linux-kernel - Re: [PATCH 3/3] perf callchain: Prune misleading callchains for self entries

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Date:	Fri, 15 Aug 2014 10:57:14 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Namhyung Kim <namhyung.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Andi Kleen <andi@...stfloor.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arun Sharma <asharma@...com>,
	Rodrigo Campos <rodrigo@...g.com.ar>
Subject: Re: [PATCH 3/3] perf callchain: Prune misleading callchains for
 self entries

Hi Jiri,

2014-08-14 (목), 16:10 +0200, Jiri Olsa:
> On Thu, Aug 14, 2014 at 03:01:40PM +0900, Namhyung Kim wrote:
> 
> SNIP
> 
> > However, with --children feature added, it now can show all callees of
> > the entry.  For example, "start_kernel" entry now can display it calls
> > rest_init and in turn cpu_idle and then cpuidle_idle_call (95.72%).
> > 
> >      6.14%     0.00%  swapper          [kernel.kallsyms]   [k] start_kernel
> >                    |
> >                     --- start_kernel
> >                         rest_init
> >                         cpu_idle
> >                        |
> >                        |--97.52%-- cpuidle_idle_call
> >                        |          cpuidle_enter_tk
> >                        |          |
> >                        |          |--99.91%-- cpuidle_wrap_enter
> >                        |          |          cpuidle_enter
> >                        |          |          intel_idle
> >                        |           --0.09%-- [...]
> >                         --2.48%-- [...]
> > 
> > Note that start_kernel has no self overhead - meaning that it never
> > get sampled by itself but constructs such a nice callgraph.  But,
> > sadly, if an entry has self overhead, callchain will get confused with
> > generated callchain (like above) and self callchains (which reversed
> > order) like the eariler example.
> > 
> > To be consistent with other entries, I'd like to make it just to show
> > a single entry - itself - like below since it doesn't have callees
> > (children) at all.  But still use the whole callchain to construct
> > children entries (like the start_kernel) as usual.
> > 
> >     40.53%    40.53%  swapper          [kernel.kallsyms]   [k] intel_idle
> >                     |
> >                     --- intel_idle
> 
> I understand the consistency point, but I think we'd loose
> usefull info by cutting this off
> 
> I guess I can run 'report -g callee' to find out who called intel_idle
> instead.. but I would not need to if the callchain stays here

Yeah, but current behavior intermixes caller-callchains and
callee-callchains together so adds confusion to users.  This is a
problem IMHO.

And with --children you can easily see the callers right above the entry
as they likely to have same or higher children overhead.

Thanks,
Namhyung


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