linux-kernel - Re: [PATCH 2/6] perf, tools: Support handling complete branch stacks as histograms v3

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Message-ID: <20140115144842.GA17358@krava.redhat.com>
Date:	Wed, 15 Jan 2014 15:48:42 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	acme@...radead.org, namhyung@...nel.org, mingo@...nel.org,
	dsahern@...il.com, fweisbec@...il.com, adrian.hunter@...el.com,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 2/6] perf, tools: Support handling complete branch stacks
 as histograms v3

On Mon, Jan 13, 2014 at 05:04:17PM -0800, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Currently branch stacks can be only shown as edge histograms for
> individual branches. I never found this display particularly useful.
> 
> This implements an alternative mode that creates histograms over complete
> branch traces, instead of individual branches, similar to how normal
> callgraphs are handled. This is done by putting it in
> front of the normal callgraph and then using the normal callgraph
> histogram infrastructure to unify them.
> 
> This way in complex functions we can understand the control flow
> that lead to a particular sample, and may even see some control
> flow in the caller for short functions.
> 
> Example (simplified, of course for such simple code this
> is usually not needed):
> 
> tcall.c:
> 
> volatile a = 10000, b = 100000, c;
> 
> __attribute__((noinline)) f2()
> {
> 	c = a / b;
> }
> 
> __attribute__((noinline)) f1()
> {
> 	f2();
> 	f2();
> }
> main()
> {
> 	int i;
> 	for (i = 0; i < 1000000; i++)
> 		f1();
> }
> 
> % perf record -b -g ./tsrc/tcall
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.044 MB perf.data (~1923 samples) ]
> % perf report --branch-history

this option is not added in this patch, I tried:
  $ perf report --call-graph=fractal,0.5,callee,function,branch

but as I already said in reply for patch 3, it's not working
please update the doc in Documentation/perf-report.txt with an example

jirka
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