linux-kernel - Re: [RFC/PATCHSET 00/15] perf report: Add support to accumulate hist periods

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Message-ID: <20120928152737.GB14215@somewhere.redhat.com>
Date:	Fri, 28 Sep 2012 17:27:52 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC/PATCHSET 00/15] perf report: Add support to accumulate hist
 periods

On Fri, Sep 28, 2012 at 02:49:55PM +0900, Namhyung Kim wrote:
> Hi Frederic,
> 
> On Fri, 28 Sep 2012 01:01:48 +0200, Frederic Weisbecker wrote:
> > When Arun was working on this, I asked him to explore if it could make sense to reuse
> > the "-b, --branch-stack"  perf report option. Because after all, this feature is doing
> > about the same than "-b" except it's using callchains instead of full branch tracing.
> > But callchains are branches. Just a limited subset of all branches taken on excecution.
> > So you can probably reuse some interface and even ground code there.
> >
> > What do you think?
> 
> Umm.. first of all, I'm not familiar with the branch stack thing.  It's
> intel-specific, right?
> 
> Also I don't understand what exactly you want here.  What kind of
> interface did you say?  Can you elaborate it bit more?

Look at commit b50311dc2ac1c04ad19163c2359910b25e16caf6
"perf report: Add support for taken branch sampling". It's doing almost
the same than you do, just using PERF_SAMPLE_BRANCH_STACK instead of
callchains.

> And AFAIK branch stack can collect much more branch information than
> just callstacks.

That's not a problem. Callchains are just a high-level filtered source of
branch samples. You don't need full branches to use "-b". Just use the flavour
of branch samples you want to make the sense you want on your branch sampling.

> Can we differentiate which is which easily?

Sure. If you have both sources in your perf.data (PERF_SAMPLE_BRANCH_STACK and
callchains), ask the user which one he wants. Otherwise defaults to what's there.

> Is there
> any limitation on using it?  What if callstacks are not sync'ed with
> branch stacks - is it possible though?

It' better to make both sources mutually exclusive. Otherwise it's going
to be over-complicated.

> 
> But I think it'd be good if the branch stack can be changed to call
> stack in general.  Did you mean this?

That's a different. We might be able to post-process branch tracing and
build a callchain on top of it (following calls and ret). May be we will
one day. But they are different issues altogether.

Thanks.
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