Re: libdw dwarf unwind issue

[Jan Kratochvil <jan.kratochvil@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 2237 bytes --]

On Tue, 03 Nov 2020 23:17:09 +0100, Jiri Olsa wrote:
> When you compiled perf with libunwind, you get:
> 
>   $ make VF=1
>   $ perf report -i perf.data --stdio
> 
>   # Children      Self  Trace output
>   # ........  ........  ..............
>   #
>      100.00%   100.00%  (5640789cdf50)
>               |
>               |--98.85%--0x7fc8eca0f554
>               |          main
>               |          main_loop_wait
>               |          qemu_poll_ns
>               |
>                --1.15%--0x7fc8ecdc2ea4
>                          iothread_run
>                          aio_poll
>                          qemu_poll_ns
> 
> 
> When you compile perf with libdw, you get:
> 
>   $ make VF=1 NO_LIBUNWIND=1
>   $ perf report -i perf.data --stdio
> 
>   # Children      Self  Trace output
>   # ........  ........  ..............
>   #
>      100.00%   100.00%  (5640789cdf50)
>               |
>               |--98.85%--0x45260879
>               |          qemu_poll_ns
>               |
>                --1.15%--qemu_poll_ns

I get after the attached fix:

# Children      Self  Trace output  
# ........  ........  ..............
#
   100.00%   100.00%  (5640789cdf50)
            |          
            |--98.85%--main
            |          main_loop_wait
            |          qemu_poll_ns
            |          
             --1.15%--iothread_run
                       aio_poll
                       qemu_poll_ns


> any idea why libdw is doing that? ;-)

Because libdw was reporting separate debuginfo file as the main file:

(lldb) p thread->process->dwfl->modulelist->main.name
(char *) $1 = 0x0000000000b52490 "/home/jkratoch/.debug/.build-id/fc/d0111443729957fa8ef9511ff5dc3e9a5ccf9d/debug"
(lldb) p thread->process->dwfl->modulelist->debug.name
(char *) $2 = 0x0000000000000000

The correct way is to report the main ELF file as the main file:

(lldb) p thread->process->dwfl->modulelist->main.name
(char *) $1 = 0x0000000000b52490 "/home/jkratoch/.debug/.build-id/fc/d0111443729957fa8ef9511ff5dc3e9a5ccf9d/elf"
(lldb) p thread->process->dwfl->modulelist->debug.name
(char *) $2 = 0x0000000000000000

In some cases it may later fill-in even the ->debug.name with "/debug" filename.


Jan Kratochvil

[-- Attachment #2: libdw-unwind.patch --]
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diff --git a/tools/perf/util/unwind-libdw.c b/tools/perf/util/unwind-libdw.c
index 7a3dbc259cec..5d66cafccde5 100644
--- a/tools/perf/util/unwind-libdw.c
+++ b/tools/perf/util/unwind-libdw.c
@@ -20,10 +20,28 @@
 
 static char *debuginfo_path;
 
+static int __find_debuginfo(Dwfl_Module *mod __maybe_unused, void **userdata,
+			    const char *modname __maybe_unused, Dwarf_Addr base __maybe_unused,
+			    const char *file_name __maybe_unused,
+			    const char *debuglink_file __maybe_unused,
+			    GElf_Word debuglink_crc __maybe_unused, char **debuginfo_file_name)
+{
+	const struct dso *dso = *userdata;
+	char filename[PATH_MAX];
+
+	assert(dso);
+
+	// We could also use dso->symsrc_filename but it may not be set yet.
+	if (dso__build_id_filename(dso, filename, sizeof(filename), true))
+		*debuginfo_file_name = strdup(filename);
+	return -1;
+}
+
 static const Dwfl_Callbacks offline_callbacks = {
-	.find_debuginfo		= dwfl_standard_find_debuginfo,
+	.find_debuginfo		= __find_debuginfo,
 	.debuginfo_path		= &debuginfo_path,
 	.section_address	= dwfl_offline_section_address,
+	// .find_elf is not set as we use dwfl_report_elf() instead.
 };
 
 static int __report_module(struct addr_location *al, u64 ip,
@@ -46,16 +64,21 @@ static int __report_module(struct addr_location *al, u64 ip,
 	mod = dwfl_addrmodule(ui->dwfl, ip);
 	if (mod) {
 		Dwarf_Addr s;
+		void **userdatap;
 
-		dwfl_module_info(mod, NULL, &s, NULL, NULL, NULL, NULL, NULL);
+		dwfl_module_info(mod, &userdatap, &s, NULL, NULL, NULL, NULL, NULL);
+		*userdatap = dso;
 		if (s != al->map->start - al->map->pgoff)
 			mod = 0;
 	}
 
-	if (!mod)
-		mod = dwfl_report_elf(ui->dwfl, dso->short_name,
-				      (dso->symsrc_filename ? dso->symsrc_filename : dso->long_name), -1, al->map->start - al->map->pgoff,
-				      false);
+	if (!mod) {
+		char filename[PATH_MAX];
+
+		if (dso__build_id_filename(dso, filename, sizeof(filename), false))
+			mod = dwfl_report_elf(ui->dwfl, dso->short_name, filename, -1,
+					      al->map->start - al->map->pgoff, false);
+	}
 
 	return mod && dwfl_addrmodule(ui->dwfl, ip) == mod ? 0 : -1;
 }