linux-kernel - Re: [PATCH 2/3] watchdog: control hard lockup detection default

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Message-ID: <53D0ED70.4070800@redhat.com>
Date:	Thu, 24 Jul 2014 13:26:40 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ulrich Obergfell <uobergfe@...hat.com>
CC:	Andrew Jones <drjones@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, dzickus@...hat.com, akpm@...ux-foundation.org,
	mingo@...hat.com
Subject: Re: [PATCH 2/3] watchdog: control hard lockup detection default

Il 24/07/2014 13:18, Ulrich Obergfell ha scritto:
>>> >> 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
>> >
>> > Why is it hard to make this show the right value? :)
>> >
>> > Paolo
> 'echo 1 > /proc/sys/kernel/nmi_watchdog' enables both - hard lockup and
> soft lockup detection. watchdog_enable_all_cpus() starts a 'watchdog/N'
> thread for each CPU. If the kernel runs on a bare metal system where the
> processor does not have a PMU, or when perf_event_create_kernel_counter()
> returns failure to watchdog_nmi_enable(), or when the kernel runs as a
> guest on a hypervisor that does not emulate a PMU, then the 'watchdog/N'
> threads are still active for soft lockup detection. Patch 2/3 essentially
> makes watchdog_nmi_enable() behave in the same way as if -ENOENT would
> have been returned by perf_event_create_kernel_counter(). This is then
> reported via a console message.
> 
>   NMI watchdog: disabled (cpu0): hardware events not enabled
> 
> It's hard say what _is_ 'the right value' (because lockup detection is
> then enabled 'partially'), regardless of whether patch 2/3 is applied
> or not.

But this means that it is not possible to re-enable softlockup detection
only.  I think that should be the effect of echo 0 + echo 1, if
hardlockup detection was disabled by either the command line or patch 3.

Paolo
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