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Date:	Fri, 25 Jul 2014 04:32:55 -0400 (EDT)
From:	Ulrich Obergfell <uobergfe@...hat.com>
To:	Andrew Jones <drjones@...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

> ----- 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
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