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Date:	Fri, 25 Jul 2014 13:25:11 +0200
From:	Andrew Jones <drjones@...hat.com>
To:	Ulrich Obergfell <uobergfe@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	dzickus@...hat.com, pbonzini@...hat.com, akpm@...ux-foundation.org,
	mingo@...hat.com
Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

On Fri, Jul 25, 2014 at 04:32:55AM -0400, Ulrich Obergfell wrote:
> > ----- Original Message -----
> > From: "Andrew Jones" <drjones@...hat.com>
> > To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org
> > Cc: uobergfe@...hat.com, dzickus@...hat.com, pbonzini@...hat.com, akpm@...ux-foundation.org, mingo@...hat.com
> > Sent: Thursday, July 24, 2014 12:13:30 PM
> > Subject: [PATCH 2/3] watchdog: control hard lockup detection default
> 
> [...]
> 
> > The running kernel still has the ability to enable/disable at any
> > time with /proc/sys/kernel/nmi_watchdog us usual. However even
> > when the default has been overridden /proc/sys/kernel/nmi_watchdog
> > will initially show '1'. To truly turn it on one must disable/enable
> > it, i.e.
> >   echo 0 > /proc/sys/kernel/nmi_watchdog
> >   echo 1 > /proc/sys/kernel/nmi_watchdog
> 
> [...]
> 
> > @@ -626,15 +665,17 @@ int proc_dowatchdog(struct ctl_table *table, int write,
> >          * disabled. The 'watchdog_running' variable check in
> >          * watchdog_*_all_cpus() function takes care of this.
> >          */
> > -        if (watchdog_user_enabled && watchdog_thresh)
> > +        if (watchdog_user_enabled && watchdog_thresh) {
> > +                watchdog_enable_hardlockup_detector(true);
> >                  err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
> > -        else
> > +        } else
> 
> [...]
> 
> 
> I just realized a possible issue in the above part of the patch:
> 
> If we would want to give the user the option to override the effect of patch 3/3
> via /proc, I think proc_dowatchdog() should enable hard lockup detection _only_
> in case of a state transition from 'NOT watchdog_running' to 'watchdog_running'.
>                                                           |
>    if (watchdog_user_enabled && watchdog_thresh) {        | need to add this
>        if (!watchdog_running) <---------------------------'
>            watchdog_enable_hardlockup_detector(true);
>        err = watchdog_enable_all_cpus(old_thresh != watchdog_thresh);
>    } else
>        ...
> 
> The additional 'if (!watchdog_running)' would _require_ the user to perform the
> sequence of commands
> 
>    echo 0 > /proc/sys/kernel/nmi_watchdog
>    echo 1 > /proc/sys/kernel/nmi_watchdog
> 
> 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?

drew
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