libct/cg/sd/v1: add SkipFreezeOnSet knob

This is helpful to kubernetes in cases it knows for sure that the freeze
is not required (since it created the systemd unit with no device
restrictions).

As the code is trivial, no tests are required.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 9a095e44db)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/v1.go

@@ -346,6 +346,11 @@ func (m *legacyManager) freezeBeforeSet(unitName string, r *configs.Resources) (
 	// Special case for SkipDevices, as used by Kubernetes to create pod
 	// cgroups with allow-all device policy).
 	if r.SkipDevices {
+		if r.SkipFreezeOnSet {
+			// Both needsFreeze and needsThaw are false.
+			return
+		}
+
 		// No need to freeze if SkipDevices is set, and either
 		// (1) systemd unit does not (yet) exist, or
 		// (2) it has DevicePolicy=auto and empty DeviceAllow list.

libcontainer/configs/cgroup_linux.go

@@ -131,4 +131,16 @@ type Resources struct {
 	//
 	// NOTE it is impossible to start a container which has this flag set.
 	SkipDevices bool `json:"-"`
+
+	// SkipFreezeOnSet is a flag for cgroup manager to skip the cgroup
+	// freeze when setting resources. Only applicable to systemd legacy
+	// (i.e. cgroup v1) manager (which uses freeze by default to avoid
+	// spurious permission errors caused by systemd inability to update
+	// device rules in a non-disruptive manner).
+	//
+	// If not set, a few methods (such as looking into cgroup's
+	// devices.list and querying the systemd unit properties) are used
+	// during Set() to figure out whether the freeze is required. Those
+	// methods may be relatively slow, thus this flag.
+	SkipFreezeOnSet bool `json:"-"`
 }