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

[Paul Turner <pjt@google.com>]

On 11/15/2011 10:41 PM, Mike Galbraith wrote:
> 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
>

Probably a good idea -- This especially sucks rocks in the sched_clock_stable==1 
case; resulting in it coming straight back out of sched_clock_cpu() and trashing 
rq->clock.

- Paul

>>> ---
>>>   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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>
>