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 A592EC433F4 for ; Tue, 18 Sep 2018 18:36:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 713AC2150B for ; Tue, 18 Sep 2018 18:36:27 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 713AC2150B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com 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 S1730334AbeISAKR (ORCPT ); Tue, 18 Sep 2018 20:10:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:45964 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729851AbeISAKR (ORCPT ); Tue, 18 Sep 2018 20:10:17 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 60735308338E; Tue, 18 Sep 2018 18:36:25 +0000 (UTC) Received: from llong.com (ovpn-122-167.rdu2.redhat.com [10.10.122.167]) by smtp.corp.redhat.com (Postfix) with ESMTP id 434C5608E5; Tue, 18 Sep 2018 18:36:21 +0000 (UTC) From: Waiman Long To: John Stultz , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, Stephen Boyd , Peter Zijlstra , Waiman Long Subject: [PATCH] clocksource: Warn if too many missing ticks are detected Date: Tue, 18 Sep 2018 14:36:14 -0400 Message-Id: <1537295774-17975-1-git-send-email-longman@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.44]); Tue, 18 Sep 2018 18:36:25 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. 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); + delta = clocksource_delta(csnow, cs->cs_last, cs->mask); cs_nsec = clocksource_cyc2ns(delta, cs->mult, cs->shift); wdlast = cs->wd_last; /* save these in case we print them */ -- 1.8.3.1