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

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Message-ID: <20151021014432.GB628@sejong>
Date:	Wed, 21 Oct 2015 10:44:32 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Arnaldo Carvalho de Melo <acme@...hat.com>
Cc:	"Wangnan (F)" <wangnan0@...wei.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, 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>,
	Stephane Eranian <eranian@...gle.com>,
	pi3orama <pi3orama@....com>
Subject: Re: [PATCH 13/16] perf callchain: Switch default to
 'graph,0.5,caller'

On Tue, Oct 20, 2015 at 11:38:16AM -0200, Arnaldo Carvalho de Melo wrote:
> Em Tue, Oct 20, 2015 at 09:23:12PM +0800, Wangnan (F) escreveu:
> > On 2015/10/6 5:03, Arnaldo Carvalho de Melo wrote:
> > >From: Arnaldo Carvalho de Melo <acme@...hat.com>
> > >Which is the most common default found in other similar tools.
>  
> > Could you please show me some example about "other similar tools"?
> > For me, in most of the case I prefer callee order because most of my
> > task is to explain the reason why some code get executed too much times
> > than expected.
>  
> > Also, I think changing default settings should be careful.
>  
> > 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.  The
> > "problem" is caused by the adding of "--children". New perf makes
> > '--children' as the default behavior at the first time it support that
> > option, but the old perf shows things similar to '--no-children'.
> > However, it is hard to explain the principle of call stack accumulation
> > and why we need '--children' to those perf users (they learned perf's
> > command line from others, and don't have enought to read perf
> > documentations or even help output. Althought the title of the first
> > column is changed to 'Children', I don't think they can understand the
> > meaning of it. I think some of them didn't even notice there's an
> > addition column in their output. They just confused and angry). Also,
> > and as you can expect, this change breaks some scripts. In those days I
> > have to make our IM tool response the information of "--no-children"
> > automatically.
> > 
> > This patch changes the default output again. Similar thing will happen
> > another time. I think this time I can make some preparation, for example,
> > prepare new script to restore old behavior?
> 
> I was bitten by the --children thing and took some time to get used to
> it, so I can relate to that...

I feel sorry about that.  I did worry about the existing users when
making the --children default and actually I didn't agree with making
it default at first. :-(


> 
> I think we should revert this change in callchain default, enough
> complaints...  Ingo, since you suggested that change, what are your
> thoughts?
> 
> Changing defaults is hard, there is also the horizontal scrolling that
> made we repurpose the right and left arrows, sigh, that one will cause
> some confusion as well...

Yeah, it worries me too.  That's why I used '<' and '>' key for
scrolling in my patch.  Maybe it's worth adding those keys again,
reverting arrows key actions, and show some message that it'll be
changed later?


> 
> It seems we'll need way more preparation for such changes, more
> infrastructure to ease the transition, questioning if the user wants
> that, etc, growing pains :-\

Yes, it reminds me of changing default push behavior in git 2.0.  We
need to provide info and wait for enough time before changing some
behavior IMHO.

Thanks,
Namhyung
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