[PATCH 3/3] cgroup: implement cgroup.subtree_populated for the default hierarchy

[Li Zefan]

On 2014/4/15 5:44, Tejun Heo wrote:
> 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 suppress release event.  This is
>   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 0 if there is no task in the cgroup and its
> descendants; otherwise, 1, 

Is cgroup.tree_populated a better name?

cgroup.subtree_control controls child cgroups only, but .subtree_populated
shows 1 if there're tasks in the cgroup or its children, so the two
are a bit inconsistent to me.

> 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.
> 
> v2: Patch description updated as per Serge.
> 
> Signed-off-by: Tejun Heo <tj at kernel.org>
> Acked-by: Serge Hallyn <serge.hallyn at ubuntu.com>
> Cc: Lennart Poettering <lennart at poettering.net>