[PATCH] fix scaled time accounting possible divide by zero

[Michael Neuling <mikey@neuling.org>]

If we get no user time allocated since the last account_system_vtime,
the system to user time ratio estimate can end up dividing by zero.

This was causing a problem noticed by Balbir Singh.

Signed-off-by: Michael Neuling <mikey@neuling.org>
---
Resent as the first version had whitespace corruption.  

Paulus: can we send this up for 2.6.24?

 arch/powerpc/kernel/time.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6-ozlabs/arch/powerpc/kernel/time.c
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/time.c
+++ linux-2.6-ozlabs/arch/powerpc/kernel/time.c
@@ -241,8 +241,9 @@ void account_system_vtime(struct task_st
 		/* deltascaled includes both user and system time.
 		 * Hence scale it based on the purr ratio to estimate
 		 * the system time */
-		deltascaled = deltascaled * get_paca()->system_time /
-			(get_paca()->system_time + get_paca()->user_time);
+		if (get_paca()->user_time)
+			deltascaled = deltascaled * get_paca()->system_time /
+			     (get_paca()->system_time + get_paca()->user_time);
 		delta += get_paca()->system_time;
 		get_paca()->system_time = 0;
 	}