freezer for cgroup v2

freezer for cgroup v2

This patchset implements freezer for cgroup v2.

It provides similar functionality as v1 freezer, but the interface conforms to the cgroup v2 interface design principles, and it provides a better user experience: tasks can be killed, ptrace works, there is no separate controller, which has to be enabled, etc.

Patches (1), (2) and (3) are some preparational work, patch (4) contains the implementation, patch (5) is a small cgroup kselftest fix, patch (6) covers freezer adds 6 new kselftests to cover the freezer functionality. Patch (7) adds corresponding docs.

v4->v3:
- fixed cgroup state transitions on task migration (by Tejun Heo)
- reading nr_descendants doesn't require taking css_set_lock anymore
- fixed docs based on Mike Rapoport's feedback
- fixed double irq lock found by Dan Carpenter

v3->v2:
- dropped TASK_FROZEN for now, frozen tasks are put into TASK_INTERRUPTIBLE state; it's probably not the final version, but the API question can be discussed separately
- don't clear TIF_SIGPENDING before going to sleep, instead add task->frozen check in signal_pending_state() and recalc_sigpending()
- cgroup-level counter are now synchronized using css_set_lock, which simplified the whole code (e.g. per-cgroup works were removed)
- the amount of comments increased significantly
- many other improvements incorporating feedback from Tejun and Oleg

v2->v1:
- fixed locking aroung calling cgroup_freezer_leave()
- added docs

Roman Gushchin (7):
cgroup: rename freezer.c into legacy_freezer.c
cgroup: implement __cgroup_task_count() helper
cgroup: protect cgroup->nr_(dying_)descendants by css_set_lock
cgroup: cgroup v2 freezer
kselftests: cgroup: don't fail on cg_kill_all() error in cg_destroy()
kselftests: cgroup: add freezer controller self-tests
cgroup: document cgroup v2 freezer interface

 Documentation/admin-guide/cgroup-v2.rst       |  27 +
 include/linux/cgroup-defs.h                   |  31 +
 include/linux/cgroup.h                        |  42 ++
 include/linux/sched.h                         |   2 +
 include/linux/sched/jobctl.h                  |   2 +
 include/linux/sched/signal.h                  |   3 +
 kernel/cgroup/Makefile                        |   4 +-
 kernel/cgroup/cgroup-internal.h               |   1 +
 kernel/cgroup/cgroup-v1.c                     |  16 -
 kernel/cgroup/cgroup.c                        | 151 +++-
 kernel/cgroup/freezer.c                       | 653 +++++++----------
 kernel/cgroup/legacy_freezer.c                | 481 ++++++++++++
 kernel/ptrace.c                               |   7 +
 kernel/signal.c                               |  58 +-
 tools/testing/selftests/cgroup/.gitignore     |   1 +
 tools/testing/selftests/cgroup/Makefile       |   2 +
 tools/testing/selftests/cgroup/cgroup_util.c  |  85 ++-
 tools/testing/selftests/cgroup/cgroup_util.h  |   7 +
 tools/testing/selftests/cgroup/test_freezer.c | 685 ++++++++++++++++++
 19 files changed, 1811 insertions(+), 447 deletions(-)
 create mode 100644 kernel/cgroup/legacy_freezer.c
 create mode 100644 tools/testing/selftests/cgroup/test_freezer.c

2.17.2

Freezer.c will contain an implementation of cgroup v2 freezer, so let's rename the v1 freezer to avoid naming conflicts.

---
 kernel/cgroup/Makefile                        | 2 +-
 kernel/cgroup/{freezer.c => legacy_freezer.c} | 0
 2 files changed, 1 insertion(+), 1 deletion(-)
 rename kernel/cgroup/{freezer.c => legacy_freezer.c} (100%)

index bfcdae896122..8d5689ca94b9 100644
@@ -1,7 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-y := cgroup.o rstat.o namespace.o cgroup-v1.o
 
-obj-$(CONFIG_CGROUP_FREEZER) += freezer.o
+obj-$(CONFIG_CGROUP_FREEZER) += legacy_freezer.o
 obj-$(CONFIG_CGROUP_PIDS) += pids.o
 obj-$(CONFIG_CGROUP_RDMA) += rdma.o
 obj-$(CONFIG_CPUSETS) += cpuset.o

similarity index 100%
rename from kernel/cgroup/freezer.c
rename to kernel/cgroup/legacy_freezer.c

2.17.2

The helper is identical to the existing cgroup_task_count() except it doesn't take the css_set_lock by itself, assuming that the caller does.

