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

[Song Liu]

Hi Jiri,

> On Jul 21, 2019, at 4:23 AM, Jiri Olsa <jolsa@xxxxxxxxxx> 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