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Date: Sat, 16 Aug 2014 11:26:31 +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
2014-08-15 (금), 21:51 +0200, Jiri Olsa:
> On Fri, Aug 15, 2014 at 10:57:14AM +0900, Namhyung Kim wrote:
> > 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.
>
> hum, where is it callee/caller mixed? with following example:
>
> ---
> void c(void)
> {
> }
>
> void b(void)
> {
> c();
> }
>
> void a(void)
> {
> b();
> }
>
> int main(void)
> {
> while(1) {
> a();
> b();
> c();
> }
> }
> ---
>
> for 'c' the current code will display:
>
> - 43.74% 43.74% t t [.] c ▒
> - __libc_start_main ▒
> - 86.33% main ▒
> 67.08% c ▒
> - 32.91% a ▒
> 99.44% c ▒
> - 0.56% b ▒
> c ▒
> 13.67% c ▒
>
> and with this patch:
>
> - 43.74% 43.74% t t [.] c ▒
> c ▒
>
>
> The 'c' callchain is still in caller order. IMO we should
> keep whole callchain here.
The problem is not in pure self entry (that has self overhead = children
overhead) and pure cumulative entry (self overhead = 0). It's in mixed
entries, please see last two examples in the description 0/3.
Thanks,
Namhyung
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