linux-kernel - Re: [PATCH V6 04/16] rv/include: Add deterministic automata monitor definition via C macros

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Date:   Thu, 21 Jul 2022 09:59:24 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Daniel Bristot de Oliveira <bristot@...nel.org>
Cc:     Wim Van Sebroeck <wim@...ux-watchdog.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marco Elver <elver@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Gabriele Paoloni <gpaoloni@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Tao Zhou <tao.zhou@...ux.dev>,
        Randy Dunlap <rdunlap@...radead.org>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-trace-devel@...r.kernel.org
Subject: Re: [PATCH V6 04/16] rv/include: Add deterministic automata monitor
 definition via C macros

On Thu, 21 Jul 2022 14:08:38 +0200
Daniel Bristot de Oliveira <bristot@...nel.org> wrote:

> On 7/20/22 22:06, Steven Rostedt wrote:
> >> +/*												\
> >> + * da_monitor_enabled_##name - checks if the monitor is enabled					\
> >> + */												\
> >> +static inline bool da_monitor_enabled_##name(void)						\
> >> +{												\  
> > Should we add a:
> > 
> > 	smp_rmb();
> > 
> > here? And then a smp_wmb() where these switches get updated?
> >  
> 
> Makes sense.
> 
> Should I also add the READ_ONCE/WRITE_ONCE? like
> 
> smp_rmb()
> READ_ONCE(var)
> 
> WRITE_ONCE(var, value)
> smp_wmb()

I'm not sure the WRITE_ONCE() is necessary with the memory barriers.
Because they should also prevent gcc from doing anything after that
barrier. As Linus once stated, most cases WRITE_ONCE() is useless, but it's
fine to keep more for annotation (as to pair with the READ_ONCE()) than for
anything that is critical.

> 
> for all these on/off knobs, or just the barriers?
> 
> > I guess how critical is it that these turn off immediately after the switch
> > is flipped?  
> 
> It is not critical to continue the execution of those that have already crossed by
> the variable. Still, waiting for the tracepoints to finish their execution before
> returning to the user-space task that disabled the variable might be a good thing.

You mean after disabling, to wait for the tracepoints that are currently
running to end?

> 
> IIRC, we can do that via RCU... like, synchronize_rcu()?

We have tracepoint_synchronize_unregister() that does that, as some
traceponits use SRCU and not RCU.

-- Steve


> 
> >> +	/* global switch */									\
> >> +	if (unlikely(!rv_monitoring_on()))							\
> >> +		return 0;									\
> >> +												\
> >> +	/* monitor enabled */									\
> >> +	if (unlikely(!rv_##name.enabled))							\
> >> +		return 0;									\
> >> +												\
> >> +	return 1;										\
> >> +}												\
> >> +												\