Re: [PATCH v2 2/2] perf evsel: Don't configure framepointer callchains on s390

[Ian Rogers]

On Thu, Mar 12, 2026 at 5:45 AM Thomas Richter <tmricht@xxxxxxxxxxxxx> wrote:
>
> On 3/12/26 07:16, Ian Rogers wrote:
> > Frame pointer callchains are not supported on s390. Ignore the option
> > and print a warning.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> > v2: Only disable user callchains as AI is telling me native "kernel"
> >     callchains are supported on s390.
> > ---
> >  tools/perf/util/evsel.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> > index bd14d9bbc91f..fa21b48cba86 100644
> > --- a/tools/perf/util/evsel.c
> > +++ b/tools/perf/util/evsel.c
> > @@ -1076,6 +1076,12 @@ static void __evsel__config_callchain(struct evsel *evsel, struct record_opts *o
> >               attr->exclude_callchain_user = 1;
> >       }
> >
> > +     if (EM_HOST == EM_S390 && (evsel->core.attr.sample_type & PERF_SAMPLE_CALLCHAIN) &&
> > +         !evsel->core.attr.exclude_callchain_user) {
> > +             pr_warning("Excluding user callchains that are not supported on s390. Try '--call-graph dwarf'\n");
> > +             evsel->core.attr.exclude_callchain_user = 1;
> > +     }
> > +
> >       if (param->defer && !attr->exclude_callchain_user)
> >               attr->defer_callchain = 1;
> >  }
>
> Ian, thanks very much.
> Your patch set helps a lot. However there is a small nit (which is mandatory). Please add these lines
>
>   evsel->core.attr.sample_type &= ~PERF_SAMPLE_CALLCHAIN;
>   evsel->core.attr.sample_type &= ~PERF_SAMPLE_REGS_USER;
>   evsel->core.attr.sample_type &= ~PERF_SAMPLE_STACK_USER;

So these lines are dropping callchain from the sample_type which means
the kernel stack won't be sampled. AI was telling me this worked, but
I'm guess it was wrong. I think rather than this it is just cleaner
never to set the bits in the perf_event_attr, more like what v1 of the
patch did:
https://lore.kernel.org/lkml/20260312031928.1494864-3-irogers@xxxxxxxxxx/

> to the new if(EM_HOST == ...) above.
> The s390 CPU Measurement sampling device driver does not check on the attr.core.exclude_callchain_user
> member, but on the sample_type bit mask. It returns -EOPNOTSUPP when this bit PERF_SAMPLE_CALLCHAIN
> is set. This solves the invocation with command line flag -g as in
>  # ./perf record -v -e cycles  -g  -- perf test -w noploop
>  ...
>  perf record: Captured and wrote 0.183 MB perf.data ]

Right because the callchain was removed from all the samples. We can't
fix old kernels (other than by using fix tags); is there a possibility
of adding the exclude_callchain_user to the s390 perf driver for the
sake of kernel callchains? It seems better than providing no
callchain.

> Also I discovered that the fallback when using --call-graph dwarf command line flag still fails:
>  # ./perf record -v -e cycles  --call-graph dwarf -- perf test -w noploop
>  ...
>  Warning:
>  Trying to fall back to excluding guest samples
>  Error:
>  Failure to open event 'cycles:H' on PMU 'cpum_cf' which will be removed.
>  cycles:H: PMU Hardware doesn't support sampling overflow-interrupts. Try 'perf stat'
>  Error:
>  Failure to open any events for recording.
>
> The reason is in __evsel__config_callchain() which calls evsel__set_sample_bit(evsel, CALLCHAIN)
> and sets the PERF_SAMPLE_CALLCHAIN bit in evsel->core.attr.sample_type. It also sets the
> member attr->exclude_callchain_user = 1 and sets bits REGS_USER and _STACK_USER.
> All three bits are not supported by s390.

I'm confused by this and your previous testing that showed the
`--call-graph dwarf` worked. You need the sampled registers for dwarf
unwinding to provide initial register values for the unwinder.

> I have modified your 2nd patch and appended it.
>
> I find all these bits in sample_type and the attr.exclude_XXX stuff very confusing. If there
> is a more consistant way of checking these feature, please let me know.

Ok, let me check it out.

> Thanks again for looking into this.

Thanks,
Ian

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