cgroup: implement cgroup.subtree_populated for the default hierarchy
cgroup users often need a way to determine when a cgroup's
subhierarchy becomes empty so that it can be cleaned up. cgroup
currently provides release_agent for it; unfortunately, this mechanism
is riddled with issues.
* It delivers events by forking and execing a userland binary
specified as the release_agent. This is a long deprecated method of
notification delivery. It's extremely heavy, slow and cumbersome to
integrate with larger infrastructure.
* There is single monitoring point at the root. There's no way to
delegate management of subtree.
* The event isn't recursive. It triggers when a cgroup doesn't have
any tasks or child cgroups. Events for internal nodes trigger only
after all children are removed. This again makes it impossible to
delegate management of subtree.
* Events are filtered from the kernel side. "notify_on_release" file
is used to subscribe to or suppres release event and events are not
generated if a cgroup becomes empty by moving the last task out of
it; however, event is generated if it becomes empty because the last
child cgroup is removed. This is inconsistent, awkward and
unnecessarily complicated and probably done this way because event
delivery itself was expensive.
This patch implements interface file "cgroup.subtree_populated" which
can be used to monitor whether the cgroup's subhierarchy has tasks in
it or not. Its value is 1 if there is no task in the cgroup and its
descendants; otherwise, 0, and kernfs_notify() notificaiton is
triggers when the value changes, which can be monitored through poll
and [di]notify.
This is a lot ligther and simpler and trivially allows delegating
management of subhierarchy - subhierarchy monitoring can block further
propgation simply by putting itself or another process in the root of
the subhierarchy and monitor events that it's interested in from there
without interfering with monitoring higher in the tree.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Serge Hallyn <serge.hallyn@ubuntu.com>
Cc: Lennart Poettering <lennart@poettering.net>
2 files changed
