On Fri, Jul 25, 2014 at 04:32:55AM -0400, Ulrich Obergfell wrote:
> > ----- Original Message -----
> > From: "Andrew Jones" <drjones@xxxxxxxxxx>
> > To: linux-kernel@xxxxxxxxxxxxxxx, kvm@xxxxxxxxxxxxxxx
> > Cc: uobergfe@xxxxxxxxxx, dzickus@xxxxxxxxxx, pbonzini@xxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx, mingo@xxxxxxxxxx
> > 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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