linux-kernel - Re: [PATCH -printk] printk, tracing: fix console tracepoint

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Date:   Fri, 15 Jul 2022 11:40:16 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Marco Elver <elver@...gle.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Berg <johannes.berg@...el.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>
Subject: Re: [PATCH -printk] printk, tracing: fix console tracepoint

On Thu 2022-07-14 11:17:49, Steven Rostedt wrote:
> Although printk is not really a fast path, you could do this and avoid the
> check when the trace event is not active:
> 
> (Not even compiled tested)
> 
> Tweaked the comment, and used raw_smp_processor_id() as I'm not sure we are
> in a preempt disabled context, and we don't care if we are not.

Makes sense.

> diff --git a/include/trace/events/printk.h b/include/trace/events/printk.h
> index 13d405b2fd8b..d0a5f63920bb 100644
> --- a/include/trace/events/printk.h
> +++ b/include/trace/events/printk.h
> @@ -7,11 +7,20 @@
>  
>  #include <linux/tracepoint.h>
>  
> -TRACE_EVENT(console,
> +TRACE_EVENT_CONDITION(console,
>  	TP_PROTO(const char *text, size_t len),
>  
>  	TP_ARGS(text, len),
>  
> +	/*
> +	 * trace_console_rcuidle() is not working in NMI. printk()
> +	 * is used more often in NMI than in rcuidle context.
> +	 * Choose the less evil solution here.
> +	 *
> +	 * raw_smp_processor_id() is reliable in rcuidle context.
> +	 */
> +	TP_CONDITION(!rcu_is_idle_cpu(raw_smp_processor_id())),
> +
>  	TP_STRUCT__entry(
>  		__dynamic_array(char, msg, len + 1)
>  	),

It looks better. I am going to test it and send as a proper patch
for review.

Thanks for help.

Best Regards,
Petr