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SubjectRe: 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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