From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755555Ab1FFQpc (ORCPT ); Mon, 6 Jun 2011 12:45:32 -0400 Received: from mo-p00-ob.rzone.de ([81.169.146.162]:55553 "EHLO mo-p00-ob.rzone.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753686Ab1FFQpb (ORCPT ); Mon, 6 Jun 2011 12:45:31 -0400 X-RZG-AUTH: :IGUXYVOIf/Z0yAghYbpIhzghmj8icP68r1arC3zTx2B9G7/X5zri/u5Y1+fsZ6BmRA== X-RZG-CLASS-ID: mo00 Message-ID: <4DED0423.4050904@die-jansens.de> Date: Mon, 06 Jun 2011 18:45:23 +0200 From: Arne Jansen User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2.17) Gecko/20110424 Thunderbird/3.1.10 MIME-Version: 1.0 To: Ingo Molnar CC: Peter Zijlstra , 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 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> <20110606161749.GA22157@elte.hu> <4DED0292.1040605@die-jansens.de> In-Reply-To: <4DED0292.1040605@die-jansens.de> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06.06.2011 18:38, Arne Jansen wrote: > On 06.06.2011 18:17, Ingo Molnar wrote: >> >> * 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? > > As long as it doesn't scramble the order of the messages, the delay > imho doesn't matter even in very printk-heavy debugging sessions. And, as important, doesn't reduce the throughput of printk. Having only 100 wakeups/s sounds like the throughput is limited to 100xsizeof(ring buffer). > >> >> This would solve a real nightmare that has plagued us ever since >> printk() has done wakeups directly - i.e. like forever. >> >> Thanks, >> >> Ingo > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/