Date:   Thu, 11 Jan 2018 15:18:16 -0700
From:   Mathieu Poirier <mathieu.poirier@...aro.org>
To:     Kim Phillips <kim.phillips@....com>
Cc:     Mark Brown <broonie@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        namhyung@...nel.org, Adrian Hunter <adrian.hunter@...el.com>,
        Mike Leach <mike.leach@....com>, suzuki.poulosi@....com,
        "Jeremiassen, Tor" <tor@...com>, Jiri Olsa <jolsa@...hat.com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 00/10] perf tools: Add support for CoreSight trace decoding

On 11 January 2018 at 14:49, Kim Phillips <kim.phillips@....com> wrote:
> On Thu, 11 Jan 2018 14:11:00 -0700
> Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
>
>> On 11 January 2018 at 10:28, Kim Phillips <kim.phillips@....com> wrote:
>> > On Thu, 11 Jan 2018 08:45:21 -0700
>> > Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
>> >
>> >> On 11 January 2018 at 05:23, Mark Brown <broonie@...nel.org> wrote:
>> >> > On Wed, Jan 10, 2018 at 06:08:21PM -0600, Kim Phillips wrote:
>> >> >> Mathieu Poirier <mathieu.poirier@...aro.org> wrote:
>> >> >
>> >> >> > Instructions on how to build and install the openCSD library are provided
>> >> >> > in the HOWTO.md of the project repository.
>> >> >
>> >> >> Usually when a perf builder sees something they need "on," they - or,
>> >> >> at least I - start querying the host's package manager for something
>> >> >> that provides it (e.g., apt search/install libopencsd), but since no
>> >> >> distro provides libopencsd, this is bad because it misleads the user.
>> >> >
>> >> > It's on the radar to push this at distros fairly soon.
>> >
>> > Adding packages to distros takes years, this patchset is being
>> > submitted for inclusion *now*.  So until then, it would greatly
>> > facilitate users if the relevant libopencsd source files were
>> > self-contained within perf from the get go.
>>
>> I do not agree with you on the front that it takes years.  On the flip
>> side it would take a significant amount of time and effort to refactor
>> the openCSD library so that it can be added to the kernel tree.  This
>
> The dtc wasn't refactored before it was added to the kernel tree.
>
>> patchset is available now with a solution that follows what has
>> already been done for dozens of other external library.  There is no
>> point in delaying the inclusion of the functionality when an
>> end-to-end solution exists.
>
> See above: I'm not necessarily suggesting the code get refactored.
>
>> >> >  Part of the
>> >> > discussion was wanting to get things to the point where the tools using
>> >> > the library were far enough along that we could be reasonably sure that
>> >
>> > Curious, what other tools are there?
>>
>> Ask around at ARM.
>
> I'm asking the person that claimed it.
>
>> >> > there weren't any problems that were going to require ABI breaks to fix
>> >> > before pushing the library at distros since ABI churn isn't nice for
>> >> > packagers to deal with.
>> >
>> > Why make perf the guinea pig?  Whatever, this doesn't preclude
>> > adding the code into the tree; it can be removed years from now when
>> > libopencsd becomes ubiquitous among distros.
>>
>> The same can be said about proceeding the other way around - the
>> openCSD library can be added to the kernel tree later if it is deemed
>> necessary.  Until then I really don't see why we'd prevent people from
>> accessing the functionality.
>
> Again, I'm not suggesting the code be refactored...
>
>> >> > There's also a bit of a chicken and egg problem
>> >> > in that it's a lot easier to get distros to package libraries that have
>> >> > users available (some are not really bothered about this of course but
>> >> > it still helps).
>> >>
>> >> Moreover including in the kernel tree every library that can
>> >> potentially be used by the perf tools simply doesn't scale.
>> >
>> > This is a trace decoder library we're talking about:  there are no
>> > others in perf's system features autodetection list.  And why wouldn't
>> > adding such libraries scale?
>>
>> I don't see why a decoder library and say, libelf, need to be treated
>> differently.
>
> libelf is a mature library based on an industry-wide standard, not to
> mention already packaged by most (all?) distros.
>
>> >>  The perf
>> >> tools project has come up with a very cleaver way to deal with
>> >> external dependencies and I don't see why the OpenCSD library should
>> >> be different.
>> >
>> > Again, the opencsd library is a decoder library:  this patchseries adds
>> > it as a package dependency (when it isn't even a package in any
>> > distro), and it's different in that it's the first decoder library to
>> > be submitted as an external dependency (i.e., not fully built-in, like
>> > Intel's, or even the Arm SPE's pending submission).
>>
>> I don't see why we absolutely need to do exactly the same as Intel.
>> The library is public and this patchset neatly integrates it with the
>> perf tools.
>
> We don't, but it'd be more efficient, upstream-acceptance-wise, but as
> you brought up above, we wouldn't be able to since we'd have to rewrite
> libopencsd to conform to upstream codingstyle, etc., so I'm suggesting
> we might look at a better enablement strategy like how the dtc works.
>
> It'd be nice if the upstream maintainers would comment on what would be
> acceptable instead of us going back and forth between each other.

Agreed.

>
>> >> >> Keeping the library external will also inevitably introduce more
>> >> >> source level synchronization problems because the perf sources being
>> >> >> built may not be compatible with their version of the library, whether
>> >> >> due to new features like new trace hardware support, or API changes.
>> >> >
>> >> > Perf users installing from source rather than from a package (who do
>> >> > tend to the more technical side even for kernel developers) already have
>> >> > to cope with potentially installing at least dwarf, gtk2, libaudit,
>> >> > libbfd, libelf, libnuma, libperl, libpython, libslang, libcrypto,
>> >> > libunwind, libdw-dwarf-unwind, zlib, lzma, bpf and OpenJDK depending on
>> >> > which features they want.  I'm not sure that adding one more library is
>> >> > going to be the end of the world here, especially once the packaging
>> >> > starts to filter through distros.  Until that happens at least people
>> >> > are no worse off for not having the feature.
>> >>
>> >> I completely agree.  Just like any other package, people that want the
>> >> very latest code need to install from source.
>> >
>> > A fully-integrated solution would work better for people, e.g., how are
>> > people supposed to know what 'latest' is when there are separate,
>> > unsynchronized git repos?
>>
>> The same applies to any of the other libraries perf is working with.
>
> The packaged libraries?  They are stable:  they don't come in the form
> of cloning a git repo and building from scratch.
>
> The decoder libraries?  They are self-contained within perf.
>
>> >> >> As Mark Brown (cc'd) mentioned on the Coresight mailing list, this may
>> >> >> be able to be done the same way the dtc is incorporated into the
>> >> >> kernel, where only its relevant sources are included and updated as
>> >> >> needed:  see linux/scripts/dtc/update-dtc-source.sh.
>> >> >
>> >> > Bear in mind that we need dtc for essentially all kernel development on
>> >> > ARM and when it was introduced it was a new requirement for existing
>> >> > systems, it's a bit of a different case here where it's an optional
>> >> > feature in an optional tool.
>> >
>> > That argument applies to Intel-PT, yet its decoder is self-contained
>> > within perf: all non-x86 perf binaries are capable of decoding PT.
>> > We'd want that for Arm Coresight where perf gets statically built to
>> > run on much more constrained systems like Android.
>>
>> Traces can't be decoded properly without the support of external
>> libraries, whether we are talking about PT or CS.
>
> Not true; perf has PT decoding self-contained.
>
> Thanks,
>
> Kim

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