linux-kernel - Re: [RFC 00/79] perf tools: Initial libperf separation

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Date:   Wed, 24 Jul 2019 07:42:50 +0000
From:   Song Liu <songliubraving@...com>
To:     Jiri Olsa <jolsa@...nel.org>
CC:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Stephane Eranian <eranian@...gle.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Alexander Shishkin" <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Andi Kleen <ak@...ux.intel.com>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Michael Petlan <mpetlan@...hat.com>
Subject: Re: [RFC 00/79] perf tools: Initial libperf separation

Hi Jiri,

> On Jul 21, 2019, at 4:23 AM, Jiri Olsa <jolsa@...nel.org> wrote:
> 
> hi,
> we have long term goal to separate some of the perf functionality
> into library. This patchset is initial effort on separating some
> of the interface.
> 
> Currently only the basic counting interface is exported, it allows
> to:
>  - create cpu/threads maps
>  - create evlist/evsel objects
>  - add evsel objects into evlist
>  - open/close evlist/evsel objects
>  - enable/disable events
>  - read evsel counts

Based on my understanding, evsel and evlist are abstractions in
perf utilities. I think most other tools that use perf UAPIs are 
not built based on these abstractions. I looked at a few internal
tools. Most of them just uses sys_perf_event_open() and struct 
perf_event_attr. I am not sure whether these tools would adopt
libperf, as libperf changes their existing concepts/abstractions.

> 
> The initial effort was to have total separation of the objects
> from perf code, but it showed not to be a good way. The amount
> of changed code was too big with high chance for regressions,
> mainly because of the code embedding one of the above objects
> statically.
> 
> We took the other approach of sharing the objects/struct details
> within the perf and libperf code. This way we can keep perf
> functionality without any major changes and the libperf users
> are still separated from the object/struct details. We can move
> to total libperf's objects separation gradually in future.

I found some duplicated logic between libperf and perf, for 
example, perf_evlist__open() and evlist__open(). Do we plan to 
merge them in the future? 

Thanks,
Song