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Date:   Tue, 18 Jul 2023 13:29:57 +0300
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Subject: Re: [PATCH v2 5/7] perf evlist: Skip dummy event sample_type check
 for evlist_config
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To:     Yang Jihong <yangjihong1@huawei.com>, peterz@infradead.org,
        mingo@redhat.com, acme@kernel.org, mark.rutland@arm.com,
        alexander.shishkin@linux.intel.com, jolsa@kernel.org,
        namhyung@kernel.org, irogers@google.com, kan.liang@linux.intel.com,
        james.clark@arm.com, tmricht@linux.ibm.com, ak@linux.intel.com,
        anshuman.khandual@arm.com, linux-kernel@vger.kernel.org,
        linux-perf-users@vger.kernel.org
References: <20230715032915.97146-1-yangjihong1@huawei.com>
 <20230715032915.97146-6-yangjihong1@huawei.com>
 <5797e5a7-a85f-4f7c-1649-88f8f9ff7a6b@intel.com>
 <44645529-0ee6-fe69-bc03-fefbc6f73d4d@huawei.com>
 <c4b7fb70-7b2e-74e9-576f-33b29e8801cd@intel.com>
 <77ea9309-da6f-f7b9-a822-b371e0f832d3@huawei.com>
From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
In-Reply-To: <77ea9309-da6f-f7b9-a822-b371e0f832d3@huawei.com>
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On 18/07/23 13:17, Yang Jihong wrote:
> Hello,
> 
> On 2023/7/18 17:56, Adrian Hunter wrote:
>> On 18/07/23 12:30, Yang Jihong wrote:
>>> 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@huawei.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?
>>
>> No.  There are additional members as defined by struct sample_id for PERF_RECORD_MMAP:
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/perf_event.h?h=v6.4#n872
>>
> I'm sorry, maybe my comments didn't make it clear.
> I mean, can we skip the "use_sample_identifier" check here?
> 
> That is, set sample_type to *XXX|PERF_SAMPLE_ID* instead of *XXX|PERF_SAMPLE_IDENTIFICATION*

In general, when there are different values of sample_type, the PERF_SAMPLE_ID is needed to determine which is which.
But PERF_SAMPLE_ID is not at a fixed position, so the sample_type is needed to find it.  That is why PERF_SAMPLE_IDENTIFIER is better.

Why do want to change it?