Also, move cgroup_task_count() implementation into kernel/cgroup/cgroup.c, as there is nothing specific to cgroup v1.

---
 kernel/cgroup/cgroup-internal.h |  1 +
 kernel/cgroup/cgroup-v1.c       | 16 ----------------
 kernel/cgroup/cgroup.c          | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 16 deletions(-)

index c950864016e2..a195328431ce 100644
@@ -226,6 +226,7 @@ int cgroup_rmdir(struct kernfs_node *kn);
 int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
 		     struct kernfs_root *kf_root);
 
+int __cgroup_task_count(const struct cgroup *cgrp);
 int cgroup_task_count(const struct cgroup *cgrp);
 
 /*

index 51063e7a93c2..6134fef07d57 100644
@@ -336,22 +336,6 @@ static struct cgroup_pidlist *cgroup_pidlist_find_create(struct cgroup *cgrp,
 	return l;
 }
 
-/**
- * cgroup_task_count - count the number of tasks in a cgroup.
- * @cgrp: the cgroup in question
- */
-int cgroup_task_count(const struct cgroup *cgrp)
-{
-	int count = 0;
-	struct cgrp_cset_link *link;
-
-	spin_lock_irq(&css_set_lock);
-	list_for_each_entry(link, &cgrp->cset_links, cset_link)
-		count += link->cset->nr_tasks;
-	spin_unlock_irq(&css_set_lock);
-	return count;
-}
-
 /*
  * Load a cgroup's pidarray with either procs' tgids or tasks' pids
  */

index e06994fd4e34..7519a4307021 100644
@@ -563,6 +563,39 @@ static void cgroup_get_live(struct cgroup *cgrp)
 	css_get(&cgrp->self);
 }
 
+/**
+ * __cgroup_task_count - count the number of tasks in a cgroup. The caller
+ * is responsible for taking the css_set_lock.
+ * @cgrp: the cgroup in question
+ */
+int __cgroup_task_count(const struct cgroup *cgrp)
+{
+	int count = 0;
+	struct cgrp_cset_link *link;
+
+	lockdep_assert_held(&css_set_lock);
+
+	list_for_each_entry(link, &cgrp->cset_links, cset_link)
+		count += link->cset->nr_tasks;
+
+	return count;
+}
+
+/**
+ * cgroup_task_count - count the number of tasks in a cgroup.
+ * @cgrp: the cgroup in question
+ */
+int cgroup_task_count(const struct cgroup *cgrp)
+{
+	int count;
+
+	spin_lock_irq(&css_set_lock);
+	count = __cgroup_task_count(cgrp);
+	spin_unlock_irq(&css_set_lock);
+
+	return count;
+}
+
 struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
 	struct cgroup *cgrp = of->kn->parent->priv;

2.17.2

The number of descendant cgroups and the number of dying descendant cgroups are currently synchronized using the cgroup_mutex.

The number of descendant cgroups will be required by the cgroup v2 freezer, which will use it to determine if a cgroup is frozen (depending on total number of descendants and number of frozen descendants). It's not always acceptable to grab the cgroup_mutex, especially from quite hot paths (e.g. exit()).

To avoid this, let's additionally synchronize these counters using the css_set_lock.

So, it's safe to read these counters with either cgroup_mutex or css_set_lock locked, and for changing both locks should be acquired.

---
 include/linux/cgroup-defs.h | 3 +++
 kernel/cgroup/cgroup.c      | 6 ++++++
 2 files changed, 9 insertions(+)

index 8fcbae1b8db0..4327fd6e8121 100644
@@ -348,6 +348,9 @@ struct cgroup {
 	 * Dying cgroups are cgroups which were deleted by a user,
 	 * but are still existing because someone else is holding a reference.
 	 * max_descendants is a maximum allowed number of descent cgroups.
+	 *
+	 * nr_descendants and nr_dying_descendants are protected
+	 * by cgroup_mutex and css_set_lock.
 	 */
 	int nr_descendants;
 	int nr_dying_descendants;

index 7519a4307021..f89dde50f693 100644
@@ -4723,9 +4723,11 @@ static void css_release_work_fn(struct work_struct *work)
 		if (cgroup_on_dfl(cgrp))
 			cgroup_rstat_flush(cgrp);
 
