[PATCH] perf evsel: Improve frame pointer unwinding warning for s390

[Jens Remus]

On s390 the kernel uses s390 back chain instead of frame pointers for
stack tracing of user space since v6.7 commit aa44433ac4ee ("s390: add
USER_STACKTRACE support").  This is because frame pointers on s390
cannot be used for stack tracing. [1]

This requires user space to maintain a s390 back chain.  For instance
user space to be built with compiler option '-mbackchain' (instead of
'-fno-omit-frame-pointer' used on other architectures, which should
better not be used on s390 [1]).

Only few distributions and users built user space with '-mbackchain'.
Therefore '--call-graph fp' may not produce the expected results.

Commit ca76fb67ebdd ("perf evlist: Improve default event for s390")
added a warning for s390 that wrongly claimed that "Framepointer
unwinding lacks kernel support".  Change the warning to hint at using
'--call-graph dwarf' if user space does not maintain a s390 back chain.

Note that '--call-graph fp' may also be useful for other applications,
such as OpenJDK maintaining a s390 back chain (does not require JVM
option '-XX:+PreserveFramePointer' on s390):

  $ perf record --call-graph fp ... -- \
    java -XX:+UnlockDiagnosticVMOptions -XX:+DumpPerfMapAtExit ...

[1]: s390: Stack tracing using Frame Pointer, Back Chain, and SFrame,
     https://conf.gnu-tools-cauldron.org/opo25/talk/Y3CVHY/

Fixes: ca76fb67ebdd ("perf evlist: Improve default event for s390")
Signed-off-by: Jens Remus <jremus@xxxxxxxxxxxxx>
---
 tools/perf/util/evsel.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index ea9fa04429f0..c4d67d1b35f3 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1080,7 +1080,8 @@ static void __evsel__config_callchain(struct evsel *evsel, const struct record_o
 
 	if (EM_HOST == EM_S390 && param->record_mode == CALLCHAIN_FP) {
 		pr_warning_once(
-			"Framepointer unwinding lacks kernel support. Use '--call-graph dwarf'\n");
+			"Use '--call-graph dwarf' if user space does not maintain a s390 back chain "
+			"(e.g. is not built with '-mbackchain').\n");
 	}
 
 	evsel__set_sample_bit(evsel, CALLCHAIN);

base-commit: f9a2394a23482bfd330911e9c8295b71724feacd
-- 
2.53.0