1 ==============
2 Cgroup Freezer
3 ==============
4
5 The cgroup freezer is useful to batch job mana
6 and stop sets of tasks in order to schedule th
7 according to the desires of a system administr
8 is often used on HPC clusters to schedule acce
9 whole. The cgroup freezer uses cgroups to desc
10 be started/stopped by the batch job management
11 a means to start and stop the tasks composing
12
13 The cgroup freezer will also be useful for che
14 of tasks. The freezer allows the checkpoint co
15 image of the tasks by attempting to force the
16 quiescent state. Once the tasks are quiescent
17 walk /proc or invoke a kernel interface to gat
18 quiesced tasks. Checkpointed tasks can be rest
19 recoverable error occur. This also allows the
20 migrated between nodes in a cluster by copying
21 to another node and restarting the tasks there
22
23 Sequences of SIGSTOP and SIGCONT are not alway
24 and resuming tasks in userspace. Both of these
25 from within the tasks we wish to freeze. While
26 blocked, or ignored it can be seen by waiting
27 SIGCONT is especially unsuitable since it can
28 programs designed to watch for SIGSTOP and SIG
29 attempting to use SIGSTOP and SIGCONT to stop
30 demonstrate this problem using nested bash she
31
32 $ echo $$
33 16644
34 $ bash
35 $ echo $$
36 16690
37
38 From a second, unrelated bash shell:
39 $ kill -SIGSTOP 16690
40 $ kill -SIGCONT 16690
41
42 <at this point 16690 exits and causes
43
44 This happens because bash can observe both sig
45 responds to them.
46
47 Another example of a program which catches and
48 signals is gdb. In fact any program designed t
49 have a problem with this method of stopping an
50
51 In contrast, the cgroup freezer uses the kerne
52 prevent the freeze/unfreeze cycle from becomin
53 being frozen. This allows the bash example abo
54 expected.
55
56 The cgroup freezer is hierarchical. Freezing a
57 tasks belonging to the cgroup and all its desc
58 cgroup has its own state (self-state) and the
59 parent (parent-state). Iff both states are THA
60 THAWED.
61
62 The following cgroupfs files are created by cg
63
64 * freezer.state: Read-write.
65
66 When read, returns the effective state of th
67 "FREEZING" or "FROZEN". This is the combined
68 If any is freezing, the cgroup is freezing (
69
70 FREEZING cgroup transitions into FROZEN stat
71 belonging to the cgroup and its descendants
72 a cgroup reverts to FREEZING from FROZEN aft
73 to the cgroup or one of its descendant cgrou
74 frozen.
75
76 When written, sets the self-state of the cgr
77 allowed - "FROZEN" and "THAWED". If FROZEN i
78 if not already freezing, enters FREEZING sta
79 descendant cgroups.
80
81 If THAWED is written, the self-state of the
82 THAWED. Note that the effective state may n
83 the parent-state is still freezing. If a cgr
84 becomes THAWED, all its descendants which ar
85 the cgroup also leave the freezing state.
86
87 * freezer.self_freezing: Read only.
88
89 Shows the self-state. 0 if the self-state is
90 This value is 1 iff the last write to freeze
91
92 * freezer.parent_freezing: Read only.
93
94 Shows the parent-state. 0 if none of the cg
95 frozen; otherwise, 1.
96
97 The root cgroup is non-freezable and the above
98 exist.
99
100 * Examples of usage::
101
102 # mkdir /sys/fs/cgroup/freezer
103 # mount -t cgroup -ofreezer freezer /sys/fs
104 # mkdir /sys/fs/cgroup/freezer/0
105 # echo $some_pid > /sys/fs/cgroup/freezer/0
106
107 to get status of the freezer subsystem::
108
109 # cat /sys/fs/cgroup/freezer/0/freezer.stat
110 THAWED
111
112 to freeze all tasks in the container::
113
114 # echo FROZEN > /sys/fs/cgroup/freezer/0/fr
115 # cat /sys/fs/cgroup/freezer/0/freezer.stat
116 FREEZING
117 # cat /sys/fs/cgroup/freezer/0/freezer.stat
118 FROZEN
119
120 to unfreeze all tasks in the container::
121
122 # echo THAWED > /sys/fs/cgroup/freezer/0/fr
123 # cat /sys/fs/cgroup/freezer/0/freezer.stat
124 THAWED
125
126 This is the basic mechanism which should do th
127 in a simple scenario.
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