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

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

On 2014/4/16 11:50, Eric W. Biederman wrote:
> Kay Sievers <kay-tD+1rO4QERM@public.gmane.org> writes:
> 
>> On Tue, Apr 15, 2014 at 7:48 PM, Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org> wrote:
>>> 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.
>>
>> The manager would add all cgroups as watches to one inotify file
>> descriptor, it should not be problem to do that.
> 
> inotify won't work on cgroupfs.
> 

Tejun's working on inotify support for cgroupfs, and I believe this patchset
has been tested, so it works.

So what do you mean by saying it won't work? Could you be more specific?