From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753812AbaIPVW5 (ORCPT ); Tue, 16 Sep 2014 17:22:57 -0400 Received: from cantor2.suse.de ([195.135.220.15]:46655 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753046AbaIPVWx (ORCPT ); Tue, 16 Sep 2014 17:22:53 -0400 Date: Tue, 16 Sep 2014 23:22:50 +0200 From: Jan Kara To: Steven Rostedt Cc: Jan Kara , Markus Trippelsdorf , Geert Uytterhoeven , "linux-kernel@vger.kernel.org" , Andrew Morton , Peter Zijlstra Subject: Re: [PATCH] printk: git rid of [sched_delayed] message for printk_deferred Message-ID: <20140916212250.GI1205@quack.suse.cz> References: <20140914050905.GA296@x4> <20140914055450.GB296@x4> <20140915163740.GA12408@x4> <20140916105547.GA1205@quack.suse.cz> <20140916144252.GB32118@x4> <20140916111331.14303381@gandalf.local.home> <20140916203510.GF1205@quack.suse.cz> <20140916170709.73ef2993@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140916170709.73ef2993@gandalf.local.home> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 16-09-14 17:07:09, Steven Rostedt wrote: > On Tue, 16 Sep 2014 22:35:10 +0200 > Jan Kara wrote: > > > On Tue 16-09-14 11:13:31, Steven Rostedt wrote: > > > On Tue, 16 Sep 2014 16:42:52 +0200 > > > Markus Trippelsdorf wrote: > > > > > > > commit 458df9fd hardcodes printk_deferred() to KERN_WARNING and inserts > > > > the string "[sched_delayed] " before the actual message. > > > > However it doesn't take into account the KERN_* prefix of the message, > > > > that now ends up in the middle of the output: > > > > > > > > [sched_delayed] ^a4CE: hpet increased min_delta_ns to 20115 nsec > > > > > > > > Fix this by just getting rid of the "[sched_delayed] " scnprintf(). > > > > > > I prefer the "[sched_delayed]" output. It lets us know that the output > > > did not come out immediately, which is important sometimes during > > > debugging. Otherwise it can confuse people, as printk is suppose to be > > > a blocking write. > > This is a common misconception about printk. It isn't a blocking write > > for ten years or more. If there happens to be someone else printing to > > console, the difference between printk() and printk_deferred() is > > marginal - it used to be bigger when scheduler had its own buffer but these > > days message is inserted in the kernel ring buffer immediately. That's why > > I don't think the prefix is useful anymore. > > For the most part it's a blocking write. Yeah, if another CPU is > writing, it wont be a blocking write, but it usually is, I know I > depend on it (when I'm debugging, I usually don't have contention > between CPUs). The important part is that they are done in order. A > delayed print, wont be in order with other printks. That is still a > crucial difference. printk_deffered() will be in order with other printks after your commit 458df9fd4815b47809875d57f42e16401674b621. Just printing to console itself will be delayed to the next timer interrupt. Or am I missing something? Honza -- Jan Kara SUSE Labs, CR