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

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Date:   Thu, 14 Jul 2022 11:17:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Petr Mladek <pmladek@...e.com>
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, 14 Jul 2022 16:53:24 +0200
Petr Mladek <pmladek@...e.com> wrote:

> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2108,7 +2108,15 @@ static u16 printk_sprint(char *text, u16 size, int facility,
>  		}
>  	}
>  
> -	trace_console_rcuidle(text, text_len);
> +	/*
> +	 * trace_console_idle() is not working in NMI. printk()
> +	 * is used more often in NMI than in rcuidle context.
> +	 * Choose the less evil solution here.
> +	 *
> +	 * smp_processor_id() is reliable in rcuidle context.
> +	 */
> +	if (!rcu_is_idle_cpu(smp_processor_id()))
> +		trace_console(text, text_len);
>  
>  	return text_len;
>  }
> -- 

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.

-- Steve

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)
 	),