linux-kernel - Re: [PATCH v2] perf evsel: Don't set sample_regs_intr/sample_regs

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Date:   Thu, 23 Jul 2020 09:01:46 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     acme@...nel.org, jolsa@...nel.org, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        Linux-kernel@...r.kernel.org, ak@...ux.intel.com,
        kan.liang@...el.com, yao.jin@...el.com, irogers@...gle.com,
        Adrian Hunter <adrian.hunter@...el.com>
Subject: Re: [PATCH v2] perf evsel: Don't set
 sample_regs_intr/sample_regs_user for dummy event

Hi Jiri, Adrian,

On 7/22/2020 7:08 PM, Jiri Olsa wrote:
> On Wed, Jul 22, 2020 at 01:00:03PM +0800, Jin, Yao wrote:
> 
> SNIP
> 
>>>>
>>>> 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@...ux.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?
> 
> I was just curious, because I did not follow the no_aux_samples
> usage in detail.. so how about a case where:
> 
>     evsel->no_aux_samples == true and evsel__is_dummy_event(evsel) = false
> 
> then the original condition will be false for non dummy event
> 
>    (opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel))
> 
> is that ok?
> 

I searched the perf source and found the no_aux_samples was only set to true in intel-pt.c. So I 
assume for the non-pt usage, the no_aux_samples is always false.

For non-pt usage,
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is equal to
(opts->sample_intr_regs && !evsel__is_dummy_event(evsel))

For pt usage, we need to consider the case that evsel__is_dummy_event(evsel) is true or false.

If evsel__is_dummy_event(evsel) is true:
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is false.
It's expected.

If evsel__is_dummy_event(evsel) is false:
(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel)) is equal to
(opts->sample_intr_regs && !evsel->no_aux_samples)
That's the current code logic.

So I think the condition "(opts->sample_intr_regs && !evsel->no_aux_samples && 
!evsel__is_dummy_event(evsel))" looks reasonable.

Adrian, please correct me if I'm wrong here.

Thanks
Jin Yao

> jirka
>