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Documentation / cgroups / freezer-subsystem.txt




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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:00 EST.

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.
8	
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.
18	
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:
27	
28		$ echo $$
29		16644
30		$ bash
31		$ echo $$
32		16690
33	
34		From a second, unrelated bash shell:
35		$ kill -SIGSTOP 16690
36		$ kill -SIGCONT 16690
37	
38		<at this point 16690 exits and causes 16644 to exit too>
39	
40	This happens because bash can observe both signals and choose how it
41	responds to them.
42	
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.
46	
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
50	expected.
51	
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
56	THAWED.
57	
58	The following cgroupfs files are created by cgroup freezer.
59	
60	* freezer.state: Read-write.
61	
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).
65	
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
70	  frozen.
71	
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
75	  descendant cgroups.
76	
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.
82	
83	* freezer.self_freezing: Read only.
84	
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".
87	
88	* freezer.parent_freezing: Read only.
89	
90	  Shows the parent-state.  0 if none of the cgroup's ancestors is
91	  frozen; otherwise, 1.
92	
93	The root cgroup is non-freezable and the above interface files don't
94	exist.
95	
96	* Examples of usage :
97	
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
102	
103	to get status of the freezer subsystem :
104	
105	   # cat /sys/fs/cgroup/freezer/0/freezer.state
106	   THAWED
107	
108	to freeze all tasks in the container :
109	
110	   # echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state
111	   # cat /sys/fs/cgroup/freezer/0/freezer.state
112	   FREEZING
113	   # cat /sys/fs/cgroup/freezer/0/freezer.state
114	   FROZEN
115	
116	to unfreeze all tasks in the container :
117	
118	   # echo THAWED > /sys/fs/cgroup/freezer/0/freezer.state
119	   # cat /sys/fs/cgroup/freezer/0/freezer.state
120	   THAWED
121	
122	This is the basic mechanism which should do the right thing for user space task
123	in a simple scenario.
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