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

[Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

Hi Tejun,

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, and kernfs_notify() notificaiton is
> triggers when the value changes, which can be monitored through poll
> and [di]notify.
> 

For the old notification mechanism, the path of the cgroup that becomes
empty will be passed to the user specified release agent. Like this:

# cat /sbin/cpuset_release_agent
#!/bin/sh
rmdir /dev/cpuset/$1

How do we achieve this using inotify?

- monitor all the cgroups, or
- monitor all the leaf cgroups, and travel cgrp->parent to delete all
  empty cgroups.
- monitor root cgroup only, and travel the whole hierarchy to find
  empy cgroups when it gets an fs event.

Seems none of them is scalible.

> 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.
>