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

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Message-ID: <20140911213027.GB1801@kernel.org>
Date:	Thu, 11 Sep 2014 18:30:27 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Namhyung Kim <namhyung@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: perf top -g -U --sort=symbol --children == lalalalala?

Em Thu, Sep 11, 2014 at 10:43:38AM -0300, Arnaldo Carvalho de Melo escreveu:
> 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.

Also, looking at the changelog entries and at tools/perf/Documentation/
the only description for --children, the default, is:

----------------
--children::
        Accumulate callchain of children to parent entry so that then can
        show up in the output.  The output will have a new "Children" column
        and will be sorted on the data.  It requires callchains are recorded.
----------------

I think that a longer/clearer entry in the 'perf record' man page is
required.

Perhaps the description got lost in a --cover-letter for the patch
series implementing it?

- Arnaldo

>  
> > > >> 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 ;-)
> 
> :-)
> 
> - Arnaldo
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