[PATCH 1/3] clockevents: Use an atomic RCU notifier for clockevents

[PATCH 1/3] clockevents: Use an atomic RCU notifier for clockevents

[Andi Kleen]

From: Andi Kleen <ak@linux.intel.com>

Use an atomic_notifier instead of a raw_notifier for the clockevents
notification.

This avoids a global lock in the idle path, is a scalability
problem. With this patch we don't have a global lock anymore
at least on systems with an always running timer.

Only broadcast_enter/exit actually do use RCU for now.

I kept all the other events under the lock because they are not
on fast paths and it was not fully clear if they are all RCU
safe or not.

Actually an alternative would be to just get rid of the notifier.
As far as I can see it only has a single client anyways. But
right now I kept it.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 kernel/time/clockevents.c |   29 +++++++++++++----------------
 1 files changed, 13 insertions(+), 16 deletions(-)

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index e4c699d..2ee6bcb 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c
@@ -25,12 +25,12 @@
 static LIST_HEAD(clockevent_devices);
 static LIST_HEAD(clockevents_released);
 
-/* Notification for clock events */
-static RAW_NOTIFIER_HEAD(clockevents_chain);
-
 /* Protection for the above */
 static DEFINE_RAW_SPINLOCK(clockevents_lock);
 
+/* Notification for clock events */
+static ATOMIC_NOTIFIER_HEAD(clockevents_chain);
+
 /**
  * clockevents_delta2ns - Convert a latch value (device ticks) to nanoseconds
  * @latch:	value to convert
@@ -137,23 +137,12 @@ int clockevents_program_event(struct clock_event_device *dev, ktime_t expires,
  */
 int clockevents_register_notifier(struct notifier_block *nb)
 {
-	unsigned long flags;
-	int ret;
-
-	raw_spin_lock_irqsave(&clockevents_lock, flags);
-	ret = raw_notifier_chain_register(&clockevents_chain, nb);
-	raw_spin_unlock_irqrestore(&clockevents_lock, flags);
-
-	return ret;
+	return atomic_notifier_chain_register(&clockevents_chain, nb);
 }
 
-/*
- * Notify about a clock event change. Called with clockevents_lock
- * held.
- */
 static void clockevents_do_notify(unsigned long reason, void *dev)
 {
-	raw_notifier_call_chain(&clockevents_chain, reason, dev);
+	atomic_notifier_call_chain(&clockevents_chain, reason, dev);
 }
 
 /*
@@ -308,6 +297,14 @@ void clockevents_notify(unsigned long reason, void *arg)
 	unsigned long flags;
 	int cpu;
 
+	/* For idle path events don't take a lock */
+
+	if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ENTER ||
+	    reason == CLOCK_EVT_NOTIFY_BROADCAST_EXIT) {
+		clockevents_do_notify(reason, arg);
+		return;
+	}
+
 	raw_spin_lock_irqsave(&clockevents_lock, flags);
 	clockevents_do_notify(reason, arg);
 
-- 
1.7.4.4

[PATCH 2/3] broadcast-tick: Move oneshot broadcast mask to per cpu variables

[Andi Kleen]

From: Andi Kleen <ak@linux.intel.com>

Avoid a global cache line hotspot in the oneshot cpu mask. Maintain
this information in per cpu variables instead.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 include/linux/tick.h         |    2 +-
 kernel/time/tick-broadcast.c |   40 ++++++++++++++++++++++++----------------
 kernel/time/timer_list.c     |   11 +++++++++--
 3 files changed, 34 insertions(+), 19 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index b232ccc..3df6e2e 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -90,7 +90,7 @@ extern struct tick_device *tick_get_broadcast_device(void);
 extern struct cpumask *tick_get_broadcast_mask(void);
 
 #  ifdef CONFIG_TICK_ONESHOT
-extern struct cpumask *tick_get_broadcast_oneshot_mask(void);
+extern void tick_get_broadcast_oneshot_mask(struct cpumask *);
 #  endif
 
 # endif /* BROADCAST */
diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index c7218d1..7f33b15 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -358,15 +358,22 @@ int tick_resume_broadcast(void)
 
 #ifdef CONFIG_TICK_ONESHOT
 
-/* FIXME: use cpumask_var_t. */
-static DECLARE_BITMAP(tick_broadcast_oneshot_mask, NR_CPUS);
+struct broadcast_cpu_state {
+	int need_oneshot;
+} ____cacheline_aligned;
+static DEFINE_PER_CPU(struct broadcast_cpu_state, state);
 
