netdev - Re: [PATCH v2 perf,bpf 08/11] perf, bpf: save btf information as headers to perf.data

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Date:   Fri, 15 Feb 2019 18:59:14 +0000
From:   Song Liu <songliubraving@...com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>
CC:     Jiri Olsa <jolsa@...nel.org>,
        Stephane Eranian <eranian@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "namhyung@...nel.org" <namhyung@...nel.org>
Subject: Re: [PATCH v2 perf,bpf 08/11] perf, bpf: save btf information as
 headers to perf.data



> On Feb 15, 2019, at 10:20 AM, Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
> 
> Em Fri, Feb 15, 2019 at 05:47:58PM +0000, Song Liu escreveu:
>> 
>> 
>>> On Feb 15, 2019, at 9:40 AM, Arnaldo Carvalho de Melo <acme@...nel.org> wrote:
>>> 
>>> Em Fri, Feb 15, 2019 at 05:25:01PM +0000, Song Liu escreveu:
>>>>> On Feb 15, 2019, at 6:26 AM, Arnaldo Carvalho de Melo <acme@...hat.com> wrote:
>>>>> Em Thu, Feb 14, 2019 at 04:00:09PM -0800, Song Liu escreveu:
>>>>>> This patch enables perf-record to save btf information as headers to
>>>>>> perf.data A new header type HEADER_BTF is introduced for this data.
>>> 
>>>>> 	Wouldn't it be better for this HEADER_BTF to be introduced
>>>>> already as an user space event, Song, see:
>>> 
>>>>> tools/perf/util/event.h
>>> 
>>>>> and:
>>> 
>>>>> tools/perf/util/event.c
>>> 
>>>>> perf_event__synthesize_cpu_map()
>>> 
>>>> BTF would be short living for short living BPF programs. I guess 
>>>> saving them as header is easier than merging them with samples. 
>>> 
>>>> What's the benefit of saving them as user space events?
>>> 
>>> When we work with pipe mode, i.e.:
>>> 
>>> 	perf record -o - | perf report -i -
>>> 
>>> and other combinations (with 'perf script', 'perf inject', etc), we need
>>> a way to pass the headers to the other side, and the way was via user
>>> space events.
>>> 
>>> This is something Stephane and Jiri have been discussing recently,
>>> probably they have more justifications, Stephane, Jiri?
>>> 
>>> - Arnaldo
>> 
>> I see. In this case, we will need some synchronization between main
>> thread and the polling thread, as they are both writing to the same
>> pipe. 
> 
> So, the whole context is that we need to have 'perf record' to start a
> thread per CPU and then read the already per-cpu mmap buffers in the
> matching thread, with the right affinity, numa settings to have the
> record phase not cause contention, etc, so it ends up dumping one stream
> per CPU in a separate file in a 'perf.data' directory instead of a
> perf.data file.
> 
> Jiri is working on that, so, if you dump one more stream into that
> directory, it would, at post processing time, be ordered together with
> the other stream, the per-cpu ones.
> 
> - Arnaldo

I see. This solution looks great. 

For this set, how about I keep this part as-is (at least for v3)? In 
this case, it will goes to the header file after Jiri's change. Once 
Jiri's work is done, I will move them into per-cpu files. 

Thanks,
Song