write back cpu.max.burst after cpu.max modified in cgroup v2

If the burst is not set, we need to read the current
value to write it back after `cpu.max` modified.

Signed-off-by: lifubang <lifubang@acmcoder.com>

libcontainer/cgroups/fs2/cpu.go

@@ -3,8 +3,10 @@ package fs2
 import (
 	"bufio"
 	"errors"
+	"fmt"
 	"os"
 	"strconv"
+	"strings"
 
 	"golang.org/x/sys/unix"
 
@@ -35,6 +37,22 @@ func setCpu(dirPath string, r *configs.Resources) error {
 		}
 	}
 
+	// If the burst is not set, we need to read the current
+	// value to write it back after `cpu.max` modified.
+	if (r.CpuQuota != 0 || r.CpuPeriod != 0) && r.CpuBurst == nil {
+		burst, _ := cgroups.ReadFile(dirPath, "cpu.max.burst")
+		if burst != "" {
+			v, err := strconv.ParseUint(strings.TrimSpace(burst), 10, 64)
+			if err != nil {
+				return fmt.Errorf("invalid value for cpu.max.burst: %w", err)
+			}
+			if v != 0 {
+				v *= 1000
+				r.CpuBurst = &v
+			}
+		}
+	}
+
 	var burst string
 	if r.CpuBurst != nil {
 		burst = strconv.FormatUint(*r.CpuBurst, 10)

libcontainer/cgroups/systemd/v2.go

@@ -486,6 +486,22 @@ func (m *UnifiedManager) Set(r *configs.Resources) error {
 		return err
 	}
 
+	// If the burst is not set, we need to read the current
+	// value to write it back after `cpu.max` modified.
+	if (r.CpuQuota != 0 || r.CpuPeriod != 0) && r.CpuBurst == nil {
+		burst, _ := cgroups.ReadFile(m.path, "cpu.max.burst")
+		if burst != "" {
+			v, err := strconv.ParseUint(strings.TrimSpace(burst), 10, 64)
+			if err != nil {
+				return fmt.Errorf("invalid value for cpu.max.burst: %w", err)
+			}
+			if v != 0 {
+				v *= 1000
+				r.CpuBurst = &v
+			}
+		}
+	}
+
 	if err := setUnitProperties(m.dbus, getUnitName(m.cgroups), properties...); err != nil {
 		return fmt.Errorf("unable to set unit properties: %w", err)
 	}