[PATCH] fix scaled time accounting possible divide by zero

[Balbir Singh]

Michael Neuling wrote:
> This fixes a problem noticed by Balbir Singh
> 
> Signed-off-by: Michael Neuling <mikey at neuling.org>
> ---
> 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
> @@ -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.

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


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL