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SubjectRe: perf report: fix off-by-one for non-activation frames
Date
On Freitag, 16. Juni 2017 08:14:56 CEST Jan Kratochvil wrote:
> 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.

Excuse me, but I'm having trouble following you. The non-GDB backtraces you
are pasting do not show srcline information. So what exactly is broken? Can
you show me the differences a bit more clearly? Maybe paste the perf output
you get now and highlight what you'd expect instead? Best would be an
accompanying test case that I can use to improve the situation, if possible?

Thanks

--
Milian Wolff | milian.wolff@kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts

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