linux-kernel - Re: [PATCH 2/3] perf tools: Add callchain order support for libunwind DWARF unwinder

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Message-ID: <B6736307-6A03-4623-A79E-99EDB0F6BD08@gmail.com>
Date:	Wed, 18 Nov 2015 18:25:15 +0900
From:	Namhyung Kim <namhyung@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>, Namhyung Kim <namhyung@...nel.org>
CC:	"Wangnan (F)" <wangnan0@...wei.com>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Milian Wolff <milian.wolff@...b.com>
Subject: Re: [PATCH 2/3] perf tools: Add callchain order support for libunwind DWARF unwinder

On November 18, 2015 5:25:25 PM GMT+09:00, Jiri Olsa <jolsa@...hat.com> wrote:
>On Wed, Nov 18, 2015 at 02:41:14PM +0900, Namhyung Kim wrote:
>
>SNIP
>
>> > I'm not sure whether we can regard this behavior changing as a
>bugfix? I
>> > think
>> > there may be some reason the original code explicitly avoid
>creating an '0'
>> > entry.
>> 
>> I think callchain value being 0 is an error or marker for the end of
>> callchain.  So it'd be better avoiding 0 entry.
>> 
>> But unfortunately, we have many 0 entries (and broken callchain after
>> them) with fp recording on optimized binaries.  I think we should
>omit
>> those callchains.
>> 
>> Maybe something like this?
>> 
>> 
>> 
>> diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
>> index 5ef90be2a249..22642c5719ab 100644
>> --- a/tools/perf/util/machine.c
>> +++ b/tools/perf/util/machine.c
>> @@ -1850,6 +1850,15 @@ static int
>thread__resolve_callchain_sample(struct thread *thread,
>>  #endif
>>  		ip = chain->ips[j];
>>  
>> +		/* callchain value inside zero page means it's broken, stop */
>> +		if (ip < 4096) {
>> +			if (callchain_param.order == ORDER_CALLER) {
>> +				callchain_cursor_reset(&callchain_cursor);
>
>hum, do we want to throw away whatever we have till now?

For caller order, yes.  

For callee order, everything after 0 value is garbage.  So we need to discard any chains before the 0 for caller IMHO.

Thanks
Namhyung


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