Make more cgroup controllers optional

Mark all noop cgroups as optional, after all there is no harm in skipping them.
Make pids cgroup optional because they are not present on Synology NAS.

Closes #6856

PiperOrigin-RevId: 409197613

runsc/cgroup/cgroup.go

@@ -57,7 +57,7 @@ var controllers = map[string]controller{
 	"devices":    &noop{},
 	"freezer":    &noop{},
 	"perf_event": &noop{},
-	"rdma":       &noop{isOptional: true},
+	"rdma":       &noop{},
 	"systemd":    &noop{},
 }
 
@@ -599,12 +599,10 @@ type controller interface {
 	skip(*specs.LinuxResources) error
 }
 
-type noop struct {
-	isOptional bool
-}
+type noop struct{}
 
 func (n *noop) optional() bool {
-	return n.isOptional
+	return true
 }
 
 func (*noop) set(*specs.LinuxResources, string) error {
@@ -612,9 +610,6 @@ func (*noop) set(*specs.LinuxResources, string) error {
 }
 
 func (n *noop) skip(*specs.LinuxResources) error {
-	if !n.isOptional {
-		panic("cgroup controller is not optional")
-	}
 	return nil
 }
 
@@ -808,8 +803,17 @@ func (*networkPrio) skip(spec *specs.LinuxResources) error {
 	return nil
 }
 
-type pids struct {
-	mandatory
+type pids struct{}
+
+func (*pids) optional() bool {
+	return true
+}
+
+func (*pids) skip(spec *specs.LinuxResources) error {
+	if spec != nil && spec.Pids != nil && spec.Pids.Limit > 0 {
+		return fmt.Errorf("Pids.Limit set but pids cgroup controller not found")
+	}
+	return nil
 }
 
 func (*pids) set(spec *specs.LinuxResources, path string) error {

runsc/cgroup/cgroup_test.go

@@ -863,6 +863,12 @@ func TestOptional(t *testing.T) {
 		spec  *specs.LinuxResources
 		err   string
 	}{
+		{
+			name:  "pids",
+			ctrlr: &pids{},
+			spec:  &specs.LinuxResources{Pids: &specs.LinuxPids{Limit: 1}},
+			err:   "Pids.Limit set but pids cgroup controller not found",
+		},
 		{
 			name:  "net-cls",
 			ctrlr: &networkClass{},