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

On Fri, Aug 7, 2026 at 3:49 AM Jens Remus <[email protected]> wrote:
>
> 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 <[email protected]>
> ---
>  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");

Thanks Jens. The text was based on:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/s390/kernel/perf_cpum_sf.c#n853
```
static int cpumsf_pmu_event_init(struct perf_event *event)
...
 /* No support for callchain, stacks and registers */
if (has_branch_stack(event) || is_callchain_event(event))
return -EOPNOTSUPP;
```
The backchain option is never tested by perf or apparently in the
kernel, but this warning is supposed to pre-warn about "not supported"
being returned when frame pointer unwinding is requested. Of course,
lacking an s390 I've never tested this. I can imagine the warning
being overly broad, but my understanding is cpum_sf is the only PMU
capable of sampling on s390.

Perhaps what is needed is additional text after the "Use '--call-graph
dwarf'". The existing initial text at least appears to match the
kernel code. I'm not clear how the lack of back chains would be
reported as an error code, and this could introduce confusion as we
have lots of stack, chain and branch related terms in the perf
codebase.

Thanks,
Ian

>         }
>
>         evsel__set_sample_bit(evsel, CALLCHAIN);
>
> base-commit: f9a2394a23482bfd330911e9c8295b71724feacd
> --
> 2.53.0
>