From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753760Ab1FFQSG (ORCPT ); Mon, 6 Jun 2011 12:18:06 -0400 Received: from mx3.mail.elte.hu ([157.181.1.138]:41049 "EHLO mx3.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751429Ab1FFQSE (ORCPT ); Mon, 6 Jun 2011 12:18:04 -0400 Date: Mon, 6 Jun 2011 18:17:49 +0200 From: Ingo Molnar To: Peter Zijlstra Cc: Arne Jansen , Linus Torvalds , mingo@redhat.com, hpa@zytor.com, linux-kernel@vger.kernel.org, efault@gmx.de, npiggin@kernel.dk, akpm@linux-foundation.org, frank.rowand@am.sony.com, tglx@linutronix.de, linux-tip-commits@vger.kernel.org Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages Message-ID: <20110606161749.GA22157@elte.hu> References: <4DEB933C.1070900@die-jansens.de> <20110605151323.GA30590@elte.hu> <1307349530.2353.7374.camel@twins> <20110606145827.GD30348@elte.hu> <1307372989.2322.136.camel@twins> <1307375227.2322.161.camel@twins> <20110606155236.GA7374@elte.hu> <1307376039.2322.164.camel@twins> <20110606160810.GA16636@elte.hu> <1307376771.2322.168.camel@twins> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1307376771.2322.168.camel@twins> User-Agent: Mutt/1.5.20 (2009-08-17) X-ELTE-SpamScore: -2.0 X-ELTE-SpamLevel: X-ELTE-SpamCheck: no X-ELTE-SpamVersion: ELTE 2.0 X-ELTE-SpamCheck-Details: score=-2.0 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.3.1 -2.0 BAYES_00 BODY: Bayes spam probability is 0 to 1% [score: 0.0000] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Mon, 2011-06-06 at 18:08 +0200, Ingo Molnar wrote: > > * Peter Zijlstra wrote: > > > > > On Mon, 2011-06-06 at 17:52 +0200, Ingo Molnar wrote: > > > > * Peter Zijlstra wrote: > > > > > > > > > Needs more staring at, preferably by someone who actually > > > > > understands that horrid mess :/ Also, this all still doesn't make > > > > > printk() work reliably while holding rq->lock. > > > > > > > > So, what about my suggestion to just *remove* the wakeup from there > > > > and use the deferred wakeup mechanism that klogd uses. > > > > > > > > That would make printk() *visibly* more robust in practice. > > > > > > That's currently done from the jiffy tick, do you want to effectively > > > delay releasing the console_sem for the better part of a jiffy? > > > > Yes, and we already do it in some other circumstances. > > We do? Yes, see the whole printk_pending logic, it delays: wake_up_interruptible(&log_wait); to the next jiffies tick. > > Can you see > > any problem with that? klogd is an utter slowpath anyway. > > but console_sem isn't klogd. We delay klogd and that's perfectly > fine, but afaict we don't delay console_sem. But console_sem is really a similar special case as klogd. See, it's about a *printk*. That's rare by definition. If someone on the console sees it he'll be startled by at least 10 msecs ;-) So delaying the wakeup to the next jiffy really fits into the same approach as we already do with &log_wait, hm? This would solve a real nightmare that has plagued us ever since printk() has done wakeups directly - i.e. like forever. Thanks, Ingo