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 X-Spam-Level: X-Spam-Status: No, score=-3.5 required=3.0 tests=BAYES_00, BUG6152_INVALID_DATE_TZ_ABSURD,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU, HEADER_FROM_DIFFERENT_DOMAINS,INVALID_DATE_TZ_ABSURD,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id BD255C433E3 for ; Thu, 23 Jul 2020 14:02:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 981C8207BB for ; Thu, 23 Jul 2020 14:02:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="t0bE2dT9"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="f4M+ARpW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729409AbgGWOCr (ORCPT ); Thu, 23 Jul 2020 10:02:47 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:58224 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727111AbgGWOCq (ORCPT ); Thu, 23 Jul 2020 10:02:46 -0400 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1595512964; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Gd/ucxZolDmpxhtFGJf0Gpvd8lew4aL+qNNCJx4SFco=; b=t0bE2dT9EJRN0sTAI0Zlg4Hkmb8V8v/1lLZpOTfdvsRj3i70sqyoyrW2u39JoERXvvQGv1 qSxvz4lW03txJ/XgDiBLl7c9z6XQsBR04idz21ndei/+s7awQz4M7GKWZkK1f0sgbFYLLR UpBiPYa3pqG1mD/nrgYHThdIJDZXWIxg9hFceks79CvC3i17MJxaaqy/1YR2fs0h0u5m27 K1o6sBGaoS5c3dyTT9iBmDO82s/j14tlcH7kjsP88hU+anrxovJOoiCA5n4hbteT3tkmo0 TWGCIvnKuzZYrMtvAWIgrrJHnm9iI3WQw+joYibt6kPeA8rotPd9EhhE0YBEhw== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1595512964; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=Gd/ucxZolDmpxhtFGJf0Gpvd8lew4aL+qNNCJx4SFco=; b=f4M+ARpWIRJFgFZbldb5kttRxUwnCdjsn8/xKQMAOc/ykNhyxX2eCsP4QOteAc4wHy2SnM KEN8FtqBysrDCiBA== To: Steven Rostedt Cc: Vincent Whitchurch , jbaron@akamai.com, mingo@redhat.com, kernel@axis.com, corbet@lwn.net, linux-kernel@vger.kernel.org, Petr Mladek , Sergey Senozhatsky Subject: Re: [PATCH] dynamic debug: allow printing to trace event In-Reply-To: <20200722112525.694880d3@oasis.local.home> References: <20200721141105.16034-1-vincent.whitchurch@axis.com> <20200721173045.540ae500@oasis.local.home> <87eep3zmg9.fsf@jogness.linutronix.de> <20200722112525.694880d3@oasis.local.home> Date: Thu, 23 Jul 2020 16:08:44 +0206 Message-ID: <87blk6cosb.fsf@jogness.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-07-22, Steven Rostedt wrote: >>> +static void dynamic_printk(unsigned int flags, const char *fmt, ...) >>> +{ >>> + if (flags & _DPRINTK_FLAGS_TRACE) { >>> + va_list args; >>> + >>> + va_start(args, fmt); >>> + /* >>> + * All callers include the KERN_DEBUG prefix to keep the >>> + * vprintk case simple; strip it out for tracing. >>> + */ >>> + dynamic_trace(fmt + strlen(KERN_DEBUG), args); >> >> Do we really need a separate tracing event for this? Why not just: >> >> ftrace_vprintk(fmt + strlen(KERN_DEBUG), args); > > It must be an event, one that can be enabled or disabled separately > from trace_printk(). > > If you are asking if this could be something like trace_printk(), > which ftrace_vprintk() is. The reason for that nasty banner when > people use trace_printk() is to keep developers from using it as their > personal debugging tool in production. > > A trace_printk() can not be discretely disabled. It's either totally > on, or totally off. And since it is used for debugging, if there's > trace_printk()s all over the kernel, you will have to deal with the > noise of everyone else's trace_printk(), making trace_printk() > useless. I understand and agree with your concern about trace_printk(). But it seems to me that trace_printk() via pr_debug() should be OK because there is discrete control per message implemented. Yes, more code is necessary to distinguish between the two, such as letting dynamic_printk use an internal function that does not trigger a splat. But I think that is reasonable. For me a trace event represents a specific point in the kernel code. But this new printk trace event, instead, represents general log redirection. I do not oppose it, but it feels like a hack to me. In contrast, simply setting a dynamic printk flag to write the message string to the trace buffer (without also activating some pseudo trace event) feels more natural. Just sharing my thoughts, as requested. John Ogness