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

["Jin, Yao" <yao.jin@linux.intel.com>]

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@linux.intel.com> 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@intel.com/
>>>
>>> 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@linux.intel.com>
>>>> ---
>>>>    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
>>>>
>>>