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=-18.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 C4341C4332B for ; Thu, 11 Mar 2021 12:22:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8ACA264FD9 for ; Thu, 11 Mar 2021 12:22:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233266AbhCKMWH (ORCPT ); Thu, 11 Mar 2021 07:22:07 -0500 Received: from mx2.suse.de ([195.135.220.15]:39532 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233319AbhCKMV4 (ORCPT ); Thu, 11 Mar 2021 07:21:56 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1615465315; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=7XJru3oceA6SYsxNHW2407YnJ691ilSchVA0HaatcT8=; b=PKEpcmj3vZqOWNopQcWEy76Svn/iepZHG4rwzp0KC1ESPXi4xYxv0tOYk02AYLQlFkkY+V SZdoWtqb0zNr8f4VYS+t9tZ3XB8UoiDXHJbn08TOizf0CwG2cVTnnG7MLcW/dbpUjD52LH dM6z2d6VLlYtZsRNc7J9RxGZye5bk3I= Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 1A9C7AC54; Thu, 11 Mar 2021 12:21:55 +0000 (UTC) From: Petr Mladek To: Thomas Gleixner , Ingo Molnar , Peter Zijlstra , Andrew Morton Cc: Laurence Oberman , Vincent Whitchurch , Michal Hocko , linux-kernel@vger.kernel.org, Petr Mladek Subject: [PATCH v2 4/7] watchdog/softlockup: Remove logic that tried to prevent repeated reports Date: Thu, 11 Mar 2021 13:21:27 +0100 Message-Id: <20210311122130.6788-5-pmladek@suse.com> X-Mailer: git-send-email 2.26.2 In-Reply-To: <20210311122130.6788-1-pmladek@suse.com> References: <20210311122130.6788-1-pmladek@suse.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The softlockup detector does some gymnastic with the variable soft_watchdog_warn. It was added by the commit 58687acba59266735ad ("lockup_detector: Combine nmi_watchdog and softlockup detector"). The purpose is not completely clear. There are the following clues. They describe the situation how it looked after the above mentioned commit: 1. The variable was checked with a comment "only warn once". 2. The variable was set when softlockup was reported. It was cleared only when the CPU was not longer in the softlockup state. 3. watchdog_touch_ts was not explicitly updated when the softlockup was reported. Without this variable, the report would normally be printed again during every following watchdog_timer_fn() invocation. The logic has got even more tangled up by the commit ed235875e2ca98 ("kernel/watchdog.c: print traces for all cpus on lockup detection"). After this commit, soft_watchdog_warn is set only when softlockup_all_cpu_backtrace is enabled. But multiple reports from all CPUs are prevented by a new variable soft_lockup_nmi_warn. Conclusion: The variable probably never worked as intended. In each case, it has not worked last many years because the softlockup was reported repeatedly after the full period defined by watchdog_thresh. The reason is that watchdog gets touched in many known slow paths, for example, in printk_stack_address(). This code is called also when printing the softlockup report. It means that the watchdog timestamp gets updated after each report. Solution: Simply remove the logic. People want the periodic report anyway. Signed-off-by: Petr Mladek --- kernel/watchdog.c | 14 ++------------ 1 file changed, 2 insertions(+), 12 deletions(-) diff --git a/kernel/watchdog.c b/kernel/watchdog.c index 6259590d6474..dc8a0bf943f5 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -179,7 +179,6 @@ static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts); static DEFINE_PER_CPU(unsigned long, watchdog_report_ts); static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer); static DEFINE_PER_CPU(bool, softlockup_touch_sync); -static DEFINE_PER_CPU(bool, soft_watchdog_warn); static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts); static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts_saved); static unsigned long soft_lockup_nmi_warn; @@ -410,19 +409,12 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) if (kvm_check_and_clear_guest_paused()) return HRTIMER_RESTART; - /* only warn once */ - if (__this_cpu_read(soft_watchdog_warn) == true) - return HRTIMER_RESTART; - if (softlockup_all_cpu_backtrace) { /* Prevent multiple soft-lockup reports if one cpu is already * engaged in dumping cpu back traces */ - if (test_and_set_bit(0, &soft_lockup_nmi_warn)) { - /* Someone else will report us. Let's give up */ - __this_cpu_write(soft_watchdog_warn, true); + if (test_and_set_bit(0, &soft_lockup_nmi_warn)) return HRTIMER_RESTART; - } } /* Start period for the next softlockup warning. */ @@ -452,9 +444,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer) add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK); if (softlockup_panic) panic("softlockup: hung tasks"); - __this_cpu_write(soft_watchdog_warn, true); - } else - __this_cpu_write(soft_watchdog_warn, false); + } return HRTIMER_RESTART; } -- 2.26.2