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

[Jin, Yao]

Hi Ian,

On 7/6/2020 8:47 AM, Ian Rogers wrote:
> 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

-IAX should be no problem. The issue only occurs on the platform with extended regs supports, such 
as ICL. So I don't know if it's suitable to add it to perf test suite.

Thanks
Jin Yao

> > > > 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