Messages in this thread | | | Date | Fri, 25 Jul 2014 04:32:55 -0400 (EDT) | From | Ulrich Obergfell <> | Subject | Re: [PATCH 2/3] watchdog: control hard lockup detection default |
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> ----- Original Message ----- > From: "Andrew Jones" <drjones@redhat.com> > To: linux-kernel@vger.kernel.org, kvm@vger.kernel.org > Cc: uobergfe@redhat.com, dzickus@redhat.com, pbonzini@redhat.com, akpm@linux-foundation.org, mingo@redhat.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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