Re: [PATCH] credit: track residual from divisions done during accounting

[David Vrabel <david.vrabel@citrix.com>]

On 25/02/13 09:29, Jan Beulich wrote:
>>>> On 22.02.13 at 18:26, Dario Faggioli <dario.faggioli@citrix.com> wrote:
>> On Mon, 2013-02-18 at 12:37 +0000, Jan Beulich wrote:
>>> --- a/xen/common/sched_credit.c
>>> +++ b/xen/common/sched_credit.c
>>>
>>> @@ -242,6 +244,7 @@ __runq_remove(struct csched_vcpu *svc)
>>>  static void burn_credits(struct csched_vcpu *svc, s_time_t now)
>>>  {
>>>      s_time_t delta;
>>> +    uint64_t val;
>>>      unsigned int credits;
>>>  
>>>      /* Assert svc is current */
>>> @@ -250,7 +253,10 @@ static void burn_credits(struct csched_v
>>>      if ( (delta = now - svc->start_time) <= 0 )
>>>          return;
>>>  
>>> -    credits = (delta*CSCHED_CREDITS_PER_MSEC + MILLISECS(1)/2) / MILLISECS(1);
>>> +    val = delta * CSCHED_CREDITS_PER_MSEC + svc->residual;
>>> +    svc->residual = do_div(val, MILLISECS(1));
>>> +    credits = val;
>>> +    ASSERT(credits == val);
>>
>> I may be missing something, but how can the assert ever be false, given
>> the assignment right before it?
> 
> val being wider than credit, this checks that there was no truncation.

ASSERT(val <= UINT_MAX);

Would be clearer.

David