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

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Date:   Wed, 29 Jul 2020 15:23:12 +0800
From:   "Jin, Yao" <yao.jin@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>,
        Adrian Hunter <adrian.hunter@...el.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
Subject: Re: [PATCH v2] perf evsel: Don't set
 sample_regs_intr/sample_regs_user for dummy event

Hi Adrian,

Could you help to check if following condition will break PT?

"(opts->sample_intr_regs && !evsel->no_aux_samples && !evsel__is_dummy_event(evsel))"

Thanks
Jin Yao

On 7/23/2020 9:01 AM, Jin, Yao wrote:
> 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
>>