From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754624Ab2DGNrw (ORCPT ); Sat, 7 Apr 2012 09:47:52 -0400 Received: from mx1.redhat.com ([209.132.183.28]:29885 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754203Ab2DGNrv (ORCPT ); Sat, 7 Apr 2012 09:47:51 -0400 Message-ID: <4F804580.9010308@redhat.com> Date: Sat, 07 Apr 2012 09:47:44 -0400 From: Prarit Bhargava User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110419 Red Hat/3.1.10-1.el6_0 Thunderbird/3.1.10 MIME-Version: 1.0 To: Thomas Gleixner CC: John Stultz , linux-kernel@vger.kernel.org, Salman Qazi , stable@kernel.org Subject: Re: [PATCH] clocksource, prevent overflow in clocksource_cyc2ns References: <1333552260-1170-1-git-send-email-prarit@redhat.com> <4F7C8C3E.1020203@us.ibm.com> <4F7C9402.3090602@redhat.com> <4F7CF094.5020201@us.ibm.com> <4F7D8FA1.1010107@redhat.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/06/2012 07:29 PM, Thomas Gleixner wrote: > On Thu, 5 Apr 2012, Prarit Bhargava wrote: > >>> >>> So what kernel version are you using? >> >> I retested using top of the linux.git tree, running >> >> echo 1 > /proc/sys/kernel/sysrq >> for i in `seq 10000`; do sleep 1000 & done >> echo t > /proc/sysrq-trigger >> >> and I no longer see a problem. However, if I increase the number of threads to >> 1000/cpu I get >> >> Clocksource %s unstable (delta = -429565427) >> Clocksource switching to hpet > > You are issuing a command which puts the kernel into a state where is > dumps data for several seconds with interrupts disabled. And you expect that > everything can cope with that? Yes actually. I do expect that everything "copes" with it. I don't find it unreasonable with system sizes increasing that functionality that has been around for years works. However, I also understand that no one expected or saw this problem -- I'm not blaming anyone or screaming "Hey! This is broken!!!". > >> If I hack in (sorry for the cut-and-paste) >> .... >> + cs_nsec = mult_frac(((csnow - cs->cs_last), cs->mult, >> + 1UL << cs->shift); >> >> - cs_nsec = clocksource_cyc2ns((csnow - cs->cs_last) & >> - cs->mask, cs->mult, cs->shift); >> then I don't see unstable messages. > > That does not make your approach more correct. The HPET wraparound > time is ~3 seconds, so you screwed everything already, when your dump > lasts longer than that. And there are clocksources which wrap way > faster. > > No, you can't fix that by hacking the timer code. A wraparound CANNOT > be fixed by hacks. > > So instead of fiddling in the victims, please fix the root cause, > i.e. that stupid sysrq-t code which should not need to have interrupts > disabled just to dump all that state. If that's not possible, send a > patch to the sysrq documentation and warn about the consequences. > > But stay away from code which is correct already. You CANNOT fix a > problem which is caused by abnormal system state by hacking the code > which is exposing the problem. > > All you do is making hot pathes more expensive with a very dubious > value. The time related calls are hotpath functions and optimized. > > Aside of that you are breaking all architectures which do not have a > native 64/32 instruction. > > This mult_frac stuff is not going to happen, period. Okay -- thanks for the info. Much appreciated. P.