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SubjectRe: [PATCH 3/3] cgroup: implement cgroup.subtree_populated for the default hierarchy
Kay Sievers <kay@vrfy.org> writes:

> On Tue, Apr 15, 2014 at 7:48 PM, Li Zefan <lizefan@huawei.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.

Eric


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