From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B0F47EB64DB for ; Wed, 14 Jun 2023 13:44:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S245064AbjFNNoA (ORCPT ); Wed, 14 Jun 2023 09:44:00 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49318 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245057AbjFNNn5 (ORCPT ); Wed, 14 Jun 2023 09:43:57 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E9D7D1FEB; Wed, 14 Jun 2023 06:43:56 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 808A063FCE; Wed, 14 Jun 2023 13:43:56 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 66290C433C0; Wed, 14 Jun 2023 13:43:55 +0000 (UTC) Date: Wed, 14 Jun 2023 09:43:53 -0400 From: Steven Rostedt To: David Laight Cc: LKML , Linux Trace Kernel , Masami Hiramatsu , Mark Rutland Subject: Re: [PATCH] tracing: Add a debug_trace_printk() function Message-ID: <20230614094353.2cf1bae5@gandalf.local.home> In-Reply-To: References: <20230612193337.0fb0d3ca@gandalf.local.home> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 14 Jun 2023 09:57:33 +0000 David Laight wrote: > From: Steven Rostedt > > Sent: 13 June 2023 00:34 > > > > From: "Steven Rostedt (Google)" > > > > While doing some tracing and kernel debugging, I found that some of my > > trace_printk()s were being lost in the noise of the other code that was > > being traced. Having a way to write trace_printk() not in the top level > > trace buffer would have been useful. > > > > There was also a time I needed to debug ftrace itself, where > > trace_printk() did not hit the paths that were being debugged. But because > > the trace that was being debugged, was going into the top level ring > > buffer, it was causing issues for seeing what is to be traced. > > > > To solve both of the above, add a debug_trace_printk() that can be used > > just like trace_printk() except that it goes into a "debug" instance > > buffer instead. This can be used at boot up as well. > ... > > +#ifdef CONFIG_FTRACE_DEBUG_PRINT > > + debug_trace = trace_array_get_by_name("debug"); > > + if (WARN_ON(!debug_trace)) > > + return; > > + trace_array_init_printk(debug_trace); > > +#endif > > I was wondering if that could be done whenever the "debug" > trace_array is created? > (perhaps only if trace_prink() has been used?) > Since (AFAICT) it could be created at any time?? > > So you wouldn't really need an extra kernel knob? > (Except to get the boot time trace diverted.) > The trace could go to the global buffer if the debug one > isn't created. I'd rather not touch trace_printk(), that would just confuse people more. Anyway, I'm not going to apply this. I have other ideas on how to accomplish this. But for now, I wanted it in my patchwork to remind me to do those other methods. In the mean time, I can just apply this patch when I need to. > > OTOH I'm missing what trace_array_init_prink() does? > It seems to just call alloc_percpu_trace_buffer() with > no arguments. > > It looks like alloc_percpu_trace_buffer() is called if there > are any trace_printk() formats in the main kernel. > Hopefully they aren't just in modules?? No, they are allocated if a module uses them too. Try it out. Load a module with trace_printk() and you'll see that banner print out. -- Steve