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-rw-r--r--Documentation/cgroups/freezer-subsystem.txt21
1 files changed, 12 insertions, 9 deletions
diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt
index c50ab58b72eb..41f37fea1276 100644
--- a/Documentation/cgroups/freezer-subsystem.txt
+++ b/Documentation/cgroups/freezer-subsystem.txt
@@ -1,4 +1,4 @@
1 The cgroup freezer is useful to batch job management system which start 1The cgroup freezer is useful to batch job management system which start
2and stop sets of tasks in order to schedule the resources of a machine 2and stop sets of tasks in order to schedule the resources of a machine
3according to the desires of a system administrator. This sort of program 3according to the desires of a system administrator. This sort of program
4is often used on HPC clusters to schedule access to the cluster as a 4is often used on HPC clusters to schedule access to the cluster as a
@@ -6,7 +6,7 @@ whole. The cgroup freezer uses cgroups to describe the set of tasks to
6be started/stopped by the batch job management system. It also provides 6be started/stopped by the batch job management system. It also provides
7a means to start and stop the tasks composing the job. 7a means to start and stop the tasks composing the job.
8 8
9 The cgroup freezer will also be useful for checkpointing running groups 9The cgroup freezer will also be useful for checkpointing running groups
10of tasks. The freezer allows the checkpoint code to obtain a consistent 10of tasks. The freezer allows the checkpoint code to obtain a consistent
11image of the tasks by attempting to force the tasks in a cgroup into a 11image of the tasks by attempting to force the tasks in a cgroup into a
12quiescent state. Once the tasks are quiescent another task can 12quiescent state. Once the tasks are quiescent another task can
@@ -16,7 +16,7 @@ recoverable error occur. This also allows the checkpointed tasks to be
16migrated between nodes in a cluster by copying the gathered information 16migrated between nodes in a cluster by copying the gathered information
17to another node and restarting the tasks there. 17to another node and restarting the tasks there.
18 18
19 Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping 19Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping
20and resuming tasks in userspace. Both of these signals are observable 20and resuming tasks in userspace. Both of these signals are observable
21from within the tasks we wish to freeze. While SIGSTOP cannot be caught, 21from within the tasks we wish to freeze. While SIGSTOP cannot be caught,
22blocked, or ignored it can be seen by waiting or ptracing parent tasks. 22blocked, or ignored it can be seen by waiting or ptracing parent tasks.
@@ -37,26 +37,29 @@ demonstrate this problem using nested bash shells:
37 37
38 <at this point 16990 exits and causes 16644 to exit too> 38 <at this point 16990 exits and causes 16644 to exit too>
39 39
40 This happens because bash can observe both signals and choose how it 40This happens because bash can observe both signals and choose how it
41responds to them. 41responds to them.
42 42
43 Another example of a program which catches and responds to these 43Another example of a program which catches and responds to these
44signals is gdb. In fact any program designed to use ptrace is likely to 44signals is gdb. In fact any program designed to use ptrace is likely to
45have a problem with this method of stopping and resuming tasks. 45have a problem with this method of stopping and resuming tasks.
46 46
47 In contrast, the cgroup freezer uses the kernel freezer code to 47In contrast, the cgroup freezer uses the kernel freezer code to
48prevent the freeze/unfreeze cycle from becoming visible to the tasks 48prevent the freeze/unfreeze cycle from becoming visible to the tasks
49being frozen. This allows the bash example above and gdb to run as 49being frozen. This allows the bash example above and gdb to run as
50expected. 50expected.
51 51
52 The freezer subsystem in the container filesystem defines a file named 52The freezer subsystem in the container filesystem defines a file named
53freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the 53freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the
54cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup. 54cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup.
55Reading will return the current state. 55Reading will return the current state.
56 56
57Note freezer.state doesn't exist in root cgroup, which means root cgroup
58is non-freezable.
59
57* Examples of usage : 60* Examples of usage :
58 61
59 # mkdir /containers/freezer 62 # mkdir /containers
60 # mount -t cgroup -ofreezer freezer /containers 63 # mount -t cgroup -ofreezer freezer /containers
61 # mkdir /containers/0 64 # mkdir /containers/0
62 # echo $some_pid > /containers/0/tasks 65 # echo $some_pid > /containers/0/tasks
@@ -94,6 +97,6 @@ things happens:
94 the freezer.state file 97 the freezer.state file
95 2) Userspace retries the freezing operation by writing "FROZEN" to 98 2) Userspace retries the freezing operation by writing "FROZEN" to
96 the freezer.state file (writing "FREEZING" is not legal 99 the freezer.state file (writing "FREEZING" is not legal
97 and returns EIO) 100 and returns EINVAL)
98 3) The tasks that blocked the cgroup from entering the "FROZEN" 101 3) The tasks that blocked the cgroup from entering the "FROZEN"
99 state disappear from the cgroup's set of tasks. 102 state disappear from the cgroup's set of tasks.