linux-kernel - Re: Notifying on empty cgroup

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Message-ID: <20140115135930.GK8782@dhcp22.suse.cz>
Date:	Wed, 15 Jan 2014 14:59:30 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	Victor Porton <porton@...od.ru>
Cc:	linux-kernel@...r.kernel.org,
	cgroups mailinglist <cgroups@...r.kernel.org>
Subject: Re: Notifying on empty cgroup

[CCing cgroups mailing list]
On Wed 15-01-14 06:12:45, Victor Porton wrote:
> I want to write software which needs to receive a signal when the cgroup 
> created by it becomes empty. (After this the empty cgroup should be deleted 
> just not to clutter the memory.)
> 
> If the kernel does not support such notifications, it should be improved. 
> This functionality is crucial for some kinds of software.
> 
> There is /sys/fs/cgroup/systemd/release_agent but I don't understand how to 
> use it. I don't understand why we would need it at all.

"1.4 What does notify_on_release do ?" in
Documentation/cgroups/cgroups.txt the kernel source doesn't help?

> Starting a binary on emptying a cgroup with the purpose to notify an other 
> binary looks like a big overkill.

the binary can do rmdir which is what you want, no?

> Also my program should work in userspace without the need to use
> release_agent which can be accessed only by root.

The release_agent is global for all groups so the program doesn't have
to care.

> Note that my work is related with sandboxing software (running a program in 
> closed environment, so that it would be unable for example to remove user's 
> files).
> 
> See also
> http://portonsoft.wordpress.com/2014/01/11/toward-robust-linux-sandbox/
> 
> -- 
> Victor Porton - http://portonvictor.org
> --
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-- 
Michal Hocko
SUSE Labs
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