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 91679C433F4 for ; Mon, 24 Sep 2018 15:36:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 52E352083A for ; Mon, 24 Sep 2018 15:36:34 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 52E352083A 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 S1730877AbeIXVjR (ORCPT ); Mon, 24 Sep 2018 17:39:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:43706 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727770AbeIXVjQ (ORCPT ); Mon, 24 Sep 2018 17:39:16 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 51FBD30001DE; Mon, 24 Sep 2018 15:36:32 +0000 (UTC) Received: from llong.com (dhcp-17-8.bos.redhat.com [10.18.17.8]) by smtp.corp.redhat.com (Postfix) with ESMTP id 3723F106A7AD; Mon, 24 Sep 2018 15:36:28 +0000 (UTC) From: Waiman Long To: John Stultz , Thomas Gleixner Cc: linux-kernel@vger.kernel.org, Stephen Boyd , Peter Zijlstra , Waiman Long Subject: [PATCH v3] clocksource: Warn if too many missing ticks are detected Date: Mon, 24 Sep 2018 11:36:15 -0400 Message-Id: <1537803375-31667-1-git-send-email-longman@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Mon, 24 Sep 2018 15:36:32 +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 invocation of the watchdog callback and print a warning once if it happens. v3: Do the check only when SYSTEM_RUNNING & print the delay as well. Signed-off-by: Waiman Long --- kernel/time/clocksource.c | 29 +++++++++++++++++++++++++++++ 1 file changed, 29 insertions(+) diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 0e6e97a..e5d2e38 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -208,11 +208,40 @@ static void clocksource_watchdog(struct timer_list *unused) u64 csnow, wdnow, cslast, wdlast, delta; int64_t wd_nsec, cs_nsec; int next_cpu, reset_pending; + static bool prev_running; /* Set if previously in SYSTEM_RUNNING */ spin_lock(&watchdog_lock); if (!watchdog_running) goto out; + /* + * 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. + * + * This check is performed only when the system is previously + * running in the SYSTEM_RUNNING state as large delay may happen + * when running in other states, especially when self-tests are + * being run. If the watchdog was previously in the running state, + * that will make sure that the current timer expiry happened in + * that state too. + */ + if (prev_running) { + unsigned long delay = jiffies - watchdog_timer.expires; + + if (delay > WATCHDOG_INTERVAL) { + pr_warn("watchdog delayed by %ld ticks!\n", delay); + WARN_ON_ONCE(1); + } + } else if (system_state == SYSTEM_RUNNING) { + prev_running = true; + } + reset_pending = atomic_read(&watchdog_reset_pending); list_for_each_entry(cs, &watchdog_list, wd_list) { -- 1.8.3.1