diff options
| -rw-r--r-- | Documentation/cgroups/cgroups.txt (renamed from Documentation/cgroups.txt) | 0 | ||||
| -rw-r--r-- | Documentation/cgroups/freezer-subsystem.txt | 99 | ||||
| -rw-r--r-- | Documentation/cpusets.txt | 2 |
3 files changed, 100 insertions, 1 deletions
diff --git a/Documentation/cgroups.txt b/Documentation/cgroups/cgroups.txt index d9014aa0eb6..d9014aa0eb6 100644 --- a/Documentation/cgroups.txt +++ b/Documentation/cgroups/cgroups.txt diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt new file mode 100644 index 00000000000..c50ab58b72e --- /dev/null +++ b/Documentation/cgroups/freezer-subsystem.txt @@ -0,0 +1,99 @@ + 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 +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 +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 +walk /proc or invoke a kernel interface to gather information about the +quiesced tasks. Checkpointed tasks can be restarted later should a +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 +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. +SIGCONT is especially unsuitable since it can be caught by the task. Any +programs designed to watch for SIGSTOP and SIGCONT could be broken by +attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can +demonstrate this problem using nested bash shells: + + $ echo $$ + 16644 + $ bash + $ echo $$ + 16690 + + From a second, unrelated bash shell: + $ kill -SIGSTOP 16690 + $ kill -SIGCONT 16990 + + <at this point 16990 exits and causes 16644 to exit too> + + 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 +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 +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 +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. + +* Examples of usage : + + # mkdir /containers/freezer + # mount -t cgroup -ofreezer freezer /containers + # mkdir /containers/0 + # echo $some_pid > /containers/0/tasks + +to get status of the freezer subsystem : + + # cat /containers/0/freezer.state + THAWED + +to freeze all tasks in the container : + + # echo FROZEN > /containers/0/freezer.state + # cat /containers/0/freezer.state + FREEZING + # cat /containers/0/freezer.state + FROZEN + +to unfreeze all tasks in the container : + + # echo THAWED > /containers/0/freezer.state + # cat /containers/0/freezer.state + THAWED + +This is the basic mechanism which should do the right thing for user space task +in a simple scenario. + +It's important to note that freezing can be incomplete. In that case we return +EBUSY. This means that some tasks in the cgroup are busy doing something that +prevents us from completely freezing the cgroup at this time. After EBUSY, +the cgroup will remain partially frozen -- reflected by freezer.state reporting +"FREEZING" when read. The state will remain "FREEZING" until one of these +things happens: + + 1) Userspace cancels the freezing operation by writing "THAWED" to + 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) + 3) The tasks that blocked the cgroup from entering the "FROZEN" + state disappear from the cgroup's set of tasks. diff --git a/Documentation/cpusets.txt b/Documentation/cpusets.txt index 47e568a9370..5c86c258c79 100644 --- a/Documentation/cpusets.txt +++ b/Documentation/cpusets.txt @@ -48,7 +48,7 @@ hooks, beyond what is already present, required to manage dynamic job placement on large systems. Cpusets use the generic cgroup subsystem described in -Documentation/cgroup.txt. +Documentation/cgroups/cgroups.txt. Requests by a task, using the sched_setaffinity(2) system call to include CPUs in its CPU affinity mask, and using the mbind(2) and |