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 18:17:17 +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 2023/7/18 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@...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?
> 
> 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*

Thanks,
Yang