 /*
  * Exposed for debugging: see timer_list.c
  */
-struct cpumask *tick_get_broadcast_oneshot_mask(void)
+void tick_get_broadcast_oneshot_mask(struct cpumask *mask)
 {
-	return to_cpumask(tick_broadcast_oneshot_mask);
+	int i;
+
+	for_each_online_cpu (i) {
+		if (per_cpu(state, i).need_oneshot)
+			cpumask_set_cpu(i, mask);
+	}
 }
 
 static int tick_broadcast_set_event(ktime_t expires, int force)
@@ -388,7 +395,7 @@ int tick_resume_broadcast_oneshot(struct clock_event_device *bc)
  */
 void tick_check_oneshot_broadcast(int cpu)
 {
-	if (cpumask_test_cpu(cpu, to_cpumask(tick_broadcast_oneshot_mask))) {
+	if (per_cpu(state, cpu).need_oneshot) {
 		struct tick_device *td = &per_cpu(tick_cpu_device, cpu);
 
 		clockevents_set_mode(td->evtdev, CLOCK_EVT_MODE_ONESHOT);
@@ -411,7 +418,9 @@ again:
 	cpumask_clear(to_cpumask(tmpmask));
 	now = ktime_get();
 	/* Find all expired events */
-	for_each_cpu(cpu, tick_get_broadcast_oneshot_mask()) {
+	for_each_online_cpu(cpu) {
+		if (!per_cpu(state, cpu).need_oneshot)
+			continue;
 		td = &per_cpu(tick_cpu_device, cpu);
 		if (td->evtdev->next_event.tv64 <= now.tv64)
 			cpumask_set_cpu(cpu, to_cpumask(tmpmask));
@@ -478,16 +487,15 @@ void tick_broadcast_oneshot_control(unsigned long reason)
 
 	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 	if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ENTER) {
-		if (!cpumask_test_cpu(cpu, tick_get_broadcast_oneshot_mask())) {
-			cpumask_set_cpu(cpu, tick_get_broadcast_oneshot_mask());
+		if (!__get_cpu_var(state).need_oneshot) {
+			__get_cpu_var(state).need_oneshot = 1;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
 			if (dev->next_event.tv64 < bc->next_event.tv64)
 				tick_broadcast_set_event(dev->next_event, 1);
 		}
 	} else {
-		if (cpumask_test_cpu(cpu, tick_get_broadcast_oneshot_mask())) {
-			cpumask_clear_cpu(cpu,
-					  tick_get_broadcast_oneshot_mask());
+		if (__get_cpu_var(state).need_oneshot) {
+			__get_cpu_var(state).need_oneshot = 0;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_ONESHOT);
 			if (dev->next_event.tv64 != KTIME_MAX)
 				tick_program_event(dev->next_event, 1);
@@ -503,7 +511,7 @@ void tick_broadcast_oneshot_control(unsigned long reason)
  */
 static void tick_broadcast_clear_oneshot(int cpu)
 {
-	cpumask_clear_cpu(cpu, tick_get_broadcast_oneshot_mask());
+	per_cpu(state, cpu).need_oneshot = 0;
 }
 
 static void tick_broadcast_init_next_event(struct cpumask *mask,
@@ -529,6 +537,7 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
 	/* Set it up only once ! */
 	if (bc->event_handler != tick_handle_oneshot_broadcast) {
 		int was_periodic = bc->mode == CLOCK_EVT_MODE_PERIODIC;
+		int i;
 
 		bc->event_handler = tick_handle_oneshot_broadcast;
 		clockevents_set_mode(bc, CLOCK_EVT_MODE_ONESHOT);
@@ -544,9 +553,8 @@ void tick_broadcast_setup_oneshot(struct clock_event_device *bc)
 		 */
 		cpumask_copy(to_cpumask(tmpmask), tick_get_broadcast_mask());
 		cpumask_clear_cpu(cpu, to_cpumask(tmpmask));
-		cpumask_or(tick_get_broadcast_oneshot_mask(),
-			   tick_get_broadcast_oneshot_mask(),
-			   to_cpumask(tmpmask));
+		for_each_cpu (i, to_cpumask(tmpmask))
+			per_cpu(state, i).need_oneshot = 1;
 
 		if (was_periodic && !cpumask_empty(to_cpumask(tmpmask))) {
 			tick_broadcast_init_next_event(to_cpumask(tmpmask),
@@ -598,7 +606,7 @@ void tick_shutdown_broadcast_oneshot(unsigned int *cpup)
 	 * Clear the broadcast mask flag for the dead cpu, but do not
 	 * stop the broadcast device!
 	 */
-	cpumask_clear_cpu(cpu, tick_get_broadcast_oneshot_mask());
+	per_cpu(state, cpu).need_oneshot = 0;
 
