From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754379AbaE1ROn (ORCPT ); Wed, 28 May 2014 13:14:43 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.226]:17532 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751518AbaE1ROm (ORCPT ); Wed, 28 May 2014 13:14:42 -0400 Date: Wed, 28 May 2014 13:14:40 -0400 From: Steven Rostedt To: LKML Cc: Peter Zijlstra , "Theodore Ts'o" , Ingo Molnar , Thomas Gleixner , Andrew Morton , Benjamin Herrenschmidt , Peter Hurley Subject: [RFC][PATCH] tracing: Print nasty banner when trace_printk() is in use Message-ID: <20140528131440.2283213c@gandalf.local.home> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.23; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-RR-Connecting-IP: 107.14.168.142:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? Not-yet-signed-off-by: Steven Rostedt --- Ted, I noticed that in fs/ext4/inline.c you use trace_printk() if INLINE_DIR_DEBUG is defined. I'm assuming that this is not a production setting and this banner should not affect you at all. Am I correct? There's some other places that use it, but those are obviously debug only. diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c index 0543169..068f453 100644 --- a/kernel/trace/trace.c +++ b/kernel/trace/trace.c @@ -1975,7 +1975,11 @@ void trace_printk_init_buffers(void) if (alloc_percpu_trace_buffer()) return; - pr_info("ftrace: Allocated trace_printk buffers\n"); + pr_warning("****************************************\n"); + pr_warning("** NOTICE NOTICE NOTICE NOTICE NOTICE **\n"); + pr_warning("** trace_printk() being used. **\n"); + pr_warning("** Allocating extra memory for it **\n"); + pr_warning("****************************************\n"); /* Expand the buffers to set size */ tracing_update_buffers();