From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B67FEECE564 for ; Tue, 18 Sep 2018 21:07:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6A8C221471 for ; Tue, 18 Sep 2018 21:07:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6A8C221471 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=linutronix.de Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730174AbeISClt (ORCPT ); Tue, 18 Sep 2018 22:41:49 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:58861 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726821AbeISClt (ORCPT ); Tue, 18 Sep 2018 22:41:49 -0400 Received: from p5492e4c1.dip0.t-ipconnect.de ([84.146.228.193] helo=nanos) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1g2NDO-0003Rg-AX; Tue, 18 Sep 2018 23:07:22 +0200 Date: Tue, 18 Sep 2018 23:07:21 +0200 (CEST) From: Thomas Gleixner To: Waiman Long cc: John Stultz , linux-kernel@vger.kernel.org, Stephen Boyd , Peter Zijlstra Subject: Re: [PATCH] clocksource: Warn if too many missing ticks are detected In-Reply-To: <1537295774-17975-1-git-send-email-longman@redhat.com> Message-ID: References: <1537295774-17975-1-git-send-email-longman@redhat.com> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Waiman, On Tue, 18 Sep 2018, Waiman Long wrote: > The clocksource watchdog, when running, is scheduled on all the CPUs in > the system sequentially on a round-robin fashion with a period of 0.5s. > A bug in the 4.18 kernel is causing missing ticks when nohz_full > is specified. Under some circumstances, this causes the watchdog to > incorrectly state that the TSC is unstable because of counter overflow > in the hpet watchdog clock source after a few minutes delay. > > That particular bug is fixed by the 4.19 commit 7059b36636beab ("sched: > idle: Avoid retaining the tick when it has been stopped"). To make it > easier to catch this kind of bug in the future, a check is added to see > if there is too much delay in the watchdog invocation and print a > warning once if it happens. I like the idea. > > Signed-off-by: Waiman Long > --- > kernel/time/clocksource.c | 13 +++++++++++++ > 1 file changed, 13 insertions(+) > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c > index 0e6e97a..2ea5db0 100644 > --- a/kernel/time/clocksource.c > +++ b/kernel/time/clocksource.c > @@ -140,6 +140,7 @@ static void inline clocksource_watchdog_unlock(unsigned long *flags) > * Interval: 0.5sec Threshold: 0.0625s > */ > #define WATCHDOG_INTERVAL (HZ >> 1) > +#define WATCHDOG_INTERNVAL_NS (NSEC_PER_SEC >> 1) > #define WATCHDOG_THRESHOLD (NSEC_PER_SEC >> 4) > > static void clocksource_watchdog_work(struct work_struct *work) > @@ -242,6 +243,18 @@ static void clocksource_watchdog(struct timer_list *unused) > wd_nsec = clocksource_cyc2ns(delta, watchdog->mult, > watchdog->shift); > > + /* > + * When the timer tick is incorrectly stopped on a CPU with > + * pending events, for example, it is possible that the > + * clocksource watchdog will stop running for a sufficiently > + * long enough time to cause overflow in the delta > + * computation leading to incorrect report of unstable clock > + * source. So print a warning if there is unusually large > + * delay (> 0.5s) in the invocation of the watchdog. That > + * can indicate a hidden bug in the timer tick code. > + */ > + WARN_ON_ONCE(!wd_nsec || wd_nsec > 2*WATCHDOG_INTERNVAL_NS); But this is using the watchdog delta to check. If that wrapped the detection is broken. I'd rather use watchdog_timer.expires and check against jiffies. That tells you how late the timer callback actually is and does not suffer any wraparound issues. Thanks, tglx