linux-kernel - [PATCH v1] perf build: Make libunwind opt-in rather than opt-out

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Message-Id: <20241026014305.233607-1-irogers@google.com>
Date: Fri, 25 Oct 2024 18:43:05 -0700
From: Ian Rogers <irogers@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, 
	Kan Liang <kan.liang@...ux.intel.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: [PATCH v1] perf build: Make libunwind opt-in rather than opt-out

Having multiple unwinding libraries makes the perf code harder to
understand and we have unused/untested code paths.

Perf made BPF support an opt-out rather than opt-in feature. As libbpf
has a libelf dependency, elfutils that provides libelf will also
provide libdw. When libdw is present perf will use libdw unwinding
rather than libunwind unwinding even if libunwind support is compiled
in.

Rather than have libunwind built into perf and never used, explicitly
disable the support and make it opt-in.

Signed-off-by: Ian Rogers <irogers@...gle.com>
Closes: https://lore.kernel.org/linux-perf-users/CAP-5=fUXkp-d7gkzX4eF+nbjb2978dZsiHZ9abGHN=BN1qAcbg@mail.gmail.com/
---
I think, once libunwind support has been disabled over multiple
releases we can look to remove the associated code. We should look to
do similar for libbfd and other support that is no longer really
tested.
---
 tools/perf/Makefile.config | 4 ++++
 tools/perf/Makefile.perf   | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index b93ed2b7623f..707d7355ff18 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -91,6 +91,10 @@ ifneq ($(SRCARCH),$(filter $(SRCARCH),x86 arm arm64 powerpc s390 csky riscv loon
   NO_LIBDW_DWARF_UNWIND := 1
 endif
 
+ifneq ($(LIBUNWIND),1)
+  NO_LIBUNWIND := 1
+endif
+
 ifeq ($(LIBUNWIND_LIBS),)
   NO_LIBUNWIND := 1
 endif
diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
index b4dee7c20ed1..d74241a15131 100644
--- a/tools/perf/Makefile.perf
+++ b/tools/perf/Makefile.perf
@@ -52,7 +52,7 @@ include ../scripts/utilities.mak
 #
 # Define NO_LIBELF if you do not want libelf dependency (e.g. cross-builds)
 #
-# Define NO_LIBUNWIND if you do not want libunwind dependency for dwarf
+# Define LIBUNWIND if you do not want libunwind dependency for dwarf
 # backtrace post unwind.
 #
 # Define NO_BACKTRACE if you do not want stack backtrace debug feature
-- 
2.47.0.163.g1226f6d8fa-goog