linux-kernel - Re: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once

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Message-ID: <20090724155041.GF5878@count0.beaverton.ibm.com>
Date:	Fri, 24 Jul 2009 08:50:41 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Ben Blum <bblum@...gle.com>
Cc:	linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org, akpm@...ux-foundation.org,
	serue@...ibm.com, lizf@...fujitsu.com, menage@...gle.com
Subject: Re: [PATCH 5/6] Makes procs file writable to move all threads by
	tgid at once

On Thu, Jul 23, 2009 at 08:22:00PM -0700, Ben Blum wrote:
> Makes procs file writable to move all threads by tgid at once
> 
> This patch adds functionality that enables users to move all threads in a
> threadgroup at once to a cgroup by writing the tgid to the 'cgroup.procs'
> file. This current implementation makes use of a rwsem that's taken for
> reading in the fork() path to prevent newly forking threads within the

There is much ado about not taking additional "global locks" in fork()
paths.

* The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
* (usually) take cgroup_mutex.  These are the two most performance
* critical pieces of code here. 
...

and as I recall cgroup_fork() doesn't ever take cgroup_mutex because it is
so performance critical. Assuming the above comments in kernel/cgroup.c
are correct then this patch adds a performance regression by introducing a
global mutex in the fork path, doesn't it?

Cheers,
	-Matt Helsley
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