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

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Message-ID: <55363CDC.4000305@fnal.gov>
Date:	Tue, 21 Apr 2015 07:04:44 -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

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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