 	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }
diff --git a/kernel/time/timer_list.c b/kernel/time/timer_list.c
index 3258455..28964ed 100644
--- a/kernel/time/timer_list.c
+++ b/kernel/time/timer_list.c
@@ -235,14 +235,21 @@ print_tickdevice(struct seq_file *m, struct tick_device *td, int cpu)
 static void timer_list_show_tickdevices(struct seq_file *m)
 {
 	int cpu;
+#ifdef CONFIG_TICK_ONESHOT
+	cpumask_var_t mask;
+#endif
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
 	print_tickdevice(m, tick_get_broadcast_device(), -1);
 	SEQ_printf(m, "tick_broadcast_mask: %08lx\n",
 		   cpumask_bits(tick_get_broadcast_mask())[0]);
 #ifdef CONFIG_TICK_ONESHOT
-	SEQ_printf(m, "tick_broadcast_oneshot_mask: %08lx\n",
-		   cpumask_bits(tick_get_broadcast_oneshot_mask())[0]);
+	if (zalloc_cpumask_var(&mask, GFP_KERNEL) == 0) {
+		tick_get_broadcast_oneshot_mask(mask);
+		SEQ_printf(m, "tick_broadcast_oneshot_mask: %08lx\n",
+			   cpumask_bits(mask)[0]);
+		free_cpumask_var(mask);
+	}
 #endif
 	SEQ_printf(m, "\n");
 #endif
-- 
1.7.4.4

[PATCH 3/3] tick-broadcast: push down tick_broadcast_lock

[Andi Kleen]

From: Andi Kleen <ak@linux.intel.com>

For the oneshot case, only take the tick_broadcast_lock when the
global device is actually changing. For the case when the new
event is only setting the wakeup to a later time than it already
is we don't need the lock.

This avoids lock contention for some special cases on systems
that don't have an always running per cpu timer. It's not a full
solution to the scalability problem there unfortunately, just
the first step.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 kernel/time/tick-broadcast.c |   18 ++++++++++++++----
 1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index 7f33b15..1576da7 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -485,23 +485,33 @@ void tick_broadcast_oneshot_control(unsigned long reason)
 
 	bc = tick_broadcast_device.evtdev;
 
-	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 	if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ENTER) {
 		if (!__get_cpu_var(state).need_oneshot) {
 			__get_cpu_var(state).need_oneshot = 1;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
-			if (dev->next_event.tv64 < bc->next_event.tv64)
+		       
+			/* Only take the lock if the events gets set earlier */
+			if (dev->next_event.tv64 < bc->next_event.tv64) {
+				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 				tick_broadcast_set_event(dev->next_event, 1);
+				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
+							   flags);		
+			}
 		}
 	} else {
 		if (__get_cpu_var(state).need_oneshot) {
 			__get_cpu_var(state).need_oneshot = 0;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_ONESHOT);
-			if (dev->next_event.tv64 != KTIME_MAX)
+
+			/* Only take the lock if the event changes */
+			if (dev->next_event.tv64 != KTIME_MAX) {
+				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 				tick_program_event(dev->next_event, 1);
+				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
+							   flags);
+			}
 		}
 	}
-	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }
 
 /*
-- 
1.7.4.4

Re: [PATCH 2/3] broadcast-tick: Move oneshot broadcast mask to per cpu variables

[Eric Dumazet]

Le vendredi 26 août 2011 à 15:32 -0700, Andi Kleen a écrit :
> From: Andi Kleen <ak@linux.intel.com>
> 
> Avoid a global cache line hotspot in the oneshot cpu mask. Maintain
> this information in per cpu variables instead.
> 
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---

>  
> -/* FIXME: use cpumask_var_t. */
> -static DECLARE_BITMAP(tick_broadcast_oneshot_mask, NR_CPUS);
> +struct broadcast_cpu_state {
> +	int need_oneshot;
> +} ____cacheline_aligned;
> +static DEFINE_PER_CPU(struct broadcast_cpu_state, state);

static DEFINE_PER_CPU_ALIGNED(...)

