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

[Paul Turner]

On 11/15/2011 10:41 PM, Mike Galbraith wrote:
> 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@xxxxxxxxxx>
> >
> > Acked-by: John Stultz<johnstul@xxxxxxxxxx>
>
> This wants a stable tag, no?
>
> 	-Mike

Probably a good idea -- This especially sucks rocks in the sched_clock_stable==1 
case; resulting in it coming straight back out of sched_clock_cpu() and trashing 
rq->clock.

- Paul

> > > ---
> > >   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@xxxxxxxxxx)
> > >    *
> > >    *			-johnstul@xxxxxxxxxx "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@xxxxxxxxxx
> > >    */
> > >
> > >   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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