linux-kernel - Re: [PATCH] watchdog: touch_nmi_watchdog should only touch local cpu not every one

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Date:	Sun, 7 Nov 2010 23:09:11 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Don Zickus <dzickus@...hat.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, akpm@...ux-foundation.org,
	sergey.senozhatsky@...il.com
Subject: Re: [PATCH] watchdog:  touch_nmi_watchdog should only touch local
	cpu not every one

On Thu, Nov 04, 2010 at 09:18:52PM -0400, Don Zickus wrote:
> I ran into a scenario where while one cpu was stuck and should have panic'd
> because of the NMI watchdog, it didn't.  The reason was another cpu was spewing
> stack dumps on to the console.  Upon investigation, I noticed that when writing
> to the console and also when dumping the stack, the watchdog is touched.
> 
> This causes all the cpus to reset their NMI watchdog flags and the 'stuck' cpu
> just spins forever.
> 
> This change causes the semantics of touch_nmi_watchdog to be changed slightly.
> Previously, I accidentally changed the semantics and we noticed there was a
> codepath in which touch_nmi_watchdog could be touched from a preemtible area.
> That caused a BUG() to happen when CONFIG_DEBUG_PREEMPT was enabled.  I believe
> it was the acpi code.
> 
> My attempt here re-introduces the change to have the touch_nmi_watchdog() code
> only touch the local cpu instead of all of the cpus.  But instead of using
> __get_cpu_var(), I use the __raw_get_cpu_var() version.
> 
> This avoids the preemption problem.  However my reasoning wasn't because I was
> trying to be lazy.  Instead I rationalized it as, well if preemption is enabled
> then interrupts should be enabled to and the NMI watchdog will have no reason
> to trigger.  So it won't matter if the wrong cpu is touched because the percpu
> interrupt counters the NMI watchdog uses should still be incrementing.
> 
> Signed-off-by: Don Zickus <dzickus@...hat.com>
> ---
>  kernel/watchdog.c |   17 ++++++++++++++++-
>  1 files changed, 16 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index dc8e168..dd0c140 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -141,6 +141,21 @@ void touch_all_softlockup_watchdogs(void)
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  void touch_nmi_watchdog(void)
>  {
> +	/*
> +	 * Using __raw here because some code paths have
> +	 * preemption enabled.  If preemption is enabled
> +	 * then interrupts should be enabled too, in which
> +	 * case we shouldn't have to worry about the watchdog
> +	 * going off.
> +	 */
> +	__raw_get_cpu_var(watchdog_nmi_touch) = true;
> +
> +	touch_softlockup_watchdog();
> +}
> +EXPORT_SYMBOL(touch_nmi_watchdog);



Did the old watchdog also touched every CPUs?

That doesn't appear to be a good thing, we may indeed miss hardlockups
because of that.

And it seems you can drop touch_all_nmi_watchdogs() as, like others
pointed out, there are no users of it.

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