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

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Message-ID: <847940EC-6E2A-4985-AE19-27758C301B5D@fb.com>
Date:   Fri, 15 Feb 2019 17:47:58 +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 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. 

Thanks,
Song