From 3b1b3f6e57064aa8f91c290fe51cda4c74642902 Mon Sep 17 00:00:00 2001 From: Li Zefan Date: Wed, 12 Nov 2008 13:26:50 -0800 Subject: freezer_cg: disable writing freezer.state of root cgroup With this change, control file 'freezer.state' doesn't exist in root cgroup, making root cgroup unfreezable. I think it's reasonable to disallow freeze tasks in the root cgroup. And then we can avoid fork overhead when freezer subsystem is compiled but not used. Also make writing invalid value to freezer.state returns EINVAL rather than EIO. This is more consistent with other cgroup subsystem. Signed-off-by: Li Zefan Acked-by: Paul Menage Cc: Cedric Le Goater Cc: Paul Menage Cc: Matt Helsley Cc: "Serge E. Hallyn" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/cgroups/freezer-subsystem.txt | 21 ++++++++++++--------- 1 file changed, 12 insertions(+), 9 deletions(-) (limited to 'Documentation/cgroups/freezer-subsystem.txt') 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 @@ - The cgroup freezer is useful to batch job management system which start +The cgroup freezer is useful to batch job management system which start and stop sets of tasks in order to schedule the resources of a machine according to the desires of a system administrator. This sort of program is 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 be started/stopped by the batch job management system. It also provides a means to start and stop the tasks composing the job. - The cgroup freezer will also be useful for checkpointing running groups +The cgroup freezer will also be useful for checkpointing running groups of tasks. The freezer allows the checkpoint code to obtain a consistent image of the tasks by attempting to force the tasks in a cgroup into a quiescent state. Once the tasks are quiescent another task can @@ -16,7 +16,7 @@ recoverable error occur. This also allows the checkpointed tasks to be migrated between nodes in a cluster by copying the gathered information to another node and restarting the tasks there. - Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping +Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping and resuming tasks in userspace. Both of these signals are observable from within the tasks we wish to freeze. While SIGSTOP cannot be caught, blocked, or ignored it can be seen by waiting or ptracing parent tasks. @@ -37,26 +37,29 @@ demonstrate this problem using nested bash shells: - This happens because bash can observe both signals and choose how it +This happens because bash can observe both signals and choose how it responds to them. - Another example of a program which catches and responds to these +Another example of a program which catches and responds to these signals is gdb. In fact any program designed to use ptrace is likely to have a problem with this method of stopping and resuming tasks. - In contrast, the cgroup freezer uses the kernel freezer code to +In contrast, the cgroup freezer uses the kernel freezer code to prevent the freeze/unfreeze cycle from becoming visible to the tasks being frozen. This allows the bash example above and gdb to run as expected. - The freezer subsystem in the container filesystem defines a file named +The freezer subsystem in the container filesystem defines a file named freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup. Reading will return the current state. +Note freezer.state doesn't exist in root cgroup, which means root cgroup +is non-freezable. + * Examples of usage : - # mkdir /containers/freezer + # mkdir /containers # mount -t cgroup -ofreezer freezer /containers # mkdir /containers/0 # echo $some_pid > /containers/0/tasks @@ -94,6 +97,6 @@ things happens: the freezer.state file 2) Userspace retries the freezing operation by writing "FROZEN" to the freezer.state file (writing "FREEZING" is not legal - and returns EIO) + and returns EINVAL) 3) The tasks that blocked the cgroup from entering the "FROZEN" state disappear from the cgroup's set of tasks. -- cgit v1.2.2