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

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Date:	Wed, 21 Oct 2015 19:57:19 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Namhyung Kim <namhyung@...nel.org>,
	Frederic Weisbecker <fweisbec@...il.com>
CC:	Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
	Chandler Carruth <chandlerc@...il.com>,
	Brendan Gregg <brendan.d.gregg@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Borislav Petkov <bp@...e.de>, David Ahern <dsahern@...il.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 13/16] perf callchain: Switch default to 'graph,0.5,caller'



On 2015/10/21 16:09, Namhyung Kim wrote:
> Hi Frederic,
>
> On Tue, Oct 20, 2015 at 07:21:16PM +0200, Frederic Weisbecker wrote:
>> On Tue, Oct 20, 2015 at 10:06:51AM -0300, Arnaldo Carvalho de Melo wrote:
>>> Em Tue, Oct 20, 2015 at 02:19:50PM +0200, Frederic Weisbecker escreveu:
>>>> On Tue, Oct 20, 2015 at 09:00:34AM -0300, Arnaldo Carvalho de Melo wrote:
>>>>> Em Mon, Oct 19, 2015 at 05:16:53PM -0700, Brendan Gregg escreveu:
>>>>> So are you advocating different defaults, one for --stdio (callee),
>>>>> another for --tui, --gtk (caller)?
>>>>> This is all configurable via ~/.perfconfig :-\
>>>>> Indeed, finding a default that is deemed adequate for most people is,
>>>>> ho-hum, difficult 8-)
>>>   
>>>> Most uses I've seen on LKML by the past involved callee because people
>>>> mostly look at the precise point where a performance issue is.
>>> A good chunk of that was because that was the default?
>> I doubt it. When you need to find the culprit of a syscall of IRQ performance issue,
>> you don't care much to see __libc_start_main() / main() on the top of your callchain.
>>
>>>   
>>>> IMHO changing that order is not a good idea. Unless many users complained
>>>> about it.
>>> Perhaps there are not that many users of callchains because the default
>>> is not what they're used to see?
>>>
>>> Motivation for the change came from a video from Chandler, that
>>> resurfaced the callchain default issue, Chandler?
>>>
>>> Anedoctally, he tweeted about it and people seemed to like it.
>> Well, I would prefer to hear from regular users than random twitter followers.
>> I could be wrong so lets ask some users first.
> Just a question.  Do you often use --children and/or '--g caller' options?

For me, I always use --no-children. However, I think it is because
I have used to --no-children and no one teach me how to utilize the
additional information --children provided. In case when result
of --no-children hard to explain I use Brendan's flame graph tool.

Thank you.

> I guess that for most kernel developers, --children is not that useful
> as you said.  But I think it can be useful for many userspace
> developers and with '-g caller' it can be even more useful. :)
>
> When '-g caller' is used, the callchains shown in a (self) entry are
> less important IMHO.  However callchains in entries generated by
> --children will show which functions are called by the entry (since
> it's reversed!) and will be more important.
>
> Thanks,
> Namhyung


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