Re: [PATCH] perf evsel: Improve callchain warning for s390

[Thomas Richter <tmricht@linux.ibm.com>]

On 9/23/26 18:37, Ian Rogers wrote:
> On Tue, Sep 22, 2026 at 7:43 AM Jens Remus <jremus@linux.ibm.com> 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 build
>> user space with '-mbackchain'.  Therefore '--call-graph fp' may in
>> practice not produce the expected results.
>>
>> Commit ca76fb67ebdd ("perf evlist: Improve default event for s390")
>> changed the default '-g' option to 'dwarf' on s390 and added a warning
>> for s390 that wrongly claimed that "Framepointer unwinding lacks kernel
>> support".  This warning resulted from a misinterpretation of comments
>> in s390 cpumsf_pmu_event_init().
>>
>> The restriction in cpumsf_pmu_event_init() applies to callchain sampling
>> with the s390 CPU Measurement Sampling Facility (CPUMSF) hardware PMU.
>> CPUMSF events, such as 'cycles', do not support callchain sampling.
>> This is independent of whether perf uses the 'fp' or 'dwarf' call-graph
>> mode.  The CPUMSF PMU provides samples collected asynchronously, making
>> it impossible to associate a callchain to the historic IPs.  Therefore
>> callchains can only be used with software events on s390.
>>
>> Remove the incorrect warning.  The kernel supports the 'fp' call-graph
>> mode on s390 by walking the back chain.  Do not replace it with a
>> hint to use 'dwarf' when the resulting callchain is incomplete.  Such a
>> suggestion could imply that 'fp' is inherently inferior.  The same
>> general limitation exists on other architectures when user space is not
>> built with frame pointers (i.e. '-fno-omit-frame-pointers').
>>
>> Instead, warn on s390 when callchain sampling is requested with an event
>> provided by the CPUMCF or CPUMSF hardware PMUs, because that combination
>> is not supported.
>>
>> Update the perf record documentation to state that 'dwarf' is the
>> default call-graph mode on s390 and that 'fp' uses back chain instead
>> of frame pointers on s390.
>>
>> 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@linux.ibm.com>
> 
> Acked-by: Ian Rogers <irogers@google.com>
> 
> Thanks,
> Ian
> 
>> ---
>>
>> Notes (jremus):
>>     Changes in v3:
>>     - Updated warning message.
>>     - Rebased on v7.3-rc4.
>>
>>  tools/perf/Documentation/perf-record.txt | 7 ++++++-
>>  tools/perf/util/evsel.c                  | 6 +++---
>>  2 files changed, 9 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/perf/Documentation/perf-record.txt b/tools/perf/Documentation/perf-record.txt
>> index 178f483140ed..dc72606cb25d 100644
>> --- a/tools/perf/Documentation/perf-record.txt
>> +++ b/tools/perf/Documentation/perf-record.txt
>> @@ -293,7 +293,7 @@ OPTIONS
>>
>>  --call-graph::
>>         Setup and enable call-graph (stack chain/backtrace) recording,
>> -       implies -g.  Default is "fp" (for user space).
>> +       implies -g.  Default is "fp" for user space (s390 uses "dwarf").
>>
>>         The unwinding method used for kernel space is dependent on the
>>         unwinder used by the active kernel configuration, i.e
>> @@ -329,6 +329,11 @@ OPTIONS
>>         enable deferred user callchain which will collect user-space callchains
>>         when the thread returns to the user space.
>>
>> +       On s390, the "fp" method uses back chain instead of frame pointers,
>> +       which requires user space to maintain a back chain (e.g. compiler
>> +       option -mbackchain).  Without a maintained back chain this produces
>> +       bogus call graphs.  In that case use "dwarf" instead.
>> +
>>  -q::
>>  --quiet::
>>         Don't print any warnings or messages, useful for scripting.
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index d4cb455f4a7d..55297b8dab37 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -1092,9 +1092,9 @@ static void __evsel__config_callchain(struct evsel *evsel, const struct record_o
>>         bool function = evsel__is_function_event(evsel);
>>         struct perf_event_attr *attr = &evsel->core.attr;
>>
>> -       if (EM_HOST == EM_S390 && param->record_mode == CALLCHAIN_FP) {
>> -               pr_warning_once(
>> -                       "Framepointer unwinding lacks kernel support. Use '--call-graph dwarf'\n");
>> +       if (EM_HOST == EM_S390 && evsel->pmu && !perf_pmu__is_software(evsel->pmu)) {
>> +               pr_warning_once("Cannot use hardware PMU 'cpum_cf' / 'cpum_sf' event with callchain. "
>> +                               "Use a software 'cpu-clock' / 'task-clock' event.\n");
>>         }
>>
>>         evsel__set_sample_bit(evsel, CALLCHAIN);
>>
>> base-commit: 93f51579e7df248780214094418f205253383cc5
>> --
>> 2.53.0
>>

Reviewed-by: Thomas Richter <tmricht@linux.ibm.com>

-- 
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
--
IBM Deutschland Research & Development GmbH

Vorsitzender des Aufsichtsrats: Wolfgang Wendt

Geschäftsführung: David Faller

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