cgroup2: add CpuMax conversion

Fix #2243

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

libcontainer/cgroups/utils.go

@@ -609,3 +609,18 @@ func ConvertCPUSharesToCgroupV2Value(cpuShares uint64) uint64 {
 	}
 	return (1 + ((cpuShares-2)*9999)/262142)
 }
+
+// ConvertCPUQuotaCPUPeriodToCgroupV2Value generates cpu.max string.
+func ConvertCPUQuotaCPUPeriodToCgroupV2Value(quota int64, period uint64) string {
+	if quota <= 0 && period == 0 {
+		return ""
+	}
+	if period == 0 {
+		// This default value is documented in https://www.kernel.org/doc/html/latest/admin-guide/cgroup-v2.html
+		period = 100000
+	}
+	if quota <= 0 {
+		return fmt.Sprintf("max %d", period)
+	}
+	return fmt.Sprintf("%d %d", quota, period)
+}