Re: [PATCH] sched: avoid unnecessary overflow in sched_clock

[Mike Galbraith <efault@gmx.de>]

On Tue, 2011-11-15 at 15:02 -0800, john stultz wrote:
> On Tue, 2011-11-15 at 14:12 -0800, Salman Qazi wrote:
> > (Added the missing signed-off-by line)
> > 
> > In hundreds of days, the __cycles_2_ns calculation in sched_clock
> > has an overflow.  cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing
> > the final value to become zero.  We can solve this without losing
> > any precision.
> > 
> > We can decompose TSC into quotient and remainder of division by the
> > scale factor, and then use this to convert TSC into nanoseconds.
> > 
> > Signed-off-by: Salman Qazi <sqazi@google.com>
> 
> Acked-by: John Stultz <johnstul@us.ibm.com>

This wants a stable tag, no?

	-Mike

> > ---
> >  arch/x86/include/asm/timer.h |   23 ++++++++++++++++++++++-
> >  1 files changed, 22 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h
> > index fa7b917..431793e 100644
> > --- a/arch/x86/include/asm/timer.h
> > +++ b/arch/x86/include/asm/timer.h
> > @@ -32,6 +32,22 @@ extern int no_timer_check;
> >   *  (mathieu.desnoyers@polymtl.ca)
> >   *
> >   *			-johnstul@us.ibm.com "math is hard, lets go shopping!"
> > + *
> > + * In:
> > + *
> > + * ns = cycles * cyc2ns_scale / SC
> > + *
> > + * Although we may still have enough bits to store the value of ns,
> > + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale,
> > + * leading to an incorrect result.
> > + *
> > + * To avoid this, we can decompose 'cycles' into quotient and remainder
> > + * of division by SC.  Then,
> > + *
> > + * ns = (quot * SC + rem) * cyc2ns_scale / SC
> > + *    = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC
> > + *
> > + *			- sqazi@google.com
> >   */
> > 
> >  DECLARE_PER_CPU(unsigned long, cyc2ns);
> > @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset);
> > 
> >  static inline unsigned long long __cycles_2_ns(unsigned long long cyc)
> >  {
> > +	unsigned long long quot;
> > +	unsigned long long rem;
> >  	int cpu = smp_processor_id();
> >  	unsigned long long ns = per_cpu(cyc2ns_offset, cpu);
> > -	ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR;
> > +	quot = (cyc >> CYC2NS_SCALE_FACTOR);
> > +	rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1);
> > +	ns += quot * per_cpu(cyc2ns, cpu) +
> > +		((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR);
> >  	return ns;
> >  }
> > 
> > 
> 
> 
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