Re: Re: [RFC] How to handle the rules engine for cgroups

From: kamezawa . hiroyu
Date: Fri Jul 18 2008 - 19:09:04 EST

----- Original Message -----
>On Fri, Jul 18, 2008 at 2:52 AM, KAMEZAWA Hiroyuki
><kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
>> For example, create a new file under memory cgroup
>> ==
>> /opt/memory_cgroup/group_A/notify_at_memory_reach_limit
>> ==
>> And a user watches the file by inotify.
>> The kernel modify modified-time of notify_at_memory_reach_limit file and ca
>> fs/notify_user.c::notify_change() against this inode. He can catchthe event
>> by inotify.
>> (I think he can also catch removal of this file, etc...)
>We've been doing something like this to handle OOMs in userspace, with
>pretty good success. The approach that we used so far was a custom
>control file tied to a wait queue, that gets woken when a cgroup
>triggers OOM, but it's a bit hacky. I've been considering some kind of
>more generic approach that could be reused by different subsystems for
>other notifications, maybe using eventfd or maybe netlink.
Hmm, eventfd is AIO's one ?
Anyway I agree we need something generic. (hopefully, reuse existing one.)

>inotify would be an option too, but that seems like it might be
>forcing ourselves into filesystem semantics too much.
At quick glance, Inotify's good points are

- can be used for any file. for example, even changes in "tasks" file can be
cathced if it modify modified-time.
- It can be queued.
- It supports ONESHOT, NONBLOCK, etc...
- All memory allocation is done by the waiter (the user).

But yes, we cannot notify other events than "there is some change".


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