linux-kernel - Re: [PATCH] tracing: Export key trace event symbols

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Message-ID: <55364052.4050707@fnal.gov>
Date:	Tue, 21 Apr 2015 07:19:30 -0500
From:	Ron Rechenmacher <ron@...l.gov>
To:	Christoph Hellwig <hch@...radead.org>
CC:	<linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] tracing: Export key trace event symbols

I see in the reference I mentioned below (https://patches.linaro.org/28821/),
and in the current mm source, that some tracepoint symbols are already EXPORTed,
but not _GPL. I do not know the fine points between "GPL-ed" and "non-GPL-ed" symbol
exporting.  Would it make a difference if my patch proposed non-GPL exporting?

Ron Rechenmacher wrote on 04/21/15 07:04:
>
>
> Christoph Hellwig wrote on 04/21/15 01:10:
>>
>> Which (in-tree) module fails with this?  I don't think anyone should
>> actually register a symbol.
>>
>
> I see you (Christoph Hellwig) have asked this question in a similar context
> (see https://patches.linaro.org/28821/).
> This question does not seem to make sense because:
> 1) the external module is not registering a _symbol_ but more
> precisely a tracepoint _function_ as the whole tracepoint system allows for
> _multiple_ functions to be called for each tracepoint declared in the kernel.
> 2) It's not the point that an in-tree module would fail.  Again, the tracepoint
> system allows for _multiple_functions_ to be defined/registered for each tracepoint
> and _in_the_earlier_kernels_(i.e. 3.10.x and many others),_external_modules_could_
> _register_ one or more _additional_functions_ to be called.
>
> IF you're specifically saying that external modules should not register additional
> tracepoint functions, my question would simply be: why do you think this?
>
> To give you an example of the usefulness of continuing to allow this (continuation
> from earlier kernels): the kernel scheduling has a tracepoint defined; of course a
> critical operation for any kernel. I use to be able to insert a module which would
> collect my own statistics on when and what switching was going on on what CPU cores.
> I can think of many other potential reasons that this would be useful for external
> modules. To think that tracepoints would only be useful for in-tree development is,
> perhaps, (not meaning to offend) short sighted.
>

-- 
Ron Rechenmacher
Engineer
Fermi National Accelerator Laboratory
Batavia, IL  60510
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