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

[Paul Turner <pjt@google.com>]

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;
>>   }
>>
>>
>
>