metric/system/cgroup: add cache for v2 entries

As multiple PIDs can use the same v2 cgroup, cache the result to reduce CPU usage and number of syscalls.

metric/system/cgroup/reader.go

@@ -23,6 +23,7 @@ import (
 	"path/filepath"
 	"strconv"
 	"strings"
+	"sync"
 
 	"github.com/elastic/elastic-agent-libs/logp"
 	"github.com/elastic/elastic-agent-system-metrics/metric/system/cgroup/cgv1"
@@ -68,7 +69,7 @@ const (
 	memoryStat  = "memory"
 )
 
-//nolint: deadcode,structcheck,unused // needed by other platforms
+// nolint: deadcode,structcheck,unused // needed by other platforms
 type mount struct {
 	subsystem  string // Subsystem name (e.g. cpuacct).
 	mountpoint string // Mountpoint of the subsystem (e.g. /cgroup/cpuacct).
@@ -85,6 +86,9 @@ type Reader struct {
 	ignoreRootCgroups        bool // Ignore a cgroup when its path is "/".
 	cgroupsHierarchyOverride string
 	cgroupMountpoints        Mountpoints // Mountpoints for each subsystem (e.g. cpu, cpuacct, memory, blkio).
+
+	// Cache to map known v2 cgroup controllerPaths to PathList.
+	v2ControllerPathCache sync.Map
 }
 
 // ReaderOptions holds options for NewReaderOptions.

metric/system/cgroup/util.go

@@ -255,6 +255,7 @@ func (r Reader) ProcessCgroupPaths(pid int) (PathList, error) {
 		if r.cgroupsHierarchyOverride != "" {
 			path = r.cgroupsHierarchyOverride
 		}
+
 		// cgroup V2
 		// cgroup v2 controllers will always start with this string
 		if strings.Contains(line, "0::/") {
@@ -275,6 +276,14 @@ the container as /sys/fs/cgroup/unified and start the system module with the hos
 				controllerPath = r.rootfsMountpoint.ResolveHostFS(filepath.Join("/sys/fs/cgroup/unified", path))
 			}
 
+			if tmp, ok := r.v2ControllerPathCache.Load(controllerPath); ok {
+				pathList, ok := tmp.(PathList)
+				if ok {
+					cPaths.V2 = pathList.V2
+					continue
+				}
+			}
+
 			cgpaths, err := os.ReadDir(controllerPath)
 			if err != nil {
 				return cPaths, fmt.Errorf("error fetching cgroupV2 controllers for cgroup location '%s' and path line '%s': %w", r.cgroupMountpoints.V2Loc, line, err)
@@ -287,6 +296,7 @@ the container as /sys/fs/cgroup/unified and start the system module with the hos
 					cPaths.V2[controllerName] = ControllerPath{ControllerPath: path, FullPath: controllerPath, IsV2: true}
 				}
 			}
+			r.v2ControllerPathCache.Store(controllerPath, cPaths)
 			// cgroup v1
 		} else {
 			subsystems := strings.Split(fields[1], ",")