linux-kernel - Re: [PATCH] perf: Add a new sort order: SORT

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Message-ID: <4F691289.7080505@fb.com>
Date:	Tue, 20 Mar 2012 16:28:09 -0700
From:	Arun Sharma <asharma@...com>
To:	Frederic Weisbecker <fweisbec@...il.com>
CC:	<linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	"Arnaldo Carvalho de Melo" <acme@...hat.com>,
	Mike Galbraith <efault@....de>,
	"Paul Mackerras" <paulus@...ba.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Stephane Eranian" <eranian@...gle.com>,
	Namhyung Kim <namhyung.kim@....com>,
	"Tom Zanussi" <tzanussi@...il.com>,
	<linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH] perf: Add a new sort order: SORT_INCLUSIVE (v4)

On 3/19/12 8:57 AM, Frederic Weisbecker wrote:

>>> Each hist have a period of 1, but the total period is 1.
>>> So the end result should be (IIUC):
>>>
>>> 100%    foo     a
>>> 100%    foo     b
>>>                  |
>>>                  --- a
>>> 100%    foo     c
>>>                  |
>>>                  --- b
>>>                      |
>>>                      --- c
>>>
>>
>> That is correct. The first column no longer adds up to 100%.
>
> So do we really want this?
>

I think so. It's a different way of presenting the data. Pie chart vs a 
bar chart of OS market share where people may be using more than one OS.

I'll post some documentation updates.

>> If we don't do this, total_period will be inflated.
>
> Yeah right I've just tried and callchains look right. I'm just puzzled
> by the percentages:
>

Thanks for testing this!

> +  98,99%  [k] execve
> +  98,99%  [k] stub_execve
> +  98,99%  [k] do_execve
> +  98,99%  [k] do_execve_common
> +  98,99%  [k] sys_execve
> +  53,12%  [k] __libc_start_main
> +  53,12%  [k] cmd_record

These look like they belong to the perf binary and are incorrectly 
classified as kernel samples. Problem is that callchain_get() is not 
populating the privilege level - it's simply propagating the privilege 
level of the sample:

+       for (i = 0; i < cursor->nr; i++) {
+               struct addr_location al_child = *al;
+
+               err = callchain_get(&iter, &al_child);

Not all fields of al_child are populated by callchain_get().

> +  53,12%  [k] T.101
> +  53,12%  [k] main
> +  53,12%  [k] run_builtin
> +  52,11%  [k] perf_evlist__prepare_workload
> +  52,09%  [k] T.1163

The rest of them look ok to me. If something doesn't make sense, please 
point me at the output of "perf script".

>
>>
>>> Also this feature reminds me a lot the -b option in perf report.
>>> Branch sorting and callchain inclusive sorting are a bit different in
>>> the way they handle the things but the core idea is the same. Callchains
>>> are branches as well.
>>>
>>
>> Yes - I kept asking why the branch stack stuff doesn't use the
>> existing callchain logic.
>
> Because I fear that loops branches could make the tree representation useless.
>

The loops could happen in callgraphs too right (eg: recursive programs)? 
The other problem in branch stacks/LBR is that they're sampled branches. 
Just because I got a sample with:

a -> b
b -> c

doesn't necessarily mean that the callchain was a -> b -> c.

I still don't have the branch stack setup working properly. But I'm now 
more sympathetic to the view that last branch sampling and callchains 
may have different representations in perf.

  -Arun
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