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

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Message-ID: <26c5c095-2aff-3c7b-80ad-7b21702a3fdd@kernel.org>
Date:   Thu, 21 Jul 2022 18:38:42 +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 7/21/22 15:59, Steven Rostedt wrote:
> 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.

Ack, I can keep for annotation.

>>
>> 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?

yes, after disabling tracepoints.

>>
>> 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.

yep, that is it!
 
-- Daniel

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