diff options
Diffstat (limited to 'Documentation/cgroups/freezer-subsystem.txt')
| -rw-r--r-- | Documentation/cgroups/freezer-subsystem.txt | 63 |
1 files changed, 42 insertions, 21 deletions
diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt index 7e62de1e59f..c96a72cbb30 100644 --- a/Documentation/cgroups/freezer-subsystem.txt +++ b/Documentation/cgroups/freezer-subsystem.txt @@ -49,13 +49,49 @@ 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. +The cgroup freezer is hierarchical. Freezing a cgroup freezes all +tasks beloning to the cgroup and all its descendant cgroups. Each +cgroup has its own state (self-state) and the state inherited from the +parent (parent-state). Iff both states are THAWED, the cgroup is +THAWED. -Note freezer.state doesn't exist in root cgroup, which means root cgroup -is non-freezable. +The following cgroupfs files are created by cgroup freezer. + +* freezer.state: Read-write. + + When read, returns the effective state of the cgroup - "THAWED", + "FREEZING" or "FROZEN". This is the combined self and parent-states. + If any is freezing, the cgroup is freezing (FREEZING or FROZEN). + + FREEZING cgroup transitions into FROZEN state when all tasks + belonging to the cgroup and its descendants become frozen. Note that + a cgroup reverts to FREEZING from FROZEN after a new task is added + to the cgroup or one of its descendant cgroups until the new task is + frozen. + + When written, sets the self-state of the cgroup. Two values are + allowed - "FROZEN" and "THAWED". If FROZEN is written, the cgroup, + if not already freezing, enters FREEZING state along with all its + descendant cgroups. + + If THAWED is written, the self-state of the cgroup is changed to + THAWED. Note that the effective state may not change to THAWED if + the parent-state is still freezing. If a cgroup's effective state + becomes THAWED, all its descendants which are freezing because of + the cgroup also leave the freezing state. + +* freezer.self_freezing: Read only. + + Shows the self-state. 0 if the self-state is THAWED; otherwise, 1. + This value is 1 iff the last write to freezer.state was "FROZEN". + +* freezer.parent_freezing: Read only. + + Shows the parent-state. 0 if none of the cgroup's ancestors is + frozen; otherwise, 1. + +The root cgroup is non-freezable and the above interface files don't +exist. * Examples of usage : @@ -85,18 +121,3 @@ to unfreeze all tasks in the container : 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 EINVAL) - 3) The tasks that blocked the cgroup from entering the "FROZEN" - state disappear from the cgroup's set of tasks. |