In message <[EMAIL PROTECTED]> you wrote: > Michael Neuling wrote: > > 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 > > =================================================================== > > --- 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.
I've not changed the math here much, just the case of user_time being zero. Is this related to this patch specifically, or something that's been wrong with these patches for a while? Mikey _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