It avoids adding an alignment requirement on .data..percpu

Re: [PATCH 3/3] tick-broadcast: push down tick_broadcast_lock

[Thomas Gleixner]

On Tue, 6 Sep 2011, Andi Kleen wrote:

> On Tue, Sep 06, 2011 at 06:19:00PM +0200, Thomas Gleixner wrote:
> 
> > 
> > There is no full solution to that problem other than using sane
> > hardware.
> 
> Not convinced.
> 
> BTW can you at least merge the first patch for the notifiers. 
> This fixes the "fixed hardware" which is currently broken too.
> 
> >  raw_spin_lock(&tick_broadcast_lock);
> >  bc->next_event = KTIME_MAX;
> >  for_each_online_cpu() {
> > 	next_event = ...;
> >  }
> >  ....                                if (dev->next_event < bc->next_event) {
> >                                        raw_spin_lock(&tick_broadcast_lock);
> > 
> >  tick_broadcast_set_event(next_event, 0);
> >    bc->next_event = next_event;
> > 
> >  raw_spin_unlock(&tick_broadcast_lock);
> >                                        tick_broadcast_set_event(dev->next_event, 1);
> > 
> > So you unconditionally set the broadcast device to dev->next_event of
> > CPU1 even if the current pending event which was evaluated on CPU0 is
> > _BEFORE_ the CPU1 event. That can cause stalls and other hard to debug
> > horror. We've been there before.
> 
> I don't understand. It only sets it if the new event is earlier.
> So it can never be set back.

If you read the above you see that the broadcast handler sets the next
event to KTIME_MAX first. Then it looks for the next event and
programs it to that expiry time.

So when the check on the other CPU happens after CPU0 does
bc->next_event = KTIME_MAX and before CPU0 finished reprogramming,
then CPU1 sees KTIME_MAX and the comparison evaluates true. So CPU1
waits for the lock which is held by CPU0 and then sets the BC device
to CPU1 next event unconditionally, which might be _AFTER_ the already
pending event which was set by CPU0.
 
> You seem to say the opposite?

No, that's racy because you do not hold the lock when doing the
comparision.
 
> > 
> > Further the unprotected comparison on 32bit is completely bogus.
> 
> Ok.  Just need a ordered read like i_size_read().
> 
> > > -			if (dev->next_event.tv64 != KTIME_MAX)
> > > +
> > > +			/* Only take the lock if the event changes */
> > > +			if (dev->next_event.tv64 != KTIME_MAX) {
> > > +				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
> > 
> > Why would you take the global lock to program the cpu local device?
> > Just because it happened to be under that lock before?
> 
> Yes, I didn't audit that code. But probably it can be dropped
> you're right.

That's not a question of auditing, it's a question of understanding
the code which you modify.

Thanks,

	tglx

Re: [PATCH 3/3] tick-broadcast: push down tick_broadcast_lock

[Andi Kleen]

On Tue, Sep 06, 2011 at 06:19:00PM +0200, Thomas Gleixner wrote:

> 
> There is no full solution to that problem other than using sane
> hardware.

Not convinced.

BTW can you at least merge the first patch for the notifiers. 
This fixes the "fixed hardware" which is currently broken too.

>  raw_spin_lock(&tick_broadcast_lock);
>  bc->next_event = KTIME_MAX;
>  for_each_online_cpu() {
> 	next_event = ...;
>  }
>  ....                                if (dev->next_event < bc->next_event) {
>                                        raw_spin_lock(&tick_broadcast_lock);
> 
>  tick_broadcast_set_event(next_event, 0);
>    bc->next_event = next_event;
> 
>  raw_spin_unlock(&tick_broadcast_lock);
>                                        tick_broadcast_set_event(dev->next_event, 1);
> 
> So you unconditionally set the broadcast device to dev->next_event of
> CPU1 even if the current pending event which was evaluated on CPU0 is
> _BEFORE_ the CPU1 event. That can cause stalls and other hard to debug
> horror. We've been there before.

I don't understand. It only sets it if the new event is earlier.
So it can never be set back.

You seem to say the opposite?

> 
> Further the unprotected comparison on 32bit is completely bogus.

Ok.  Just need a ordered read like i_size_read().

> > -			if (dev->next_event.tv64 != KTIME_MAX)
> > +
> > +			/* Only take the lock if the event changes */
> > +			if (dev->next_event.tv64 != KTIME_MAX) {
> > +				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
> 
> Why would you take the global lock to program the cpu local device?
> Just because it happened to be under that lock before?

Yes, I didn't audit that code. But probably it can be dropped
you're right.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.

Re: [PATCH 3/3] tick-broadcast: push down tick_broadcast_lock

[Thomas Gleixner]

On Mon, 29 Aug 2011, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
> 
> For the oneshot case, only take the tick_broadcast_lock when the
> global device is actually changing. For the case when the new
> event is only setting the wakeup to a later time than it already
> is we don't need the lock.
> 
> This avoids lock contention for some special cases on systems
> that don't have an always running per cpu timer. It's not a full
> solution to the scalability problem there unfortunately, just
> the first step.

There is no full solution to that problem other than using sane
hardware.

> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
>  kernel/time/tick-broadcast.c |   18 ++++++++++++++----
>  1 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
> index 54a5977..7e748fb 100644
> --- a/kernel/time/tick-broadcast.c
> +++ b/kernel/time/tick-broadcast.c
> @@ -485,23 +485,33 @@ void tick_broadcast_oneshot_control(unsigned long reason)
>  
>  	bc = tick_broadcast_device.evtdev;
>  
> -	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
>  	if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ENTER) {
>  		if (!__get_cpu_var(state).need_oneshot) {
>  			__get_cpu_var(state).need_oneshot = 1;
>  			clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
> -			if (dev->next_event.tv64 < bc->next_event.tv64)
> +		       
> +			/* Only take the lock if the events gets set earlier */
> +			if (dev->next_event.tv64 < bc->next_event.tv64) {

That's racy and broken.

CPU0                                 CPU1

tick_handle_oneshot_broadcast()

 raw_spin_lock(&tick_broadcast_lock);
 bc->next_event = KTIME_MAX;
 for_each_online_cpu() {
	next_event = ...;
 }
 ....                                if (dev->next_event < bc->next_event) {
                                       raw_spin_lock(&tick_broadcast_lock);

 tick_broadcast_set_event(next_event, 0);
   bc->next_event = next_event;

 raw_spin_unlock(&tick_broadcast_lock);
                                       tick_broadcast_set_event(dev->next_event, 1);

So you unconditionally set the broadcast device to dev->next_event of
CPU1 even if the current pending event which was evaluated on CPU0 is
_BEFORE_ the CPU1 event. That can cause stalls and other hard to debug
horror. We've been there before.

Further the unprotected comparison on 32bit is completely bogus.

> +				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
>  				tick_broadcast_set_event(dev->next_event, 1);
> +				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
> +							   flags);		
> +			}
>  		}
>  	} else {
>  		if (__get_cpu_var(state).need_oneshot) {
>  			__get_cpu_var(state).need_oneshot = 0;
>  			clockevents_set_mode(dev, CLOCK_EVT_MODE_ONESHOT);
> -			if (dev->next_event.tv64 != KTIME_MAX)
> +
> +			/* Only take the lock if the event changes */
> +			if (dev->next_event.tv64 != KTIME_MAX) {
> +				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);

Why would you take the global lock to program the cpu local device?
Just because it happened to be under that lock before?

>  				tick_program_event(dev->next_event, 1);
> +				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
> +							   flags);

Thanks,

	tglx

[PATCH 3/3] tick-broadcast: push down tick_broadcast_lock

[Andi Kleen]

From: Andi Kleen <ak@linux.intel.com>

For the oneshot case, only take the tick_broadcast_lock when the
global device is actually changing. For the case when the new
event is only setting the wakeup to a later time than it already
is we don't need the lock.

This avoids lock contention for some special cases on systems
that don't have an always running per cpu timer. It's not a full
solution to the scalability problem there unfortunately, just
the first step.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
---
 kernel/time/tick-broadcast.c |   18 ++++++++++++++----
 1 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index 54a5977..7e748fb 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c
@@ -485,23 +485,33 @@ void tick_broadcast_oneshot_control(unsigned long reason)
 
 	bc = tick_broadcast_device.evtdev;
 
-	raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 	if (reason == CLOCK_EVT_NOTIFY_BROADCAST_ENTER) {
 		if (!__get_cpu_var(state).need_oneshot) {
 			__get_cpu_var(state).need_oneshot = 1;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_SHUTDOWN);
-			if (dev->next_event.tv64 < bc->next_event.tv64)
+		       
+			/* Only take the lock if the events gets set earlier */
+			if (dev->next_event.tv64 < bc->next_event.tv64) {
+				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 				tick_broadcast_set_event(dev->next_event, 1);
+				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
+							   flags);		
+			}
 		}
 	} else {
 		if (__get_cpu_var(state).need_oneshot) {
 			__get_cpu_var(state).need_oneshot = 0;
 			clockevents_set_mode(dev, CLOCK_EVT_MODE_ONESHOT);
-			if (dev->next_event.tv64 != KTIME_MAX)
+
+			/* Only take the lock if the event changes */
+			if (dev->next_event.tv64 != KTIME_MAX) {
+				raw_spin_lock_irqsave(&tick_broadcast_lock, flags);
 				tick_program_event(dev->next_event, 1);
+				raw_spin_unlock_irqrestore(&tick_broadcast_lock, 
+							   flags);
+			}
 		}
 	}
-	raw_spin_unlock_irqrestore(&tick_broadcast_lock, flags);
 }
 
 /*
-- 
1.7.4.4