Re: [RFC PATCH 06/30] s390: Introduce cputime64_to_nsecs()

[Frederic Weisbecker <fweisbec@gmail.com>]

On Mon, Dec 01, 2014 at 01:24:52PM +0100, Heiko Carstens wrote:
> On Fri, Nov 28, 2014 at 07:23:36PM +0100, Frederic Weisbecker wrote:
> > This will be needed for the conversion of kernel stat to nsecs.
> > 
> > Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> > Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
> > Cc: Ingo Molnar <mingo@kernel.org>
> > Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Rik van Riel <riel@redhat.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Tony Luck <tony.luck@intel.com>
> > Cc: Wu Fengguang <fengguang.wu@intel.com>
> > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> > ---
> >  arch/s390/include/asm/cputime.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h
> > index 820b38a..75ba96f 100644
> > --- a/arch/s390/include/asm/cputime.h
> > +++ b/arch/s390/include/asm/cputime.h
> > @@ -59,6 +59,11 @@ static inline cputime64_t jiffies64_to_cputime64(const u64 jif)
> >  	return (__force cputime64_t)(jif * (CPUTIME_PER_SEC / HZ));
> >  }
> > 
> > +static inline u64 cputime64_to_nsecs(cputime64_t cputime)
> > +{
> > +	return (__force u64)cputime * CPUTIME_PER_USEC * NSEC_PER_USEC;
> > +}
> > +
> 
> This is incorrect. You probably wanted to write something like
> 
> 	return (__force u64)cputime / CPUTIME_PER_USEC * NSEC_PER_USEC; ?

You're right :-)

> 
> However we would still lose a lot of precision.
> The correct algorithm to convert from cputime to nanoseconds can be found in
> tod_to_ns() - see arch/s390/include/asm/timex.h
> 
> And if you see that rather complex algorithm, I doubt we want to have the
> changes you propose. We need to have that calculation three times for each
> irq (user, system and steal time) and would still have worse precision than
> we have right now. Not talking about the additional wasted cpu cycles...

Yeah indeed. So probably it could be better to accumulate the time in cputime_t
and flush it as nsecs on tick.

> 
> But I guess Martin wanted to comment on your patches anyway ;)
>