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

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Date:	Wed, 18 Nov 2015 09:25:25 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	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 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?

jirka
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