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

[Kay Sievers]

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

Kay
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