Messages in this thread | | | Subject | Re: [PATCH] sched: avoid unnecessary overflow in sched_clock | From | Mike Galbraith <> | Date | Wed, 16 Nov 2011 07:41:45 +0100 |
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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; > > } > > > > > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/
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