linux-kernel - Re: [PATCH v2 5/7] perf evlist: Skip dummy event sample_type check for evlist

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Date:   Tue, 18 Jul 2023 17:30:48 +0800
From:   Yang Jihong <yangjihong1@...wei.com>
To:     Adrian Hunter <adrian.hunter@...el.com>, <peterz@...radead.org>,
        <mingo@...hat.com>, <acme@...nel.org>, <mark.rutland@....com>,
        <alexander.shishkin@...ux.intel.com>, <jolsa@...nel.org>,
        <namhyung@...nel.org>, <irogers@...gle.com>,
        <kan.liang@...ux.intel.com>, <james.clark@....com>,
        <tmricht@...ux.ibm.com>, <ak@...ux.intel.com>,
        <anshuman.khandual@....com>, <linux-kernel@...r.kernel.org>,
        <linux-perf-users@...r.kernel.org>
Subject: Re: [PATCH v2 5/7] perf evlist: Skip dummy event sample_type check
 for evlist_config

Hello,

On 2023/7/17 22:41, Adrian Hunter wrote:
> On 15/07/23 06:29, Yang Jihong wrote:
>> The dummp event does not contain sampls data. Therefore, sample_type does
>> not need to be checked.
>>
>> Currently, the sample id format of the actual sampling event may be changed
>> after the dummy event is added.
>>
>> Signed-off-by: Yang Jihong <yangjihong1@...wei.com>
>> ---
>>   tools/perf/util/record.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/tools/perf/util/record.c b/tools/perf/util/record.c
>> index 9eb5c6a08999..0240be3b340f 100644
>> --- a/tools/perf/util/record.c
>> +++ b/tools/perf/util/record.c
>> @@ -128,6 +128,13 @@ void evlist__config(struct evlist *evlist, struct record_opts *opts, struct call
>>   		evlist__for_each_entry(evlist, evsel) {
>>   			if (evsel->core.attr.sample_type == first->core.attr.sample_type)
>>   				continue;
>> +
>> +			/*
>> +			 * Skip the sample_type check for the dummy event
>> +			 * because it does not have any samples anyway.
>> +			 */
>> +			if (evsel__is_dummy_event(evsel))
>> +				continue;
> 
> Sideband event records have "ID samples" so the sample type still matters.
> 
Okay, will remove this patch in next version.

Can I ask a little more about this?

Use PERF_SAMPLE_IDENTIFICATION instead of PERF_SAMPLE_ID because for 
samples of type PERF_RECORD_SAMPLE, there may be different record 
formats due to different *sample_type* settings, so the fixed SAMPLE_ID 
  location mode PERF_SAMPLE_NAME is required here.

However, for the sideband event, the samples of the PERF_RECORD_SAMPLE 
type is not recorded (only PERF_RECORD_MMAP, PERF_RECORD_COMM, and so 
on). Therefore, the "use sample identifier "check can be skipped here.

That's my understanding of PERF_SAMPLE_IDENTIFICATION . If there is any 
error, please help to correct it.

*Sideband event records have "ID samples" so the sample type still matters.*

Does this mean that sideband will also record samples of type 
PERF_RECORD_SAMPLE? What exactly is the sampling data?

Thanks,
Yang