linux-kernel - Re: [PATCH -tip 6/7] perf bts trace: print function+offset

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Message-ID: <20101206024244.GA2418@nowhere>
Date:	Mon, 6 Dec 2010 03:42:47 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Akihiro Nagai <akihiro.nagai.hw@...achi.com>
Cc:	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH -tip  6/7] perf bts trace: print function+offset

On Fri, Dec 03, 2010 at 01:00:15PM +0900, Akihiro Nagai wrote:
> Provide the function to print function+offset.
> And, set it as the default behavior of 'perf bts trace'.
> To use this function, users can also specify the option '-s' or '--symbol'.
> 
> Example: 'perf bts -as trace'
> This command prints address and function+offset.
> 
> Output sample:
> address            function+offset
> 0xffffffff8146fe0e irq_return+0x0         => 0x00007fd4038e3b20 _start+0x0
> ...
> 0x000000380661ee79 __libc_start_main+0xf9 => 0x00000000004035c3 main+0x0
> 0xffffffff8146ef4e irq_return+0x0         => 0x00000000004035c3 main+0x0
> 0x00000000004035e8 main+0x25              => 0x000000000040bca0 set_program_name+0x0
> 0xffffffff8146ef4e irq_return+0x0         => 0x000000000040bca0 set_program_name+0x0
> 0x000000000040bcae set_program_name+0xe   => 0x00000000004023d0 strrchr@...+0x0
> 0x00000000004023d0 strrchr@...+0x0        => 0x00000000004023d6 strrchr@...+0x6
> ...
> 0x0000000000403e0c main+0x849             => 0x00000000004021f0 exit@...+0x0
> 0x00000000004021f0 exit@...+0x0           => 0x00000000004021f6 exit@...+0x6
> 0x00000000004021fb exit@...+0xb           => 0x00000000004020d0 _init+0x18
> 0x00000000004020d6 _init+0x1e             => 0x00000038062149d0 _dl_runtime_resolve+0x0
> ...

There is another kind of mode that I suspect would be very useful: something like
a userspace function graph tracer (you can have a look into the kernel function
graph tracer we have in ftrace to get an overview).

So, the idea would be to rebuild the whole function call flow:

main() {
	func1()
	func2() {
		func3()
	}
}


This would require to deref the instructions into the dso addresses from
the bts trace, keep only the "call" and the "ret" (there would be a small arch
backend for this mode) and rebuild the whole tree of calls.
We already have all the dso address mapping API in place with perf.

I really think this could be a very useful tool. Also we can even later
expand this to the branches, as Peter suggested. But starting with calls
would a very great start already.

Thanks.
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