
+
+var onlineCPU struct {
+	once sync.Once
+	err  error
+	num  int
+}
+
+// OnlineCPUs returns the number of currently online CPUs
+// Logical CPU numbers must be of the form 0-n
+func OnlineCPUs() (int, error) {
+	onlineCPU.once.Do(func() {
+		onlineCPU.num, onlineCPU.err = parseCPUs("/sys/devices/system/cpu/online")
+	})
+
+	return onlineCPU.num, onlineCPU.err
+}
+
+// parseCPUs parses the number of cpus from sysfs,
+// in the format of "/sys/devices/system/cpu/{possible,online,..}.
+// Logical CPU numbers must be of the form 0-n
+func parseCPUs(path string) (int, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return 0, err
+	}
+	defer file.Close()
+
+	var low, high int
+	n, _ := fmt.Fscanf(file, "%d-%d", &low, &high)
+	if n < 1 || low != 0 {
+		return 0, errors.Wrapf(err, "%s has unknown format", path)
+	}
+	if n == 1 {
+		high = low
+	}
+
+	// cpus is 0 indexed
+	return high + 1, nil
+}