linux-kernel - Re: [RFC][PATCH] tracing: Print nasty banner when trace

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Date:	Wed, 28 May 2014 19:22:39 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Theodore Ts'o <tytso@....edu>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Peter Hurley <peter@...leysoftware.com>
Subject: Re: [RFC][PATCH] tracing: Print nasty banner when trace_printk() is
 in use

On Wed, May 28, 2014 at 01:14:40PM -0400, Steven Rostedt wrote:
> 
> trace_printk() is used to debug fast paths within the kernel. Places
> that gets called in any context (interrupt or NMI) or thousands of
> times a second. Something you do not want to do with a printk().
> 
> In order to make it completely lockless as it needs a temporary buffer
> to handle some of the string formatting, a page is created per cpu for
> every context (four per cpu; normal, softirq, irq, NMI).
> 
> Since trace_printk() should only be used for debugging purposes,
> there's no reason to waste memory on these buffers on a production
> system. That means, trace_printk() should never be used unless a
> developer is debugging their kernel. There's macro magic to allocate
> the buffers if trace_printk() is used anywhere in the kernel.
> 
> To help enforce that trace_printk() isn't used outside of development,
> when it is used, a nasty banner is displayed on bootup (or when a module
> is loaded that uses trace_printk() and the kernel core does not).
> 
> Here's the banner:
> 
>  ****************************************
>  ** NOTICE NOTICE NOTICE NOTICE NOTICE **
>  ** trace_printk() being used.         **
>  ** Allocating extra memory for it     **
>  ****************************************
> 
> Hmm, maybe I should add "Not for production use" to scare people even
> more?

Does that really stop people from doing stupid? Wouldn't it be better to
make sure nobody merges a trace_printk() user in mainline? You can set
up a commit hook and check for +.*trace_printk or so.

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