Re: [PATCH v2 3/4] perf build: Use libtraceevent from the system

[Namhyung Kim]

On Wed, Nov 30, 2022 at 12:13 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
>
> On Wed, Nov 30, 2022 at 11:05 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> >
> > On Tue, Nov 29, 2022 at 10:30 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> > >
> > > Remove the LIBTRACEEVENT_DYNAMIC and LIBTRACEFS_DYNAMIC. If
> > > libtraceevent isn't installed or NO_LIBTRACEEVENT=1 is passed to the
> > > build, don't compile in libtraceevent and libtracefs support. This
> > > also disables CONFIG_TRACE that controls "perf
> > > trace". CONFIG_TRACEEVENT is used to control enablement in
> > > Build/Makefiles, HAVE_LIBTRACEEVENT is used in C code. Without
> > > HAVE_LIBTRACEEVENT tracepoints are disabled and as such the commands
> > > kmem, kwork, lock, sched and timechart are removed. The majority of
> > > commands continue to work including "perf test".
> >
> > Maybe we can have a different approach.  I guess the trace data
> > access is isolated then we can make dummy interfaces when there's
> > no libtraceevent.  This way we don't need to touch every command
> > and let it fail when it's asked.
>
> Sounds like a worthwhile refactor that can land on top of this change.
>
> > The motivation is that we should be able to run the sub-commands
> > as much as possible.  In fact, we could run 'record' part only on the
> > target machine and pass the data to the host for analysis with a
> > full-fledged perf.  Also some commands like 'perf lock contention'
> > can run with or without libtraceevent (using BPF only).
>
> The issue here is that perf lock contention will use evsel__new_tp and
> internally that uses libtraceevent. As such it is removed without
> HAVE_LIBTRACEEVENT. Without the evsel there's not much perf lock
> contention can do, so rather than litter the code with
> HAVE_LIBTRACEEVENT and for it to be broken, I made the choice just to
> remove it from the no libtraceevent build for now.

I don't think it needs evsel__new_tp() when BPF is used.
The BPF program is attached to the raw tracepoint without
perf_event and the result is written to the BPF map.

>
> I think it is worth pursuing these patches in the shape they are in so
> that we can land the removal of tools/lib/traceevent and ensure the
> migration away from an out-of-date version of that library.

Yeah, I agree that we should remove the stale libtraceevent but
I'd like to do it with minimal changes in the perf code base.
Let me take a look at this.

Thanks,
Namhyung