libct/cg/fs: don't write cpu_burst twice on ENOENT

If CPU burst knob is non-existent, the current implementation (added in
commit e158483) still tries to set it again after setting the new CPU
quota, which is useless (and we have to ignore ENOENT again).

Fix this.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/fs/cpu.go

@@ -89,9 +89,11 @@ func (s *CpuGroup) Set(path string, r *configs.Resources) error {
 	if r.CpuBurst != nil {
 		burst = strconv.FormatUint(*r.CpuBurst, 10)
 		if err := cgroups.WriteFile(path, "cpu.cfs_burst_us", burst); err != nil {
-			// this is a special trick for burst feature, the current systemd and low version of kernel will not support it.
-			// So, an `no such file or directory` error would be raised, and we can ignore it .
-			if !errors.Is(err, unix.ENOENT) {
+			if errors.Is(err, unix.ENOENT) {
+				// If CPU burst knob is not available (e.g.
+				// older kernel), ignore it.
+				burst = ""
+			} else {
 				// Sometimes when the burst to be set is larger
 				// than the current one, it is rejected by the kernel
 				// (EINVAL) as old_quota/new_burst exceeds the parent
@@ -117,9 +119,7 @@ func (s *CpuGroup) Set(path string, r *configs.Resources) error {
 		}
 		if burst != "" {
 			if err := cgroups.WriteFile(path, "cpu.cfs_burst_us", burst); err != nil {
-				if !errors.Is(err, unix.ENOENT) {
-					return err
-				}
+				return err
 			}
 		}
 	}