lkml.org 
[lkml]   [2011]   [Nov]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH] sched: avoid unnecessary overflow in sched_clock
From
Date
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;
> }
>
>




\
 
 \ /
  Last update: 2011-11-16 00:05    [W:0.400 / U:0.040 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site