Messages in this thread | | | Subject | Re: [PATCH] sched: avoid unnecessary overflow in sched_clock | From | john stultz <> | Date | Tue, 15 Nov 2011 15:02:03 -0800 |
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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>
> --- > 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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