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=-15.8 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,URIBL_BLOCKED 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 436C0C4332D for ; Thu, 11 Mar 2021 21:23:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1900064F9A for ; Thu, 11 Mar 2021 21:23:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229607AbhCKVW3 (ORCPT ); Thu, 11 Mar 2021 16:22:29 -0500 Received: from mail.kernel.org ([198.145.29.99]:42004 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230341AbhCKVWG (ORCPT ); Thu, 11 Mar 2021 16:22:06 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 263AE64F95; Thu, 11 Mar 2021 21:22:05 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1615497725; bh=1KBn+/tzwpK0c8ijLkSaK5iP3v7+rvech44ribcGVU8=; h=Date:From:To:Subject:From; b=vlg4j8eH/TifqbxUiY1FqFlsr6jpSuLcvymbA4sCU2pX/fNxW9sQouOYvcOx+5s/6 A4nZfho+GMEjkrwMzRJxGVmmDEbe1yrqjxu61566qYI0ucBIqRgXwXwUQuB9Xld5pH q0lI6RmiIGc/L4sERuDZYVmLkJGRDY6WyAqynJyA= Date: Thu, 11 Mar 2021 13:22:04 -0800 From: akpm@linux-foundation.org To: loberman@redhat.com, mhocko@suse.com, mingo@kernel.org, mm-commits@vger.kernel.org, peterz@infradead.org, pmladek@suse.com, tglx@linutronix.de, vincent.whitchurch@axis.com Subject: + =?US-ASCII?Q?watchdog-softlockup-remove-logic-that-tried-to-prevent-re?= =?US-ASCII?Q?peated-reports.patch?= added to -mm tree Message-ID: <20210311212204.YEXSD0Z2T%akpm@linux-foundation.org> User-Agent: s-nail v14.8.16 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Precedence: bulk Reply-To: linux-kernel@vger.kernel.org List-ID: X-Mailing-List: mm-commits@vger.kernel.org The patch titled Subject: watchdog/softlockup: remove logic that tried to prevent repeated reports has been added to the -mm tree. Its filename is watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports.patch This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports.patch and later at https://ozlabs.org/~akpm/mmotm/broken-out/watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Petr Mladek Subject: watchdog/softlockup: remove logic that tried to prevent repeated reports 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. Link: https://lkml.kernel.org/r/20210311122130.6788-5-pmladek@suse.com Signed-off-by: Petr Mladek Cc: Ingo Molnar Cc: Laurence Oberman Cc: Michal Hocko Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: Vincent Whitchurch Signed-off-by: Andrew Morton --- kernel/watchdog.c | 14 ++------------ 1 file changed, 2 insertions(+), 12 deletions(-) --- a/kernel/watchdog.c~watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports +++ a/kernel/watchdog.c @@ -179,7 +179,6 @@ static DEFINE_PER_CPU(unsigned long, wat 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_tim 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_tim 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; } _ Patches currently in -mm which might be from pmladek@suse.com are watchdog-rename-__touch_watchdog-to-a-better-descriptive-name.patch watchdog-explicitly-update-timestamp-when-reporting-softlockup.patch watchdog-softlockup-report-the-overall-time-of-softlockups.patch watchdog-softlockup-remove-logic-that-tried-to-prevent-repeated-reports.patch watchdog-fix-barriers-when-printing-backtraces-from-all-cpus.patch watchdog-cleanup-handling-of-false-positives.patch