Re: perf report: fix off-by-one for non-activation frames

From: Jan Kratochvil <jan.kratochvil@redhat.com>
To: Milian Wolff <milian.wolff@kdab.com>
Cc: Linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	Arnaldo Carvalho de Melo <acme@redhat.com>,
	David Ahern <dsahern@gmail.com>,
	Namhyung Kim <namhyung@kernel.org>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Yao Jin <yao.jin@linux.intel.com>, Jiri Olsa <jolsa@redhat.com>
Subject: Re: perf report: fix off-by-one for non-activation frames
Date: Fri, 16 Jun 2017 08:14:56 +0200	[thread overview]
Message-ID: <20170616061456.GA2472@host1.jankratochvil.net> (raw)
In-Reply-To: <20170515150444.6841-1-milian.wolff@kdab.com>

On Mon, 15 May 2017 17:04:44 +0200, Milian Wolff wrote:

commit 1982ad48fc82c284a5cc55697a012d3357e84d01
Author: Milian Wolff <milian.wolff@kdab.com>
Date:   Wed May 24 15:21:25 2017 +0900

> --- a/tools/perf/util/unwind-libdw.c
> +++ b/tools/perf/util/unwind-libdw.c
> @@ -168,12 +168,16 @@ frame_callback(Dwfl_Frame *state, void *arg)
...
> +	if (!isactivation)
> +		--pc;
> +

FYI I find it as a regression a bit:

perf-4.11.4-200.fc25.x86_64
                  30c563 gdb_main (/usr/libexec/gdb)
                   fae48 main (/usr/libexec/gdb)
   0x000055555564ee43 <+51>:    callq  0x55555585f340 <gdb_main(captured_main_args*)>
   0x000055555564ee48 <+56>:    mov    0x18(%rsp),%rcx

perf-4.12.0-0.rc5.git0.1.fc27.x86_64
                  39e32e gdb_main (/usr/libexec/gdb)
                  10b6fa main (/usr/libexec/gdb)
   0x000055555565f6f6 <+54>:    callq  0x5555558f17a0 <gdb_main(captured_main_args*)>
   0x000055555565f6fb <+59>:    mov    0x18(%rsp),%rcx

In backtraces it is correct to show the source line of the calling line - as
perf does now after your fix - but one still should report PC address of the
start of the next instruction.  At least this is what debuggers are used to
do:

#9  gdb_main (args=0x7fffffffe2e0) at ../../gdb/main.c:1257
#10 0x000055555565f6fb in main (argc=<optimized out>, argv=<optimized out>) at ../../gdb/gdb.c:40
   0x000055555565f6f6 <+54>:	callq  0x5555558f17a0 <gdb_main(captured_main_args*)>
=> 0x000055555565f6fb <+59>:	mov    0x18(%rsp),%rcx
Line 40 of "../../gdb/gdb.c" starts at address 0x55555565f6f6 <main(int, char**)+54> and ends at 0x55555565f6fb <main(int, char**)+59>.
Line 41 of "../../gdb/gdb.c" starts at address 0x55555565f6fb <main(int, char**)+59> and ends at 0x55555565f715.

You see "gdb.c:40" and 0x000055555565f6fb in the backtrace despite
0x55555565f6fb is already line 41.

This is also why elfutils reports separately PC and 'isactivation' flag.
Instead of just reporting decreased PC.

Jan