linux-kernel - [PATCH 2/2] perf unwind: Do not fail due to missing unwind support

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Date:   Tue,  4 Jul 2017 16:30:00 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Jiri Olsa <jolsa@...nel.org>,
        David Ahern <dsahern@...il.com>, He Kuang <hekuang@...wei.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: [PATCH 2/2] perf unwind: Do not fail due to missing unwind support

From: Jiri Olsa <jolsa@...nel.org>

We currently fail the MMAP event processing if we don't have the MMAP
event's specific arch unwind support compiled in.

That's wrong and can lead to unresolved mmaps in report output for 32bit
binaries on 64bit server, like in this example on x86_64 server:

  $ cat ex.c
  int main(int argc, char **argv)
  {
          while (1) {}
  }
  $ gcc -o ex -m32 ex.c
  $ perf record ./ex
  ^C[ perf record: Woken up 2 times to write data ]
  [ perf record: Captured and wrote 0.371 MB perf.data (9322 samples) ]

Before:
  $ perf report --stdio

  SNIP

  # Overhead  Command  Shared Object     Symbol
  # ........  .......  ................  ......................
  #
     100.00%  ex       [unknown]         [.] 0x00000000080483de
       0.00%  ex       [unknown]         [.] 0x00000000f76dba4f
       0.00%  ex       [unknown]         [.] 0x00000000f76e4c11
       0.00%  ex       [unknown]         [.] 0x00000000f76daa30

After:
  $ perf report --stdio

  SNIP

  # Overhead  Command  Shared Object  Symbol
  # ........  .......  .............  ...............
  #
     100.00%  ex       ex             [.] main
       0.00%  ex       ld-2.24.so     [.] _dl_start
       0.00%  ex       ld-2.24.so     [.] do_lookup_x
       0.00%  ex       ld-2.24.so     [.] _start

The fix is not to fail, just warn if there's not unwind support compiled
in.

Reported-by: Michael Lyle <mlyle@...e.org>
Signed-off-by: Jiri Olsa <jolsa@...nel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@...hat.com>
Cc: David Ahern <dsahern@...il.com>
Cc: He Kuang <hekuang@...wei.com>
Cc: Namhyung Kim <namhyung@...nel.org>
Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Link: http://lkml.kernel.org/r/20170704131131.27508-1-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
---
 tools/perf/util/unwind-libunwind.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/perf/util/unwind-libunwind.c b/tools/perf/util/unwind-libunwind.c
index 6d542a4e0648..8aef572d0889 100644
--- a/tools/perf/util/unwind-libunwind.c
+++ b/tools/perf/util/unwind-libunwind.c
@@ -50,7 +50,7 @@ int unwind__prepare_access(struct thread *thread, struct map *map,
 
 	if (!ops) {
 		pr_err("unwind: target platform=%s is not supported\n", arch);
-		return -1;
+		return 0;
 	}
 out_register:
 	unwind__register_ops(thread, ops);
-- 
2.9.4