libct/cg: GetAllPids: optimize for go 1.16+

filepath.WalkDir function, introduced in Go 1.16, doesn't do stat(2)
on every entry, and is therefore somewhat faster (see below).

Since we have to support Go 1.15, keep the old version for backward
compatibility.

Add a quick benchmark, which shows approximately 3x improvement:

        $ go1.15.15 test -bench AllPid -run xxx .
	BenchmarkGetAllPids-4   	      48	  23528839 ns/op

        $ go version
        go version go1.16.6 linux/amd64
        $ go test -bench AllPid -run xxx .
	BenchmarkGetAllPids-4   	     147	   7700170 ns/op

(Unrelated but worth noting -- go 1.17rc2 is pushing it even further)

        $ go1.17rc2 test -bench AllPid -run xxx .
	BenchmarkGetAllPids-4   	     164	   6820994 ns/op

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

libcontainer/cgroups/getallpids.go

@@ -1,22 +1,21 @@
-// +build linux
+// +build linux,go1.16
 
 package cgroups
 
 import (
-	"os"
+	"io/fs"
 	"path/filepath"
 )
 
-// GetAllPids returns all pids, that were added to cgroup at path and to all its
-// subcgroups.
+// GetAllPids returns all pids from the cgroup identified by path, and all its
+// sub-cgroups.
 func GetAllPids(path string) ([]int, error) {
 	var pids []int
-	// collect pids from all sub-cgroups
-	err := filepath.Walk(path, func(p string, info os.FileInfo, iErr error) error {
+	err := filepath.WalkDir(path, func(p string, d fs.DirEntry, iErr error) error {
 		if iErr != nil {
 			return iErr
 		}
-		if !info.IsDir() {
+		if !d.IsDir() {
 			return nil
 		}
 		cPids, err := readProcsFile(p)

libcontainer/cgroups/getallpids_go115.go

@@ -0,0 +1,30 @@
+// +build linux,!go1.16
+
+package cgroups
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// GetAllPids returns all pids, that were added to cgroup at path and to all its
+// subcgroups.
+func GetAllPids(path string) ([]int, error) {
+	var pids []int
+	// collect pids from all sub-cgroups
+	err := filepath.Walk(path, func(p string, info os.FileInfo, iErr error) error {
+		if iErr != nil {
+			return iErr
+		}
+		if !info.IsDir() {
+			return nil
+		}
+		cPids, err := readProcsFile(p)
+		if err != nil {
+			return err
+		}
+		pids = append(pids, cPids...)
+		return nil
+	})
+	return pids, err
+}

libcontainer/cgroups/getallpids_test.go

@@ -0,0 +1,19 @@
+// +build linux
+
+package cgroups
+
+import (
+	"testing"
+)
+
+func BenchmarkGetAllPids(b *testing.B) {
+	total := 0
+	for i := 0; i < b.N; i++ {
+		i, err := GetAllPids("/sys/fs/cgroup")
+		if err != nil {
+			b.Fatal(err)
+		}
+		total += len(i)
+	}
+	b.Logf("iter: %d, total: %d", b.N, total)
+}