+		spin_lock_irq(&css_set_lock);
 		for (tcgrp = cgroup_parent(cgrp); tcgrp;
 		     tcgrp = cgroup_parent(tcgrp))
 			tcgrp->nr_dying_descendants--;
+		spin_unlock_irq(&css_set_lock);
 
 		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);
 		cgrp->id = -1;

@@ -4943,12 +4945,14 @@ static struct cgroup *cgroup_create(struct cgroup *parent)
 	if (ret)
 		goto out_psi_free;
 
+	spin_lock_irq(&css_set_lock);
 	for (tcgrp = cgrp; tcgrp; tcgrp = cgroup_parent(tcgrp)) {
 		cgrp->ancestor_ids[tcgrp->level] = tcgrp->id;
 
 		if (tcgrp != cgrp)
 			tcgrp->nr_descendants++;
 	}
+	spin_unlock_irq(&css_set_lock);
 
 	if (notify_on_release(parent))
 		set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

@@ -5233,10 +5237,12 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	if (parent && cgroup_is_threaded(cgrp))
 		parent->nr_threaded_children--;
 
+	spin_lock_irq(&css_set_lock);
 	for (tcgrp = cgroup_parent(cgrp); tcgrp; tcgrp = cgroup_parent(tcgrp)) {
 		tcgrp->nr_descendants--;
 		tcgrp->nr_dying_descendants++;
 	}
+	spin_unlock_irq(&css_set_lock);
 
 	cgroup1_check_for_release(parent);
 

2.17.2

Sure. Thanks!

Cgroup v1 implements the freezer controller, which provides an ability to stop the workload in a cgroup and temporarily free up some resources (cpu, io, network bandwidth and, potentially, memory) for some other tasks. Cgroup v2 lacks this functionality.

This patch implements freezer for cgroup v2.

Cgroup v2 freezer tries to put tasks into a state similar to jobctl stop. This means that tasks can be killed, ptraced (using PTRACE_SEIZE*), and interrupted. It is possible to attach to a frozen task, get some information (e.g. read registers) and detach. It's also possible to migrate a frozen tasks to another cgroup.

This differs cgroup v2 freezer from cgroup v1 freezer, which mostly tried to imitate the system-wide freezer. However uninterruptible sleep is fine when all tasks are going to be frozen (hibernation case), it's not the acceptable state for some subset of the system.

Cgroup v2 freezer is not supporting freezing kthreads.
If a non-root cgroup contains kthread, the cgroup still can be frozen, but the kthread will remain running, the cgroup will be shown as non-frozen, and the notification will not be delivered.

* PTRACE_ATTACH is not working because non-fatal signal delivery is blocked in frozen state.

There are some interface differences between cgroup v1 and cgroup v2 freezer too, which are required to conform the cgroup v2 interface design principles:
1) There is no separate controller, which has to be turned on:
the functionality is always available and is represented by cgroup.freeze and cgroup.events cgroup control files.
2) The desired state is defined by the cgroup.freeze control file. Any hierarchical configuration is allowed.
3) The interface is asynchronous. The actual state is available using cgroup.events control file ("frozen" field). There are no dedicated transitional states.
4) It's allowed to make any changes with the cgroup hierarchy (create new cgroups, remove old cgroups, move tasks between cgroups) no matter if some cgroups are frozen.

---
 include/linux/cgroup-defs.h  |  28 +++
 include/linux/cgroup.h       |  42 +++++
 include/linux/sched.h        |   2 +
 include/linux/sched/jobctl.h |   2 +
 include/linux/sched/signal.h |   3 +
 kernel/cgroup/Makefile       |   2 +-
 kernel/cgroup/cgroup.c       | 112 +++++++++++-
 kernel/cgroup/freezer.c      | 318 +++++++++++++++++++++++++++++++++++
 kernel/ptrace.c              |   7 +
 kernel/signal.c              |  58 +++++--
 10 files changed, 556 insertions(+), 18 deletions(-)
 create mode 100644 kernel/cgroup/freezer.c

index 4327fd6e8121..23a99be114b8 100644
@@ -64,6 +64,12 @@ enum {
 	 * specified at mount time and thus is implemented here.
 	 */
 	CGRP_CPUSET_CLONE_CHILDREN,
+
+	/* Control group has to be frozen. */
+	CGRP_FREEZE,
+
+	/* Cgroup is frozen. */
+	CGRP_FROZEN,
 };
 
