linux-kernel - Re: [PATCH 3/3] cgroup: implement cgroup.subtree

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Date:	Tue, 15 Apr 2014 21:15:10 -0700
From:	Kay Sievers <kay@...y.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Li Zefan <lizefan@...wei.com>, rlove@...ve.org,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	containers@...ts.linux-foundation.org, serge.hallyn@...ntu.com,
	LKML <linux-kernel@...r.kernel.org>,
	Lennart Poettering <lennart@...ttering.net>,
	eparis@...isplace.org, Tejun Heo <tj@...nel.org>,
	cgroups@...r.kernel.org, john@...nmccutchan.com
Subject: Re: [PATCH 3/3] cgroup: implement cgroup.subtree_populated for the
 default hierarchy

On Tue, Apr 15, 2014 at 8:50 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Kay Sievers <kay@...y.org> writes:
>
>> On Tue, Apr 15, 2014 at 7:48 PM, Li Zefan <lizefan@...wei.com> 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.

Inotify on kernfs will work.

Kay
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