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

[Andi Kleen <andi@firstfloor.org>]

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