linux-kernel - Re: [PATCH v7 1/4] perf report: Support LLVM for addr2line()

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Date: Thu, 30 May 2024 09:58:09 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: "Steinar H. Gunderson" <sesse@...gle.com>
Cc: acme@...nel.org, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, irogers@...gle.com, 
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH v7 1/4] perf report: Support LLVM for addr2line()

On Sun, May 26, 2024 at 11:22 AM Steinar H. Gunderson <sesse@...glecom> wrote:
>
> In addition to the existing support for libbfd and calling out to
> an external addr2line command, add support for using libllvm directly.
> This is both faster than libbfd, and can be enabled in distro builds
> (the LLVM license has an explicit provision for GPLv2 compatibility).
> Thus, it is set as the primary choice if available.
>
> As an example, running perf report on a medium-size profile with
> DWARF-based backtraces took 58 seconds with LLVM, 78 seconds with
> libbfd, 153 seconds with external llvm-addr2line, and I got tired
> and aborted the test after waiting for 55 minutes with external
> bfd addr2line (which is the default for perf as compiled by distributions
> today). Evidently, for this case, the bfd addr2line process needs
> 18 seconds (on a 5.2 GHz Zen 3) to load the .debug ELF in question,
> hits the 1-second timeout and gets killed during initialization,
> getting restarted anew every time. Having an in-process addr2line
> makes this much more robust.
>
> As future extensions, libllvm can be used in many other places where
> we currently use libbfd or other libraries:
>
>  - Symbol enumeration (in particular, for PE binaries).
>  - Demangling (including non-Itanium demangling, e.g. Microsoft
>    or Rust).
>  - Disassembling (perf annotate).
>
> However, these are much less pressing; most people don't profile
> PE binaries, and perf has non-bfd paths for ELF. The same with
> demangling; the default _cxa_demangle path works fine for most
> users. Disassembling is coming in a later patch in the series;
> however do note that while bfd objdump can be slow on large binaries,
> it is possible to use --objdump=llvm-objdump to get the speed benefits.
> (It appears LLVM-based demangling is very simple, should we want
> that.)
>
> Tested with LLVM 14, 15, 16, 18 and 19. For some reason, LLVM 12 was not
> correctly detected using feature_check, and thus was not tested.
>
> Signed-off-by: Steinar H. Gunderson <sesse@...gle.com>
> Tested-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> ---
>  tools/build/Makefile.feature       |   1 +
>  tools/perf/Makefile.config         |  15 ++++
>  tools/perf/builtin-version.c       |   1 +
>  tools/perf/util/Build              |   1 +
>  tools/perf/util/llvm-c-helpers.cpp | 134 +++++++++++++++++++++++++++++
>  tools/perf/util/llvm-c-helpers.h   |  49 +++++++++++
>  tools/perf/util/srcline.c          |  57 +++++++++++-
>  7 files changed, 257 insertions(+), 1 deletion(-)
>  create mode 100644 tools/perf/util/llvm-c-helpers.cpp
>  create mode 100644 tools/perf/util/llvm-c-helpers.h

Can you please add something to tests/mask so that
we can test the build with and without libllvm easily?

Thanks,
Namhyung