linux-kernel - Re: Module loading/unloading and "The Stop Machine"

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Date:	Fri, 22 Feb 2008 10:38:54 +0900
From:	Tejun Heo <htejun@...il.com>
To:	Max Krasnyanskiy <maxk@...lcomm.com>
CC:	rusty@...tcorp.com.au, Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Module loading/unloading and "The Stop Machine"

Max Krasnyanskiy wrote:
> Thanks for the info. I guess I missed that from the code. In any case
> that seems like a pretty heavy refcounting mechanism. In a sense that
> every time something is loaded or unloaded entire machine freezes,
> potentially for several milliseconds. Normally it's not a big deal. But
> once you get more and more CPUs and/or start using realtime apps this
> becomes a big deal.

Module loading doesn't involve stop_machine last time I checked.  It's a
big deal when unloading a module but it's actually a very good trade off
because it makes much hotter path (module_get/put) much cheaper.  If
your application can't stand stop_machine, simply don't unload a module.

> And it's plain broken for the use case that I mentioned
> during CPU isolation discussions. ie When user-space thread(s) prevent
> stopmachine kthread from running, in which
> case machine simply hangs until those user-space threads exit.

This I don't know nothing about. :-)

-- 
tejun
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