From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752849AbbALKGA (ORCPT ); Mon, 12 Jan 2015 05:06:00 -0500 Received: from www.linutronix.de ([62.245.132.108]:36495 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752186AbbALKF6 (ORCPT ); Mon, 12 Jan 2015 05:05:58 -0500 Date: Mon, 12 Jan 2015 11:05:29 +0100 (CET) From: Thomas Gleixner To: Linus Torvalds cc: Dave Jones , Chris Mason , Mike Galbraith , Ingo Molnar , Peter Zijlstra , =?ISO-8859-15?Q?D=E2niel_Fraga?= , Sasha Levin , "Paul E. McKenney" , Linux Kernel Mailing List , Suresh Siddha , Oleg Nesterov , Peter Anvin Subject: Re: frequent lockups in 3.18rc4 In-Reply-To: Message-ID: References: <20141219145528.GC13404@redhat.com> <20141221223204.GA9618@codemonkey.org.uk> User-Agent: Alpine 2.11 (DEB 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 21 Dec 2014, Linus Torvalds wrote: > On Sun, Dec 21, 2014 at 2:32 PM, Dave Jones wrote: > > On Sun, Dec 21, 2014 at 02:19:03PM -0800, Linus Torvalds wrote: > > > > > > And finally, and stupidly, is there any chance that you have anything > > > accessing /dev/hpet? > > > > Not knowingly at least, but who the hell knows what systemd has its > > fingers in these days. > > Actually, it looks like /dev/hpet doesn't allow write access. > > I can do the mmap(/dev/mem) thing and access the HPET by hand, and > when I write zero to it I immediately get something like this: > > Clocksource tsc unstable (delta = -284317725450 ns) > Switched to clocksource hpet > > just to confirm that yes, a jump in the HPET counter would indeed give > those kinds of symptoms:blaming the TSC with a negative delta in the > 0-300s range, even though it's the HPET that is broken. > > And if the HPET then occasionally jumps around afterwards, it would > show up as ktime_get() occasionally going backwards, which in turn > would - as far as I can tell - result in exactly that pseudo-infirnite > loop with timers. > > Anyway, any wild kernel pointer access *could* happen to just hit the > HPET and write to the main counter value, although I'd personally be > more inclined to blame BIOS/SMM kind of code playing tricks with > time.. We do have a few places where we explicitly write the value on > purpose, but they are in the HPET init code, and in the clocksource > resume code, so they should not be involved. Right. > Thomas - have you had reports of HPET breakage in RT circles, the same > way BIOSes have been tinkering with TSC? Not that I'm aware of. > Also, would it perhaps be a good idea to make "ktime_get()" save the > last time in a percpu variable, and warn if time ever goes backwards > on a particular CPU? A percpu thing should be pretty cheap, even if > we write to it every time somebody asks for time.. That should be simple enough to implement. Thanks, tglx