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

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Date:	Fri, 15 Aug 2014 21:51:39 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Namhyung Kim <namhyung@...nel.org>
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

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.

jirka
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