linux-kernel - Re: [PATCH 13/16] perf callchain: Switch default to 'graph,0.5,caller'

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Date:	Thu, 22 Oct 2015 10:46:55 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>, linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Chandler Carruth <chandlerc@...il.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	pi3orama <pi3orama@....com>
Subject: Re: [PATCH 13/16] perf callchain: Switch default to
 'graph,0.5,caller'

* Arnaldo Carvalho de Melo <acme@...hat.com> wrote:

> > 5) --no-children
> > 
> > I agree that 'perf top -g --no-children' looks more intuitive than 'perf top -g'.
> 
> So, what do you propose, to switch back the default to --no-children, for both 
> tools, top and report? Now that I am getting used to it... ;-)

Heh ;-) So I'm only thinking out loud, trying to find the most intuitive initial 
screen to display. Expert users can configure their output any which way they want 
it to be, I'm not worried about them.

It's casual and in particular first-time users we should be worried about most - 
if they try the '-g' option in record, what will they first see in 'perf report' 
output?

I think the best output method would be to include only the 'highest level' parent 
symbols, with all children summed up under the parent's entry. Isn't the new 
'graph,0.5,caller' default very close to that?

But what confuses me about the output is the same that confused Wangnan's users:

  "This is my story: after switching to new version of perf, in a period of time 
   there are plenty of perf users in my company be confused by the first column of 
   'perf report' because the sum of the percentage listed there is much higher than 
   100%. They find me because they think this is a bug in perf which breaks their 
   routinely profiling work."

So this is suboptimal.

The first column is 'Children', which should show the sum of all child overhead - 
but if a child overhead was already included under a parent, it should never show 
up under another parent's entry. I.e. the first column should only contain the 
highest level entries, no sub-entries.

But what we do currently is:

  Children      Self  Command        Shared Object       Symbol
-   70.41%     0.00%  cc1            cc1                 [.] toplev_main
   - toplev_main
      + __libc_start_main
-   70.38%     0.00%  cc1            libc-2.20.so        [.] __libc_start_main
   + __libc_start_main

i.e. even though '__libc_start_main' is a child of 'toplev_main', it's still 
included on the 'overview' page.

Is there an output method that can do what I suggest above?

( Having both 'children' and 'self' columns in itself is intuitive IMHO: it shows
  that an entry that is shown does not directly have overhead at that level, a
  child call of it has that overhead. )

Thanks,

	Ingo
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