Messages in this thread | | | From | Paul Turner <> | Subject | Re: [PATCH] sched: avoid unnecessary overflow in sched_clock | Date | Tue, 15 Nov 2011 16:07:38 -0800 |
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On 11/15/2011 03:02 PM, 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> >
Reviewed-by: Paul Turner <pjt@google.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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