linux-kernel - Re: [PATCH 2/2] tracing/function-return-tracer: Call prepare_ftrace

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Date:	Mon, 17 Nov 2008 19:58:09 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] tracing/function-return-tracer: Call prepare_ftrace_return
 by registers

Ingo Molnar a écrit :
> * Frédéric Weisbecker <fweisbec@...il.com> wrote:
> 
>> 2008/11/13 Ingo Molnar <mingo@...e.hu>:
>>> hm, function-exit is a quite bad name i think that tells nothing to
>>> the user. I like "function-cost tracer" because that tells the user
>>> what it's all about in the end.
>>>
>>> Or perhaps we could name it the "callgraph" tracer? (as opposed to the
>>> simpler function tracer which traces function entries) Note that we
>>> could use the output to build function call coverage graphs.
>> But you can build a call graph with the function tracer, that what 
>> does the script draw_trace.py in a bit loosely way for example.
> 
> yes, but not reliably so - there's no guaranteed callgraph structure. 
> With entry tracing we have entry+parent events, but especially across 
> longer callchains there's no truly guaranteed way to preserve the full 
> graph.


That's right, with the depth, we could have a more safe result for the call graph.

>> IMHO, function cost measurement or call graphs are particular uses 
>> that can be made of this engine. You can also use it to trace 
>> function return values for example.
> 
> yes. The mockup output has place for that.


The problem with this is that we don't know in advance if the return value fits in 32
or 64 bits...

>> So perhaps naming it by thinking on the purpose it could be use at 
>> most would be better that its "general" or "potential" purpose. I 
>> don't know...
> 
> i suggested "full-function" tracer name before, but that sounds a bit 
> quirky too.
> 
> Perhaps this should be the function-tracer, and the entry tracer would 
> be the function-entry tracer?
> 
> 	Ingo

I still wonder about the name we should choose...
Renaming function to function_entry seems to me a bit wrong because usual function
tracing happens in entry...

Why not two-pass function tracer?

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