linux-kernel - Re: On trace_*_rcuidle functions in modules

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Message-ID: <20200415025748.GV17661@paulmck-ThinkPad-P72>
Date:   Tue, 14 Apr 2020 19:57:49 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Saravana Kannan <saravanak@...gle.com>,
        Todd Kjos <tkjos@...gle.com>, Stephen Boyd <sboyd@...nel.org>
Subject: Re: On trace_*_rcuidle functions in modules

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?

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

							Thanx, Paul