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

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Message-ID: <3c0a4cb5-f88f-ec5f-e614-d1e8ceb036c2@kernel.org>
Date:   Thu, 21 Jul 2022 14:08:38 +0200
From:   Daniel Bristot de Oliveira <bristot@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.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 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()

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.

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

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