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

[Song Liu]

> On Jul 24, 2019, at 1:33 AM, Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> 
> On Wed, Jul 24, 2019 at 07:42:50AM +0000, Song Liu wrote:
>> 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
> 
> AFAICS some abstraction is needed to carry on the needed stuff
> like mmaps, counts, group links, PMU details (type, cpus..)
> 
>> 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.
> 
> well, besides that we wanted to do this separation for tools/* sake,
> I think that once libperf shares more interface on sampling and pmu
> events parsing, it will be considerable choice also for out of the
> tree tools

Yeah, in tree tools would benefit from it for sure. And they should
also motivate out of the tree tools to use libperf. 

> 
>>> 
>>> 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? 
> 
> yea, as I wrote in the perf_evsel__open patch changelog:
> 
>  It's a simplified version of evsel__open without fallback
>  stuff. We can try to merge it in the future to libperf,
>  but it has many glitches.

I was reading the code in your git tree and missed the change 
log. 

Thanks for the explanations. 

Song