Re: [PATCH] perf evsel: Don't set sample_regs_intr/sample_regs_user for dummy event

[Ian Rogers]

On Fri, Jul 3, 2020 at 5:31 PM Jin, Yao <yao.jin@xxxxxxxxxxxxxxx> wrote:
>
> Hi Jiri,
>
> On 7/3/2020 7:00 PM, Jiri Olsa wrote:
> > On Fri, Jul 03, 2020 at 08:42:15AM +0800, Jin Yao wrote:
> >> Since commit 0a892c1c9472 ("perf record: Add dummy event during system wide synthesis"),
> >> a dummy event is added to capture mmaps.
> >>
> >> But if we run perf-record as,
> >>
> >>   # perf record -e cycles:p -IXMM0 -a -- sleep 1
> >>   Error:
> >>   dummy:HG: PMU Hardware doesn't support sampling/overflow-interrupts. Try 'perf stat'
> >>

Sorry for the breakage caused by modifying the dummy event. Could we
add a test to cover the issue? Perhaps in tools/perf/tests/shell/.
Trying to reproduce with a register on my skylakex on a 5.6.14 kernel
with:

$ perf record -e cycles:p -IAX -a -- sleep 1

succeeds.

Thanks,
Ian

> >> The issue is, if we enable the extended regs (-IXMM0), but the
> >> pmu->capabilities is not set with PERF_PMU_CAP_EXTENDED_REGS, the kernel
> >> will return -EOPNOTSUPP error.
> >>
> >> See following code pieces.
> >>
> >> /* in kernel/events/core.c */
> >> static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
> >>
> >> {
> >>      ....
> >>      if (!(pmu->capabilities & PERF_PMU_CAP_EXTENDED_REGS) &&
> >>          has_extended_regs(event))
> >>              ret = -EOPNOTSUPP;
> >>      ....
> >> }
> >>
> >> For software dummy event, the PMU should be not set with
> >> PERF_PMU_CAP_EXTENDED_REGS. But unfortunately in current code, the dummy
> >> event has possibility to be set with PERF_REG_EXTENDED_MASK bit.
> >>
> >> In evsel__config, /* tools/perf/util/evsel.c */
> >>
> >> if (opts->sample_intr_regs) {
> >>      attr->sample_regs_intr = opts->sample_intr_regs;
> >> }
> >>
> >> If we use -IXMM0, the attr>sample_regs_intr will be set with
> >> PERF_REG_EXTENDED_MASK bit.
> >>
> >> It doesn't make sense to set attr->sample_regs_intr for a
> >> software dummy event.
> >>
> >> This patch adds dummy event checking before setting
> >> attr->sample_regs_intr.
> >>
> >> After:
> >>    # ./perf record -e cycles:p -IXMM0 -a -- sleep 1
> >>    [ perf record: Woken up 1 times to write data ]
> >>    [ perf record: Captured and wrote 0.413 MB perf.data (45 samples) ]
> >
> > LGTM, Adrian (cc-ed) just added another check to the same place,
> > but it looks like both of them should be there:
> >
> >    https://lore.kernel.org/lkml/20200630133935.11150-2-adrian.hunter@xxxxxxxxx/
> >
> > jirka
> >
>
> Thanks Jiri! Yes, it looks like both of checks should be added here.
>
> So do I post v2 (just rebase) once Adrian's patch gets merged?
>
> Thanks
> Jin Yao
>
> >>
> >> Fixes: 0a892c1c9472 ("perf record: Add dummy event during system wide synthesis")
> >> Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
> >> ---
> >>   tools/perf/util/evsel.c | 4 ++--
> >>   1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> >> index 96e5171dce41..df3315543e86 100644
> >> --- a/tools/perf/util/evsel.c
> >> +++ b/tools/perf/util/evsel.c
> >> @@ -1020,12 +1020,12 @@ void evsel__config(struct evsel *evsel, struct record_opts *opts,
> >>      if (callchain && callchain->enabled && !evsel->no_aux_samples)
> >>              evsel__config_callchain(evsel, opts, callchain);
> >>
> >> -    if (opts->sample_intr_regs) {
> >> +    if (opts->sample_intr_regs && !is_dummy_event(evsel)) {
> >>              attr->sample_regs_intr = opts->sample_intr_regs;
> >>              evsel__set_sample_bit(evsel, REGS_INTR);
> >>      }
> >>
> >> -    if (opts->sample_user_regs) {
> >> +    if (opts->sample_user_regs && !is_dummy_event(evsel)) {
> >>              attr->sample_regs_user |= opts->sample_user_regs;
> >>              evsel__set_sample_bit(evsel, REGS_USER);
> >>      }
> >> --
> >> 2.17.1
> >>
> >