[PATCH] watchdog: touch_nmi_watchdog should only touch local cpu not every one

[PATCH] watchdog: touch_nmi_watchdog should only touch local cpu not every one

[Don Zickus]

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@redhat.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);
+
+void touch_all_nmi_watchdogs(void)
+{
 	if (watchdog_enabled) {
 		unsigned cpu;
 
@@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
 	}
 	touch_softlockup_watchdog();
 }
-EXPORT_SYMBOL(touch_nmi_watchdog);
+EXPORT_SYMBOL(touch_all_nmi_watchdogs);
 
 #endif
 
-- 
1.7.2.3

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

[Sergey Senozhatsky]

[-- Attachment #1: Type: text/plain, Size: 1181 bytes --]

On (11/04/10 21:18), Don Zickus wrote:
>  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);
> +
> +void touch_all_nmi_watchdogs(void)
> +{
>  	if (watchdog_enabled) {
>  		unsigned cpu;
>  
> @@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
>  	}
>  	touch_softlockup_watchdog();
>  }
> -EXPORT_SYMBOL(touch_nmi_watchdog);
> +EXPORT_SYMBOL(touch_all_nmi_watchdogs);
>  

Hello,
Seems like no one is actually calling touch_all_nmi_watchdogs, as for now. 
Right?


Minor nit

	touch_all_nmi_watchdogs:
	...
	for_each_present_cpu(cpu) {
		if (per_cpu(watchdog_nmi_touch, cpu) != true)
			per_cpu(watchdog_nmi_touch, cpu) = true;
	}


which is, I belive, could be simplified to 
	for_each_present_cpu(cpu) {
		per_cpu(watchdog_nmi_touch, cpu) = true;
	}



	Sergey

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

[Andrew Morton]

On Fri, 5 Nov 2010 15:51:18 +0200
Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:

> On (11/04/10 21:18), Don Zickus wrote:
> >  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);
> > +
> > +void touch_all_nmi_watchdogs(void)
> > +{
> >  	if (watchdog_enabled) {
> >  		unsigned cpu;
> >  
> > @@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
> >  	}
> >  	touch_softlockup_watchdog();
> >  }
> > -EXPORT_SYMBOL(touch_nmi_watchdog);
> > +EXPORT_SYMBOL(touch_all_nmi_watchdogs);
> >  
> 
> Hello,
> Seems like no one is actually calling touch_all_nmi_watchdogs, as for now. 
> Right?

Yes, there doesn't seem a lot of point in adding the interface unless
we have callers.

> 
> Minor nit
> 
> 	touch_all_nmi_watchdogs:
> 	...
> 	for_each_present_cpu(cpu) {
> 		if (per_cpu(watchdog_nmi_touch, cpu) != true)
> 			per_cpu(watchdog_nmi_touch, cpu) = true;
> 	}
> 
> 
> which is, I belive, could be simplified to 
> 	for_each_present_cpu(cpu) {
> 		per_cpu(watchdog_nmi_touch, cpu) = true;
> 	}

We sometimes do this trick to avoid dirtying lots of cachelines which
already held the correct value.  It'll be extra-benefical when dealing
with other CPU's data, I expect.

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

[Frederic Weisbecker]

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@redhat.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.

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

[Don Zickus]

On Fri, Nov 05, 2010 at 12:58:55PM -0700, Andrew Morton wrote:
> On Fri, 5 Nov 2010 15:51:18 +0200
> Sergey Senozhatsky <sergey.senozhatsky@gmail.com> wrote:
> 
> > On (11/04/10 21:18), Don Zickus wrote:
> > >  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);
> > > +
> > > +void touch_all_nmi_watchdogs(void)
> > > +{
> > >  	if (watchdog_enabled) {
> > >  		unsigned cpu;
> > >  
> > > @@ -151,7 +166,7 @@ void touch_nmi_watchdog(void)
> > >  	}
> > >  	touch_softlockup_watchdog();
> > >  }
> > > -EXPORT_SYMBOL(touch_nmi_watchdog);
> > > +EXPORT_SYMBOL(touch_all_nmi_watchdogs);
> > >  
> > 
> > Hello,
> > Seems like no one is actually calling touch_all_nmi_watchdogs, as for now. 
> > Right?
> 
> Yes, there doesn't seem a lot of point in adding the interface unless
> we have callers.

Yeah I wasn't sure how to deal with that.  It didn't seem like any of the
callers was relying on the fact that touch_nmi_watchdog() touched
everyone.  I just provided it as an option in case I misread someone's use
of the touch_nmi_watchdog.

I'll repost and remove it then.

Thanks for the feedback.

Cheers,
Don

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

[Don Zickus]

On Sun, Nov 07, 2010 at 11:09:11PM +0100, Frederic Weisbecker wrote:
> > +	/*
> > +	 * 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?

Yeah, unfortunately.

Cheers,
Don

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

[Don Zickus]

On Thu, Dec 05, 2013 at 12:42:24PM -0800, Ben Zhang 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.

Hi Andrew,

I'm ok with this patch, though it does alter the behaviour of how
touch_nmi_watchdog works.  For the most part I don't think most callers
need to touch all of the watchdogs (on each cpu).  Perhaps a corner case
will pop up (the scheduler?? to mimic touch_all_softlockup_watchdogs() ).

But this does address an issue where if a system is locked up and one cpu
is spewing out useful debug messages (or error messages), the hard lockup
will fail to go off.  We have seen this on RHEL also.

I wouldn't mind see this get soaked in linux-next if possible.  And see
what falls out.

Cheers,
Don

> 
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> Signed-off-by: Ben Zhang <benzh@chromium.org>
> ---
>  kernel/watchdog.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> index 4431610..613f611 100644
> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -158,14 +158,14 @@ void touch_all_softlockup_watchdogs(void)
>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>  void touch_nmi_watchdog(void)
>  {
> -	if (watchdog_user_enabled) {
> -		unsigned cpu;
> -
> -		for_each_present_cpu(cpu) {
> -			if (per_cpu(watchdog_nmi_touch, cpu) != true)
> -				per_cpu(watchdog_nmi_touch, cpu) = true;
> -		}
> -	}
> +	/*
> +	 * 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);
> -- 
> 1.8.5.1
> 

[PATCH] watchdog: touch_nmi_watchdog should only touch local cpu not every one

[Ben Zhang]

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@redhat.com>
Signed-off-by: Ben Zhang <benzh@chromium.org>
---
 kernel/watchdog.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 4431610..613f611 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -158,14 +158,14 @@ void touch_all_softlockup_watchdogs(void)
 #ifdef CONFIG_HARDLOCKUP_DETECTOR
 void touch_nmi_watchdog(void)
 {
-	if (watchdog_user_enabled) {
-		unsigned cpu;
-
-		for_each_present_cpu(cpu) {
-			if (per_cpu(watchdog_nmi_touch, cpu) != true)
-				per_cpu(watchdog_nmi_touch, cpu) = true;
-		}
-	}
+	/*
+	 * 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);
-- 
1.8.5.1