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

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Date:   Tue, 4 Aug 2020 10:06:56 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     "Jin, Yao" <yao.jin@...ux.intel.com>, 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
Subject: Re: [PATCH v2] perf evsel: Don't set
 sample_regs_intr/sample_regs_user for dummy event

On 29/07/20 10:23 am, Jin, Yao wrote:
> 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))"

Sorry for slow response - I've been away.

This is fine.  It will not break PT.

no_aux_samples is useful for evsels that have been added by the code rather
than requested by the user.  For old kernels PT adds sched_switch tracepoint
to track context switches (before the current context switch event was
added) and having auxiliary sample information unnecessarily uses up space
in the perf buffer.

Acked-by: Adrian Hunter <adrian.hunter@...el.com>

> 
> 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
>>>