 /* cgroup_root->flags */

@@ -316,6 +322,25 @@ struct cgroup_rstat_cpu {
 	struct cgroup *updated_next;		/* NULL iff not on the list */
 };
 
+struct cgroup_freezer_state {
+	/* Should the cgroup and its descendants be frozen. */
+	bool freeze;
+
+	/* Should the cgroup actually be frozen? */
+	int e_freeze;
+
+	/* Fields below are protected by css_set_lock */
+
+	/* Number of frozen descendant cgroups */
+	int nr_frozen_descendants;
+
+	/* Number of tasks to freeze */
+	int nr_tasks_to_freeze;
+
+	/* Number of frozen tasks */
+	int nr_frozen_tasks;
+};
+
 struct cgroup {
 	/* self css with NULL ->ss, points back to this cgroup */
 	struct cgroup_subsys_state self;

@@ -450,6 +475,9 @@ struct cgroup {
 	/* If there is block congestion on this cgroup. */
 	atomic_t congestion_count;
 
+	/* Used to store internal freezer state */
+	struct cgroup_freezer_state freezer;
+
 	/* ids of the ancestors at each level including self */
 	int ancestor_ids[];
 };

index a51c13c2b1a0..6e82c88998f5 100644
@@ -731,6 +731,8 @@ struct task_struct {
 #ifdef CONFIG_CGROUPS
 	/* disallow userland-initiated cgroup migration */
 	unsigned			no_cgroup_migration:1;
+	/* task is frozen by the cgroup freezer */
+	unsigned			frozen:1;
 #endif
 #ifdef CONFIG_BLK_CGROUP
 	/* to be used once the psi infrastructure lands upstream. */

index 98228bd48aee..fa067de9f1a9 100644
@@ -18,6 +18,7 @@ struct task_struct;
 #define JOBCTL_TRAP_NOTIFY_BIT	20	/* trap for NOTIFY */
 #define JOBCTL_TRAPPING_BIT	21	/* switching to TRACED */
 #define JOBCTL_LISTENING_BIT	22	/* ptracer is listening for events */
+#define JOBCTL_TRAP_FREEZE_BIT	23	/* trap for cgroup freezer */
 
 #define JOBCTL_STOP_DEQUEUED	(1UL << JOBCTL_STOP_DEQUEUED_BIT)
 #define JOBCTL_STOP_PENDING	(1UL << JOBCTL_STOP_PENDING_BIT)

@@ -26,6 +27,7 @@ struct task_struct;
 #define JOBCTL_TRAP_NOTIFY	(1UL << JOBCTL_TRAP_NOTIFY_BIT)
 #define JOBCTL_TRAPPING		(1UL << JOBCTL_TRAPPING_BIT)
 #define JOBCTL_LISTENING	(1UL << JOBCTL_LISTENING_BIT)
+#define JOBCTL_TRAP_FREEZE	(1UL << JOBCTL_TRAP_FREEZE_BIT)
 
 #define JOBCTL_TRAP_MASK	(JOBCTL_TRAP_STOP | JOBCTL_TRAP_NOTIFY)
 #define JOBCTL_PENDING_MASK	(JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)

index 13789d10a50e..c02dd6c782cc 100644
@@ -8,6 +8,7 @@
 #include <linux/sched/jobctl.h>
 #include <linux/sched/task.h>
 #include <linux/cred.h>
+#include <linux/cgroup.h>
 
 /*
  * Types defining task->signal and task->sighand and APIs using them:

