1 The cgroup freezer is useful to batch job management system which start
2 and stop sets of tasks in order to schedule the resources of a machine
3 according to the desires of a system administrator. This sort of program
4 is often used on HPC clusters to schedule access to the cluster as a
5 whole. The cgroup freezer uses cgroups to describe the set of tasks to
6 be started/stopped by the batch job management system. It also provides
7 a means to start and stop the tasks composing the job.
9 The cgroup freezer will also be useful for checkpointing running groups
10 of tasks. The freezer allows the checkpoint code to obtain a consistent
11 image of the tasks by attempting to force the tasks in a cgroup into a
12 quiescent state. Once the tasks are quiescent another task can
13 walk /proc or invoke a kernel interface to gather information about the
14 quiesced tasks. Checkpointed tasks can be restarted later should a
15 recoverable error occur. This also allows the checkpointed tasks to be
16 migrated between nodes in a cluster by copying the gathered information
17 to another node and restarting the tasks there.
19 Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping
20 and resuming tasks in userspace. Both of these signals are observable
21 from within the tasks we wish to freeze. While SIGSTOP cannot be caught,
22 blocked, or ignored it can be seen by waiting or ptracing parent tasks.
23 SIGCONT is especially unsuitable since it can be caught by the task. Any
24 programs designed to watch for SIGSTOP and SIGCONT could be broken by
25 attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can
26 demonstrate this problem using nested bash shells:
34 From a second, unrelated bash shell:
38 <at this point 16690 exits and causes 16644 to exit too>
40 This happens because bash can observe both signals and choose how it
43 Another example of a program which catches and responds to these
44 signals is gdb. In fact any program designed to use ptrace is likely to
45 have a problem with this method of stopping and resuming tasks.
47 In contrast, the cgroup freezer uses the kernel freezer code to
48 prevent the freeze/unfreeze cycle from becoming visible to the tasks
49 being frozen. This allows the bash example above and gdb to run as
52 The cgroup freezer is hierarchical. Freezing a cgroup freezes all
53 tasks beloning to the cgroup and all its descendant cgroups. Each
54 cgroup has its own state (self-state) and the state inherited from the
55 parent (parent-state). Iff both states are THAWED, the cgroup is
58 The following cgroupfs files are created by cgroup freezer.
60 * freezer.state: Read-write.
62 When read, returns the effective state of the cgroup - "THAWED",
63 "FREEZING" or "FROZEN". This is the combined self and parent-states.
64 If any is freezing, the cgroup is freezing (FREEZING or FROZEN).
66 FREEZING cgroup transitions into FROZEN state when all tasks
67 belonging to the cgroup and its descendants become frozen. Note that
68 a cgroup reverts to FREEZING from FROZEN after a new task is added
69 to the cgroup or one of its descendant cgroups until the new task is
72 When written, sets the self-state of the cgroup. Two values are
73 allowed - "FROZEN" and "THAWED". If FROZEN is written, the cgroup,
74 if not already freezing, enters FREEZING state along with all its
77 If THAWED is written, the self-state of the cgroup is changed to
78 THAWED. Note that the effective state may not change to THAWED if
79 the parent-state is still freezing. If a cgroup's effective state
80 becomes THAWED, all its descendants which are freezing because of
81 the cgroup also leave the freezing state.
83 * freezer.self_freezing: Read only.
85 Shows the self-state. 0 if the self-state is THAWED; otherwise, 1.
86 This value is 1 iff the last write to freezer.state was "FROZEN".
88 * freezer.parent_freezing: Read only.
90 Shows the parent-state. 0 if none of the cgroup's ancestors is
93 The root cgroup is non-freezable and the above interface files don't
98 # mkdir /sys/fs/cgroup/freezer
99 # mount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezer
100 # mkdir /sys/fs/cgroup/freezer/0
101 # echo $some_pid > /sys/fs/cgroup/freezer/0/tasks
103 to get status of the freezer subsystem :
105 # cat /sys/fs/cgroup/freezer/0/freezer.state
108 to freeze all tasks in the container :
110 # echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state
111 # cat /sys/fs/cgroup/freezer/0/freezer.state
113 # cat /sys/fs/cgroup/freezer/0/freezer.state
116 to unfreeze all tasks in the container :
118 # echo THAWED > /sys/fs/cgroup/freezer/0/freezer.state
119 # cat /sys/fs/cgroup/freezer/0/freezer.state
122 This is the basic mechanism which should do the right thing for user space task
123 in a simple scenario.