Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753098AbaG3Nnr (ORCPT ); Wed, 30 Jul 2014 09:43:47 -0400 Received: from mx1.redhat.com ([209.132.183.28]:1949 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751059AbaG3Nnp (ORCPT ); Wed, 30 Jul 2014 09:43:45 -0400 Date: Wed, 30 Jul 2014 09:43:42 -0400 From: Don Zickus To: Andrew Jones Cc: Ulrich Obergfell , linux-kernel@vger.kernel.org, kvm@vger.kernel.org, pbonzini@redhat.com, akpm@linux-foundation.org, mingo@redhat.com Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default Message-ID: <20140730134342.GA7959@redhat.com> References: <1406196811-5384-1-git-send-email-drjones@redhat.com> <1406196811-5384-3-git-send-email-drjones@redhat.com> <615371508.17867577.1406277175913.JavaMail.zimbra@redhat.com> <20140725112510.GA3456@hawk.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140725112510.GA3456@hawk.usersys.redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 25, 2014 at 01:25:11PM +0200, Andrew Jones wrote: > > to enable hard lockup detection explicitly. > > > > I think changing the 'watchdog_thresh' while 'watchdog_running' is true should > > _not_ enable hard lockup detection as a side-effect, because a user may have a > > 'sysctl.conf' entry such as > > > > kernel.watchdog_thresh = ... > > > > or may only want to change the 'watchdog_thresh' on the fly. > > > > I think the following flow of execution could cause such undesired side-effect. > > > > proc_dowatchdog > > if (watchdog_user_enabled && watchdog_thresh) { > > > > watchdog_enable_hardlockup_detector > > hardlockup_detector_enabled = true > > > > watchdog_enable_all_cpus > > if (!watchdog_running) { > > ... > > } else if (sample_period_changed) > > update_timers_all_cpus > > for_each_online_cpu > > update_timers > > watchdog_nmi_disable > > ... > > watchdog_nmi_enable > > > > watchdog_hardlockup_detector_is_enabled > > return true > > > > enable perf counter for hard lockup detection > > > > Regards, > > > > Uli > > Nice catch. Looks like this will need a v2. Paolo, do we have a > consensus on the proc echoing? Or should that be revisited in the v2 as > well? As discussed privately, how about something like this to handle that case: (applied on top of these patches) Cheers, Don diff --git a/kernel/watchdog.c b/kernel/watchdog.c index 34eca29..027fb6c 100644 --- a/kernel/watchdog.c +++ b/kernel/watchdog.c @@ -666,7 +666,12 @@ int proc_dowatchdog(struct ctl_table *table, int write, * watchdog_*_all_cpus() function takes care of this. */ if (watchdog_user_enabled && watchdog_thresh) { - watchdog_enable_hardlockup_detector(true); + /* + * Prevent a change in watchdog_thresh accidentally overriding + * the enablement of the hardlockup detector. + */ + if (watchdog_user_enabled != old_enabled) + watchdog_enable_hardlockup_detector(true); err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh); } else watchdog_disable_all_cpus(); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/