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

[Song Liu]

> On Jul 24, 2019, at 6:50 AM, Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx> wrote:
> 
> Em Wed, Jul 24, 2019 at 07:42:50AM +0000, Song Liu escreveu:
>>> On Jul 21, 2019, at 4:23 AM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> 
>>> 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.
> 
> Right, and for now we're just trying to have something that is not so
> tied to perf and could possibly be useful outside tools/perf/ when the
> need arises for whatever new tool or pre-existing one.
> 
> There are features there that may be interesting to use outside perf,
> time will tell.

Thanks for the explanation. This is not an easy task. :)

> 
>>> 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? 
> 
> He is just slowly moving things to a public libperf while keeping perf
> working, in the end the goal is to have as much stuff that is not
> super specific to some of the existing perf tools
> (tools/perf/builtin-*.c) in libperf as possible.
> 
> It is still early in this effort, that is why he is still leaving it in
> tools/perf/lib/ and not in tools/lib/perf/ :-)

I saw that discussion. It is a good strategy. 

Thanks,
Song