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

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Message-ID: <47BE2C7B.4000507@gmail.com>
Date:	Fri, 22 Feb 2008 10:59:23 +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:
> Tejun Heo wrote:
>> 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.
> 
> static struct module *load_module(void __user *umod,
>                                  unsigned long len,
>                                  const char __user *uargs)
> {
>      ...
> 
>      /* Now sew it into the lists so we can get lockdep and oops
>         * info during argument parsing.  Noone should access us, since
>         * strong_try_module_get() will fail. */
>        stop_machine_run(__link_module, mod, NR_CPUS);
>      ...
> }

Ah... right.  That part doesn't have anything to do with module
reference counting as the comment suggests and can probably be removed
by updating how kallsyms synchronize against module load/unload.

> I actually rarely unload modules. The way I notice the problem in first
> place is when things started hanging when tun driver was autoloaded or
> when fs automounts triggered some auto loading.
> These days it's kind hard to have a semi-general purpose machine without
> module loading :).

Yeap, agreed.

-- 
tejun
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