linux-kernel - Re: perf top -g -U --sort=symbol --children == lalalalala?

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Message-ID: <878ulpmhbt.fsf@sejong.aot.lge.com>
Date:	Fri, 12 Sep 2014 16:24:54 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Mike Galbraith <umgwanakikbuti@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf top -g -U --sort=symbol --children == lalalalala?

Hi Arnaldo and Mike,

On Thu, 11 Sep 2014 10:43:38 -0300, Arnaldo Carvalho de Melo wrote:
> Em Thu, Sep 11, 2014 at 02:43:12PM +0200, Mike Galbraith escreveu:
>> On Thu, 2014-09-11 at 17:09 +0900, Namhyung Kim wrote: 
>> > On Tue, Sep 9, 2014 at 10:37 PM, Arnaldo Carvalho de Melo
>> > <acme@...stprotocols.net> wrote:
>> > > Em Tue, Sep 09, 2014 at 01:54:01PM +0200, Mike Galbraith escreveu:
>> > >> Seems the now default on --children thingy doesn't like -U much.
>> > >
>> > > Namhyung, can you please take a look at this?
>> > 
>> > So what is the problem here?
>> 
>> Well, if you don't see anything wrong, I guess nothing at all.
>
> :-)
>
> I think that when we decide that it is so better to change defaults like
> we did this time, we should be required to add a big fat warning (a
> --tui popup, use the first lines on --stdio, etc) about why the default
> was changed and allow quick, easy opt out, restoring previous behaviour
> after the user, being warned, knows what to expect, tries it, and then
> is in a better position to decide if keeping the new default is what is
> desired.

So the problem is that why it turned on --children option by default,
right?  I thought you mentioned there's a problem with -U option and I
couldn't figure out what it is.


>  
>> > >> Samples: 5K of event 'cycles', Event count (approx.): 2268660922
>> > >>   Children      Self  Symbol
>> > >> +   46.42%     0.04%  [k] system_call_fastpath
>  
>> I'll just turn it off until I figure out what cool stuff this is telling
>> me. why that symbol becomes the number one hit, and why total% > 100.
>  
>> To me, it looks like top smoked it's breakfast, went to lala land ;-)
>
> Yeah, its confusing, I'll let Namhyung explain it ;-)

Now I have three persion yell at me for this change. :)

When this change was developping, Ingo said it'd be better if it looks
like output of sysprof as it's more popular for most (userland?) guys.
You can see the discussion in the following links:

  https://lkml.org/lkml/2013/10/31/97
  https://lkml.org/lkml/2013/11/1/85

The children field is a cumulative total overhead (for its all
children/callee) so sum of them would be more than 100%.  And as Ingo
requested it sorts the output entries using children overhead so that
one can easily see higher level view of performance bottle-neck.

Thanks,
Namhyung
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