Improve performance by for pid stats (cgroups1) re-using readuint

Benchstat results

➜  cgroups git:(main) ✗ benchstat old.txt new.txt
goos: linux
goarch: amd64
pkg: github.com/containerd/cgroups/v3/cgroup1
cpu: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
           │   old.txt   │              new.txt               │
           │   sec/op    │   sec/op     vs base               │
TestPids-8   19.32µ ± 4%   17.39µ ± 2%  -9.97% (p=0.000 n=10)

           │   old.txt   │              new.txt               │
           │    B/op     │    B/op     vs base                │
TestPids-8   1344.0 ± 0%   624.0 ± 0%  -53.57% (p=0.000 n=10)

           │  old.txt   │              new.txt               │
           │ allocs/op  │ allocs/op   vs base                │
TestPids-8   13.00 ± 0%   11.00 ± 0%  -15.38% (p=0.000 n=10)
➜  cgroups git:(main) ✗

Signed-off-by: Manu Gupta <manugupt1@gmail.com>

cgroup1/pids.go

@@ -20,7 +20,6 @@ import (
 	"os"
 	"path/filepath"
 	"strconv"
-	"strings"
 
 	v1 "github.com/containerd/cgroups/v3/cgroup1/stats"
 	specs "github.com/opencontainers/runtime-spec/specs-go"
@@ -67,16 +66,10 @@ func (p *pidsController) Stat(path string, stats *v1.Metrics) error {
 	if err != nil {
 		return err
 	}
-	var max uint64
-	maxData, err := os.ReadFile(filepath.Join(p.Path(path), "pids.max"))
+	max, err := readUint(filepath.Join(p.Path(path), "pids.max"))
 	if err != nil {
 		return err
 	}
-	if maxS := strings.TrimSpace(string(maxData)); maxS != "max" {
-		if max, err = parseUint(maxS, 10, 64); err != nil {
-			return err
-		}
-	}
 	stats.Pids = &v1.PidsStat{
 		Current: current,
 		Limit:   max,

cgroup1/pids_test.go

@@ -186,3 +186,47 @@ func TestPidsOverflowMax(t *testing.T) {
 		t.Fatal("expected not nil err")
 	}
 }
+
+func BenchmarkTestPids(b *testing.B) {
+
+	mock, err := newMock(b)
+	if err != nil {
+		b.Fatal(err)
+	}
+	defer func() {
+		if err := mock.delete(); err != nil {
+			b.Errorf("failed delete: %v", err)
+		}
+	}()
+
+	pids := NewPids(mock.root)
+	if pids == nil {
+		b.Fatal("pids is nil")
+	}
+	resources := specs.LinuxResources{}
+	resources.Pids = &specs.LinuxPids{}
+	resources.Pids.Limit = int64(10)
+	err = pids.Create("test", &resources)
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	current := filepath.Join(mock.root, "pids", "test", "pids.current")
+	if err = os.WriteFile(
+		current,
+		[]byte(strconv.Itoa(5)),
+		defaultFilePerm,
+	); err != nil {
+		b.Fatal(err)
+	}
+
+	b.ReportAllocs()
+
+	for i := 0; i < b.N; i++ {
+		metrics := v1.Metrics{}
+		err = pids.Stat("test", &metrics)
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}

cgroup1/utils.go

@@ -137,7 +137,7 @@ func readUint(path string) (uint64, error) {
 	}
 	defer f.Close()
 
-	b := make([]byte, 128) // Chose 128 as some files have leading/trailing whitespaces and alignment
+	b := make([]byte, 32) // Chose 32 as some files have leading/trailing whitespaces and alignment
 	n, err := f.Read(b)
 	if err != nil {
 		return 0, err