fix scaled time accounting possible divide by zero

This fixes a problem noticed by Balbir Singh

Signed-off-by Michael Neuling <[email protected]>

---
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
===================================================================

arch/powerpc/kernel/time.c +0 -0

 		delta += get_paca()->system_time;
 		get_paca()->system_time = 0;
 	}

Quoted 14 lines · Michael Neuling

This fixes a problem noticed by Balbir Singh

Signed-off-by Michael Neuling <[email protected]>

---
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
===================================================================

arch/powerpc/kernel/time.c void account_system_vtime(struct task_st · line 249

Quoted 8 lines

arch/powerpc/kernel/time.c +3 -2

@@ -244,8 +244,9 @@ 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);

Hi, Michael,

I'd be doubly careful with scaled multiplication approach, we tried this for CFS (please see task_utime() and task_stime() and the fixes that went around it). We ran into problems were due to multiplication rounding errors, we would see stime and utime go back after a period of time.

Please see http://kerneltrap.org/mailarchive/linux-kernel/2007/10/16/344377

Our problems were made severe by the fact that sum_exec_runtime and stime/utime accounting occured differently. stime/utime were sampled at jiffy boundaries and hence could we incorrect. I think we need to use rounding to ensure that ac_scaled*time never goes back due to rounding errors.