@@ -368,6 +369,8 @@ static inline int signal_pending_state(long state, struct task_struct *p)
 		return 0;
 	if (!signal_pending(p))
 		return 0;
+	if (unlikely(cgroup_task_frozen(p)))
+		return __fatal_signal_pending(p);
 
 	return (state & TASK_INTERRUPTIBLE) || __fatal_signal_pending(p);
 }

index 8d5689ca94b9..5d7a76bfbbb7 100644
@@ -1,5 +1,5 @@
 # SPDX-License-Identifier: GPL-2.0
-obj-y := cgroup.o rstat.o namespace.o cgroup-v1.o
+obj-y := cgroup.o rstat.o namespace.o cgroup-v1.o freezer.o
 
 obj-$(CONFIG_CGROUP_FREEZER) += legacy_freezer.o
 obj-$(CONFIG_CGROUP_PIDS) += pids.o

index 80b34dffdfb9..5dfd2bd09fab 100644
@@ -410,6 +410,13 @@ static int ptrace_attach(struct task_struct *task, long request,
 
 	spin_lock(&task->sighand->siglock);
 
+	/*
+	 * If the process is frozen, let's wake it up to give it a chance
+	 * to enter the ptrace trap.
+	 */
+	if (cgroup_task_frozen(task))
+		wake_up_process(task);
+
 	/*
 	 * If the task is already STOPPED, set JOBCTL_TRAP_STOP and
 	 * TRAPPING, and kick it so that it transits to TRACED.  TRAPPING

2.17.2

To be honest, I fail to understand this patch. At least after a quick glance, I will try to read it again tomorrow but so far I do not even understand the desired semantics wrt signals/ptrace.

		if (JOBCTL_TRAP_MASK)
			do_jobctl_trap();
		else if (JOBCTL_TRAP_FREEZE)
			do_jobctl_freeze();

Sorry, it's a leftover from one of the previous versions. Fixed.

So, the idea here is to make ptrace traps and fatal signals working, but non-fatal shouldn't be delivered.

As soon as the frozen task is looping in the signal delivery loop, it's fine, it's not going anywhere.

Without the change above the task is getting out of schedule() immediately, if any signal is pending (including non-fatals). So it becomes a busy-loop.

It's a good point. So I need an additional synchronization around checking/setting the JOBCTL_TRAP_FREEZE?

Sure.

Makes sense. Will follow this path in v5.

Thank you!

If the cgroup destruction races with an exit() of a belonging process(es), cg_kill_all() may fail. It's not a good reason to make cg_destroy() fail and leave the cgroup in place, potentially causing next test runs to fail.

---
 tools/testing/selftests/cgroup/cgroup_util.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

index 14c9fe284806..eba06f94433b 100644
@@ -227,9 +227,7 @@ int cg_destroy(const char *cgroup)
 retry:
 	ret = rmdir(cgroup);
 	if (ret && errno == EBUSY) {
-		ret = cg_killall(cgroup);
-		if (ret)
-			return ret;
+		cg_killall(cgroup);
 		usleep(100);
 		goto retry;
 	}

2.17.2

This patch implements six tests for the freezer controller for cgroup v2:
1) a simple test, which aims to freeze and unfreeze a cgroup with 100 processes
2) a more complicated tree test, which creates a hierarchy of cgroups, puts some processes in some cgroups, and tries to freeze and unfreeze different parts of the subtree
3) a forkbomb test: the test aims to freeze a forkbomb running in a cgroup, kill all tasks in the cgroup and remove the cgroup without the unfreezing.
4) rmdir test: the test creates two nested cgroups, freezes the parent one, checks that the child can be successfully removed, and a new child can be created
5) migration tests: the test checks migration of a task between frozen cgroups: from a frozen to a running, from a running to a frozen, and from a frozen to a frozen.
6) ptrace test: the test checks that it's possible to attach to a process in a frozen cgroup, get some information and detach, and the cgroup will remain frozen.

Expected output:

  $ ./test_freezer
  ok 1 test_cgfreezer_simple
  ok 2 test_cgfreezer_tree
  ok 3 test_cgfreezer_forkbomb
  ok 4 test_cgrreezer_rmdir
  ok 5 test_cgfreezer_migrate
  ok 6 test_cgfreezer_ptrace

---
 tools/testing/selftests/cgroup/.gitignore     |   1 +
 tools/testing/selftests/cgroup/Makefile       |   2 +
 tools/testing/selftests/cgroup/cgroup_util.c  |  81 ++-
 tools/testing/selftests/cgroup/cgroup_util.h  |   7 +
 tools/testing/selftests/cgroup/test_freezer.c | 685 ++++++++++++++++++
 5 files changed, 775 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/cgroup/test_freezer.c

index adacda50a4b2..7f9835624793 100644
@@ -1,2 +1,3 @@
 test_memcontrol
 test_core
+test_freezer

index 23fbaa4a9630..8d369b6a2069 100644
@@ -5,8 +5,10 @@ all:
 
