linux-kernel - Re: [PATCH v3 07/12] perf record: init data file at mmap buffer object

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Date:   Mon, 1 Mar 2021 16:33:01 +0300
From:   "Bayduraev, Alexey V" <alexey.v.bayduraev@...ux.intel.com>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     Alexei Budankov <abudankov@...wei.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Antonov <alexander.antonov@...ux.intel.com>
Subject: Re: [PATCH v3 07/12] perf record: init data file at mmap buffer
 object

On 01.03.2021 14:44, Namhyung Kim wrote:
> Hello,
> 
> On Mon, Mar 1, 2021 at 8:16 PM Bayduraev, Alexey V
> <alexey.v.bayduraev@...ux.intel.com> wrote:
>>
>> Hi,
>>
>> On 20.11.2020 13:49, Namhyung Kim wrote:
>>> On Mon, Nov 16, 2020 at 03:19:41PM +0300, Alexey Budankov wrote:
>>
>> <SNIP>
>>
>>>>
>>>> @@ -1400,8 +1417,12 @@ static int record__mmap_read_evlist(struct record *rec, struct evlist *evlist,
>>>>      /*
>>>>       * Mark the round finished in case we wrote
>>>>       * at least one event.
>>>> +     *
>>>> +     * No need for round events in directory mode,
>>>> +     * because per-cpu maps and files have data
>>>> +     * sorted by kernel.
>>>
>>> But it's not just for single cpu since task can migrate so we need to
>>> look at other cpu's data too.  Thus we use the ordered events queue
>>> and round events help to determine when to flush the data.  Without
>>> the round events, it'd consume huge amount of memory during report.
>>>
>>> If we separate tracking records and process them first, we should be
>>> able to process samples immediately without sorting them in the
>>> ordered event queue.  This will save both cpu cycles and memory
>>> footprint significantly IMHO.
>>>
>>> Thanks,
>>> Namhyung
>>>
>>
>> As far as I understand, to split tracing records (FORK/MMAP/COMM) into
>> a separate file, we need to implement a runtime trace decoder on the
>> perf-record side to recognize such tracing records coming from the kernel.
>> Is that what you mean?
> 
> No, I meant separating the mmap buffers so that the record process
> can save the data without decoding.
> 

Thanks,

Do you think this can be implemented only on the user side by creating a dummy
event and manipulating by mmap/comm/task flags of struct perf_event_attr?
Or some changes on the kernel side are necessary?

Regards,
Alexey

>>
>> IMHO this can be tricky to implement and adds some overhead that can lead
>> to possible data loss. Do you have any other ideas how to optimize memory
>> consumption on perf-report side without a runtime trace decoder?
>> Maybe "round events" would somehow help in directory mode?
>>
>> BTW In our tool we use another approach: two-pass trace file loading.
>> The first loads tracing records, the second loads samples.
> 
> Yeah, something like that.  With the separated data, we can do it
> more efficiently IMHO.
> 
> Thanks,
> Namhyung
>