Re: [PATCH v2] perf evsel: Don't set sample_regs_intr/sample_regs_user for dummy event

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

Hi Jiri,

On 7/20/2020 5:17 PM, Jiri Olsa wrote:
> On Mon, Jul 20, 2020 at 09:00:13AM +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'
>>
>> 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:
>>
>> /* 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 not be set with
>> PERF_PMU_CAP_EXTENDED_REGS. But unfortunately now, 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 and attr->sample_regs_user.
>>
>> 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) ]
>>
>>   v2:
>>   ---
>>   Rebase to perf/core
>>
>> 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 | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
>> index 9aa51a65593d..11794d3b7879 100644
>> --- a/tools/perf/util/evsel.c
>> +++ b/tools/perf/util/evsel.c
>> @@ -1014,12 +1014,14 @@ 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 && !evsel->no_aux_samples) {
>> +	if (opts->sample_intr_regs && !evsel->no_aux_samples &&
>> +	    !evsel__is_dummy_event(evsel)) {
> 
> hum, I thought it'd look something like this:
> 
>    if (opts->sample_intr_regs && (!evsel->no_aux_samples || !evsel__is_dummy_event(evsel))
> 
> but I'm not sure how no_aux_samples flag works exactly.. so it might be
> correct.. just making sure ;-)
> 
> cc-ing Adrian
> 
> jirka
> 
> 

no_aux_samples is set to false by default and it's only set to true by pt, right?

So most of the time, !evsel->no_aux_samples is always true.

if (opts->sample_intr_regs && (!evsel->no_aux_samples || !evsel__is_dummy_event(evsel)) {
	attr->sample_regs_intr = opts->sample_intr_regs;
	evsel__set_sample_bit(evsel, REGS_INTR);
}

So even if the evsel is dummy event, the condition check is true. :(

Or maybe I misunderstand anything?

Thanks
Jin Yao

>>   		attr->sample_regs_intr = opts->sample_intr_regs;
>>   		evsel__set_sample_bit(evsel, REGS_INTR);
>>   	}
>>   
>> -	if (opts->sample_user_regs && !evsel->no_aux_samples) {
>> +	if (opts->sample_user_regs && !evsel->no_aux_samples &&
>> +	    !evsel__is_dummy_event(evsel)) {
>>   		attr->sample_regs_user |= opts->sample_user_regs;
>>   		evsel__set_sample_bit(evsel, REGS_USER);
>>   	}
>> -- 
>> 2.17.1
>>
>