 TEST_GEN_PROGS = test_memcontrol
 TEST_GEN_PROGS += test_core
+TEST_GEN_PROGS += test_freezer
 
 include ../lib.mk
 
 $(OUTPUT)/test_memcontrol: cgroup_util.c
 $(OUTPUT)/test_core: cgroup_util.c
+$(OUTPUT)/test_freezer: cgroup_util.c

index 9ac8b7958f83..8ee63c00a668 100644
@@ -18,6 +18,7 @@ static inline int values_close(long a, long b, int err)
 extern int cg_find_unified_root(char *root, size_t len);
 extern char *cg_name(const char *root, const char *name);
 extern char *cg_name_indexed(const char *root, const char *name, int index);
+extern char *cg_control(const char *cgroup, const char *control);
 extern int cg_create(const char *cgroup);
 extern int cg_destroy(const char *cgroup);
 extern int cg_read(const char *cgroup, const char *control,

@@ -32,6 +33,7 @@ extern int cg_write(const char *cgroup, const char *control, char *buf);
 extern int cg_run(const char *cgroup,
 		  int (*fn)(const char *cgroup, void *arg),
 		  void *arg);
+extern int cg_enter(const char *cgroup, int pid);
 extern int cg_enter_current(const char *cgroup);
 extern int cg_run_nowait(const char *cgroup,
 			 int (*fn)(const char *cgroup, void *arg),

@@ -41,3 +43,8 @@ extern int alloc_pagecache(int fd, size_t size);
 extern int alloc_anon(const char *cgroup, void *arg);
 extern int is_swap_enabled(void);
 extern int set_oom_adj_score(int pid, int score);
+extern int cg_for_all_procs(const char *cgroup, int (*fn)(int pid, void *arg),
+			    void *arg);
+extern int cg_wait_for_proc_count(const char *cgroup, int count);
+extern int cg_killall(const char *cgroup);
+extern char proc_read_text(int pid, const char *item, char *buf, size_t size);

2.17.2

Describe cgroup v2 freezer interface in the cgroup v2 admin guide.

---
 Documentation/admin-guide/cgroup-v2.rst | 27 +++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

index 07e06136a550..f8335e26b362 100644
@@ -864,6 +864,8 @@ All cgroup core files are prefixed with "cgroup."
 	  populated
 		1 if the cgroup or its descendants contains any live
 		processes; otherwise, 0.
+	  frozen
+		1 if the cgroup is frozen; otherwise, 0.
 
   cgroup.max.descendants
 	A read-write single value files.  The default is "max".

@@ -897,6 +899,31 @@ All cgroup core files are prefixed with "cgroup."
 		A dying cgroup can consume system resources not exceeding
 		limits, which were active at the moment of cgroup deletion.
 
+  cgroup.freeze
+	A read-write single value file which exists on non-root cgroups.
+	Allowed values are "0" and "1". The default is "0".
+
+	Writing "1" to the file causes freezing of the cgroup and all
+	descendant cgroups. This means that all belonging processes will
+	be stopped and will not run until the cgroup will be explicitly
+	unfrozen. Freezing of the cgroup may take some time; when this action
+	is completed, the "frozen" value in the cgroup.events control file
+	will be updated to "1" and the corresponding notification will be
+	issued.
+
+	A cgroup can be frozen either by its own settings, or by settings
+	of any ancestor cgroups. If any of ancestor cgroups is frozen, the
+	cgroup will remain frozen.
+
+	Processes in the frozen cgroup can be killed by a fatal signal.
+	They also can enter and leave a frozen cgroup: either by an explicit
+	move by a user, or if freezing of the cgroup races with fork().
+	If a process is moved to a frozen cgroup, it stops. If a process is
+	moved out of a frozen cgroup, it becomes running.
+
+	Frozen status of a cgroup doesn't affect any cgroup tree operations:
+	it's possible to delete a frozen (and empty) cgroup, as well as
+	create new sub-cgroups.
 
 Controllers
 ===========

2.17.2