Re: On trace_*_rcuidle functions in modules

[John Stultz <john.stultz@linaro.org>]

On Tue, Apr 14, 2020 at 7:57 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Apr 14, 2020 at 07:20:01PM -0700, John Stultz wrote:
> > Hey folks,
> >   So recently I was looking at converting some drivers to be loadable
> > modules instead of built-in only, and one of my patches just landed in
> > -next and started getting build error reports.
> >
> > It ends up, recently in the merge window, the driver I was converting
> > to module switched a trace_*() function to trace_*_rcuidle() to fix a
> > bug.  Now when building as a module, if tracing is configured on, it
> > can't seem to find the trace_*_rcuidle() symbol.
> >
> > This is because, as you are aware, we don't declare trace_*_rcuidle
> > functions in modules - and haven't for quite some time:
> >   https://lore.kernel.org/lkml/20120905062306.GA14756@leaf/
> >
> > I wanted to better understand the background rationale for that patch,
> > to understand if not exporting the rcu_idle_exit and rcu_idle_enter,
> > calls was because they weren't used or if it was a more intentional
> > decision to avoid allowing modules to use them.
> >
> > Would it be reasonable to revisit that patch? Or is there some
> > recommended alternative solution?
>
> I will defer to Steven and Josh on the rationale.  (Cowardly of me,
> I know!)
>
> What I do is to maintain a wrapper for tracepoints within a built-in
> portion of RCU, export the wrapper, and invoke the wrapper from the
> rcutorture module.  Maybe you can do something similar?

That feels a little hackish, but I guess if there isn't a better option...

> But why would a module be invoked from the idle loop?  Is the module
> supplying an idle driver or some such?

The driver (qcom rpmh driver) registers a cpu_pm notifier callback,
which gets called when entering idle.

thanks
-john