Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751631AbaLCT41 (ORCPT ); Wed, 3 Dec 2014 14:56:27 -0500 Received: from mail-ig0-f171.google.com ([209.85.213.171]:39976 "EHLO mail-ig0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750798AbaLCT40 (ORCPT ); Wed, 3 Dec 2014 14:56:26 -0500 MIME-Version: 1.0 In-Reply-To: References: <547bbe36.48548c0a.105c.779c@mx.google.com> <20141201191431.GA17385@linux.vnet.ibm.com> <547ccf74.a5198c0a.25de.26d9@mx.google.com> <20141201230339.GA20487@ret.masoncoding.com> <1417529606.3924.26.camel@maggy.simpson.net> <1417540493.21136.3@mail.thefacebook.com> <20141203184111.GA32005@redhat.com> <20141203190045.GB32005@redhat.com> Date: Wed, 3 Dec 2014 11:56:25 -0800 X-Google-Sender-Auth: 3NUiZKncPgzbZudYN4_iUx-gzKs Message-ID: Subject: Re: frequent lockups in 3.18rc4 From: John Stultz To: Linus Torvalds Cc: Dave Jones , Chris Mason , Mike Galbraith , Ingo Molnar , Peter Zijlstra , =?UTF-8?Q?D=C3=A2niel_Fraga?= , Sasha Levin , "Paul E. McKenney" , Linux Kernel Mailing List , Thomas Gleixner Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Dec 3, 2014 at 11:25 AM, Linus Torvalds wrote: > On Wed, Dec 3, 2014 at 11:00 AM, Dave Jones wrote: >> >> So right after sending my last mail, I rebooted, and restarted the run >> on the same kernel again. >> >> As I was writing this mail, this happened. >> >> [ 524.420897] NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [trinity-c178:20182] >> >> and that's all that made it over the console. I couldn't log in via ssh, >> and thought "ah-ha, so it IS bad". I walked over to reboot it, and >> found I could actually log in on the console. check out this dmesg.. >> >> [ 503.683055] Clocksource tsc unstable (delta = -95946009388 ns) >> [ 503.692038] Switched to clocksource hpet >> [ 524.420897] NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s! [trinity-c178:20182] > > Interesting. That whole NMI watchdog thing happens pretty much 22s > after the "TSC unstable" message. > > Have you ever seen that TSC issue before? The watchdog relies on > comparing get_timestamp() differences, so if the timestamp was > incorrect... > > Maybe that whole "clocksource_watchdog()" is bogus. That delta is > about 96 seconds, sounds very odd. I'm not seeing how the TSC could > actually scew up that badly, so I'd almost be more likely to blame the > "watchdog" clock. > > I don't know. This piece of code: > > delta = clocksource_delta(wdnow, cs->wd_last, watchdog->mask); > > makes no sense to me. Shouldn't it be > > delta = clocksource_delta(wdnow, watchdog->wd_last, watchdog->mask); So we store wdnow value in the cs->wd_last a few lines below, so I don't think that's problematic. I do recall seeing problematic watchdog behavior back in the day w/ PREEMPT_RT when a high priority task really starved the watchdog for a long time. When we came back the hpet had wrapped, making the wd_delta look quite small relative to the TSC delta, causing improper disqualification of the TSC. But in that case the watchdog would disqualify the TSC after the stall, and here the stall is happening right afterwards. So I'm not sure. I'll look around for some other suspects though. The nohz ntp improvments might be high on my list there, since it was a 3.17 item. Will dig. thanks -john -- 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/