Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753952AbdFWICO (ORCPT ); Fri, 23 Jun 2017 04:02:14 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:58855 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751240AbdFWICK (ORCPT ); Fri, 23 Jun 2017 04:02:10 -0400 Date: Fri, 23 Jun 2017 10:01:55 +0200 (CEST) From: Thomas Gleixner To: Don Zickus cc: Kan Liang , linux-kernel@vger.kernel.org, mingo@kernel.org, akpm@linux-foundation.org, babu.moger@oracle.com, atomlin@redhat.com, prarit@redhat.com, torvalds@linux-foundation.org, peterz@infradead.org, eranian@google.com, acme@redhat.com, ak@linux.intel.com, stable@vger.kernel.org Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups In-Reply-To: <20170622154450.2lua7fdmigcixldw@redhat.com> Message-ID: References: <20170621144118.5939-1-kan.liang@intel.com> <20170622154450.2lua7fdmigcixldw@redhat.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2318 Lines: 62 On Thu, 22 Jun 2017, Don Zickus wrote: > On Wed, Jun 21, 2017 at 11:53:57PM +0200, Thomas Gleixner wrote: > > On Wed, 21 Jun 2017, kan.liang@intel.com wrote: > > > We now have more and more systems where the Turbo range is wide enough > > > that the NMI watchdog expires faster than the soft watchdog timer that > > > updates the interrupt tick the NMI watchdog relies on. > > > > > > This problem was originally added by commit 58687acba592 > > > ("lockup_detector: Combine nmi_watchdog and softlockup detector"). > > > Previously the NMI watchdog would always check jiffies, which were > > > ticking fast enough. But now the backing is quite slow so the expire > > > time becomes more sensitive. > > > > And slapping a factor 3 on the NMI period is the wrong answer to the > > problem. The simple solution would be to increase the hrtimer frequency, > > but that's not really desired either. > > > > Find an untested patch below, which should cure the issue. > > A simple low pass filter. It compiles. :-) I don't think I have knowledge > to test it. Kan? Yes, and it has an interesting twist. It's only working once we have switched to TSC as clocksource. As long as jiffies are the clocksource, this will miserably fail because when the hrtimer interrupt is not delivered jiffies wont be incremented either and the NMI will say: Oh. not enough time elapsed. Lather, rinse and repeat. One simple way to fix this is with the delta patch below. Thanks, tglx 8<-------------------------- --- a/kernel/watchdog_hld.c +++ b/kernel/watchdog_hld.c @@ -72,6 +72,7 @@ EXPORT_SYMBOL(touch_nmi_watchdog); #ifdef CONFIG_HARDLOCKUP_CHECK_TIMESTAMP static DEFINE_PER_CPU(ktime_t, last_timestamp); +static DEFINE_PER_CPU(unsigned int, nmi_rearmed); static ktime_t watchdog_hrtimer_sample_threshold __read_mostly; void watchdog_update_hrtimer_threshold(u64 period) @@ -105,8 +106,11 @@ static bool watchdog_check_timestamp(voi ktime_t delta, now = ktime_get_mono_fast_ns(); delta = now - __this_cpu_read(last_timestamp); - if (delta < watchdog_hrtimer_sample_threshold) - return false; + if (delta < watchdog_hrtimer_sample_threshold) { + if (__this_cpu_inc_return(nmi_rearmed) < 10) + return false; + } + __this_cpu_write(nmi_rearmed, 0); __this_cpu_write(last_timestamp, now); return true; }