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Date: Wed, 15 Jul 2009 11:32:52 +0530
From: Balbir Singh <balbir@linux.vnet.ibm.com>
To: Paul Menage <menage@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
       Vladislav Buzov <vbuzov@embeddedalley.com>,
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	<containers@lists.linux-foundation.org>,
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Subject: Re: [PATCH 1/1] Memory usage limit notification addition to memcg
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* menage@google.com <menage@google.com> [2009-07-13 15:15:45]:

> On Tue, Jul 7, 2009 at 5:56 PM, KAMEZAWA
> Hiroyuki<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> > I know people likes to wait for file descriptor to get notification in these days.
> > Can't we have "event" file descriptor in cgroup layer and make it reusable for
> > other purposes ?
> 
> I agree - rather than having to add a separate "wait for value to
> cross X threshold" file for each numeric usage value that people might
> be concerned about, it would be better to have a generic way to do it
> for any file. Given that this is a userspace API, it would be better
> to work out at least the generic API first, even if the initial
> implementation isn't generic.
> 
> Properties that it should support include:
> 
> - notification when a value crosses above or below a given threshold
> (which would include binary cases such as OOM notification where the
> value cross from "not-OOM" to "OOM"
> 
> - independent thresholds for different waiters
> 
> - epoll support (by using eventfd?)
> 
> - automatic wakeup when a cgroup is removed
> 
> - maybe optional wakeup when a thread attach occurs?
>

Please don't forget netlink, we've already got a genetlink socket for
cgroups in the form og cgroupstats.
 
> - not require more than read permissions on the file containing the
> value being monitored
> 
> I guess there are a few possible ways this could be exposed to userspace:
> 
> 1) new ioctl on cgroups files. simple but probably not popular
> 
> 2) new system call. maybe the cleanest, but involves changing every
> arch and is hard to script
> 
> 3) new per-cgroup file to control these e.g:
>   - create an eventfd
>   - open the control file to be monitored
>   - write the "<event_fd>, <control_fd> <threshold> to
> cgroup.event_control to link them together
> flexible and scriptable but maybe a clumsy interface in general
> 
> Paul

-- 
	Balbir
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