cgroup2: cpu shares: follow the behavior of runc v1.3.2

cgroup2/utils.go

@@ -146,6 +146,35 @@ func parseCgroupFromReader(r io.Reader) (string, error) {
 	return "", fmt.Errorf("cgroup path not found")
 }
 
+// ConvertCPUSharesToCgroupV2Value converts CPU shares, used by cgroup v1,
+// to CPU weight, used by cgroup v2.
+//
+// Cgroup v1 CPU shares has a range of [2^1...2^18], i.e. [2...262144],
+// and the default value is 1024.
+//
+// Cgroup v2 CPU weight has a range of [10^0...10^4], i.e. [1...10000],
+// and the default value is 100.
+//
+// Taken from https://github.com/opencontainers/cgroups/blob/v0.0.5/utils.go#L417-L441
+// (Apache License 2.0)
+func ConvertCPUSharesToCgroupV2Value(cpuShares uint64) uint64 {
+	// The value of 0 means "unset".
+	if cpuShares == 0 {
+		return 0
+	}
+	if cpuShares <= 2 {
+		return 1
+	}
+	if cpuShares >= 262144 {
+		return 10000
+	}
+	l := math.Log2(float64(cpuShares))
+	// Quadratic function which fits min, max, and default.
+	exponent := (l*l+125*l)/612.0 - 7.0/34.0
+
+	return uint64(math.Ceil(math.Pow(10, exponent)))
+}
+
 // ToResources converts the oci LinuxResources struct into a
 // v2 Resources type for use with this package.
 //
@@ -159,7 +188,7 @@ func ToResources(spec *specs.LinuxResources) *Resources {
 			Mems: cpu.Mems,
 		}
 		if shares := cpu.Shares; shares != nil {
-			convertedWeight := 1 + ((*shares-2)*9999)/262142
+			convertedWeight := ConvertCPUSharesToCgroupV2Value(*shares)
 			resources.CPU.Weight = &convertedWeight
 		}
 		if period := cpu.Period; period != nil {

cgroup2/utils_test.go

@@ -75,6 +75,38 @@ full avg10=1.00 avg60=1.01 avg300=1.00 total=157622356`
 	assert.Equal(t, &st.Full, &expected.Full)
 }
 
+// TestConvertCPUSharesToCgroupV2Value tests the ConvertCPUSharesToCgroupV2Value function.
+// Taken from https://github.com/opencontainers/cgroups/blob/v0.0.5/utils_test.go#L537-L564
+// (Apache License 2.0)
+func TestConvertCPUSharesToCgroupV2Value(t *testing.T) {
+	const (
+		sharesMin = 2
+		sharesMax = 262144
+		sharesDef = 1024
+		weightMin = 1
+		weightMax = 10000
+		weightDef = 100
+		unset     = 0
+	)
+	cases := map[uint64]uint64{
+		unset: unset,
+
+		sharesMin - 1: weightMin,     // Below the minimum (out of range).
+		sharesMin:     weightMin,     // Minimum.
+		sharesMin + 1: weightMin + 1, // Just above the minimum.
+		sharesDef:     weightDef,     // Default.
+		sharesMax - 1: weightMax,     // Just below the maximum.
+		sharesMax:     weightMax,     // Maximum.
+		sharesMax + 1: weightMax,     // Above the maximum (out of range).
+	}
+	for shares, want := range cases {
+		got := ConvertCPUSharesToCgroupV2Value(shares)
+		if got != want {
+			t.Errorf("ConvertCPUSharesToCgroupV2Value(%d): got %d, want %d", shares, got, want)
+		}
+	}
+}
+
 func TestToResources(t *testing.T) {
 	var (
 		quota  int64  = 8000
@@ -84,7 +116,7 @@ func TestToResources(t *testing.T) {
 		mem  int64 = 300
 		swap int64 = 500
 	)
-	weight := 1 + ((shares-2)*9999)/262142
+	weight := ConvertCPUSharesToCgroupV2Value(shares)
 	res := specs.LinuxResources{
 		CPU:    &specs.LinuxCPU{Quota: &quota, Period: &period, Shares: &shares},
 		Memory: &specs.LinuxMemory{Limit: &mem, Swap: &swap},