merge branch 'pr-2791'

Kir Kolyshkin (1):
  cgroupv1 freezer: thaw to increase freeze chances

LGTMs: @cyphar @dqminh
Closes #2791

libcontainer/cgroups/fs/freezer.go

@@ -12,6 +12,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/cgroups/fscommon"
 	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
 
@@ -38,13 +39,19 @@ func (s *FreezerGroup) Set(path string, cgroup *configs.Cgroup) error {
 		// (either via fork/clone or by writing new PIDs to
 		// cgroup.procs).
 		//
-		// The number of retries below is chosen to have a decent
-		// chance to succeed even in the worst case scenario (runc
-		// pause/unpause with parallel runc exec).
+		// The numbers below are chosen to have a decent chance to
+		// succeed even in the worst case scenario (runc pause/unpause
+		// with parallel runc exec).
 		//
 		// Adding any amount of sleep in between retries did not
 		// increase the chances of successful freeze.
 		for i := 0; i < 1000; i++ {
+			if i%50 == 49 {
+				// Briefly thawing the cgroup also helps.
+				_ = fscommon.WriteFile(path, "freezer.state", string(configs.Thawed))
+				time.Sleep(10 * time.Millisecond)
+			}
+
 			if err := fscommon.WriteFile(path, "freezer.state", string(configs.Frozen)); err != nil {
 				return err
 			}
@@ -58,6 +65,9 @@ func (s *FreezerGroup) Set(path string, cgroup *configs.Cgroup) error {
 			case "FREEZING":
 				continue
 			case string(configs.Frozen):
+				if i > 1 {
+					logrus.Debugf("frozen after %d retries", i)
+				}
 				return nil
 			default:
 				// should never happen