Re: [Xen-devel] [PATCH] xen, credit2: Avoid extra c2t calcuation in csched_runtime

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.01.13 at 16:55, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> On 24/01/13 15:45, Jan Beulich wrote:
>>>>> On 24.01.13 at 16:24, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
>>> +    /* FIXME: See if we can eliminate this conversion if we know time
>>> +     * will be outside (MIN,MAX).  Probably requires pre-calculating
>>> +     * credit values of MIN,MAX per vcpu, since each vcpu burns credit
>>> +     * at a different rate. */
>> The obvious situation is when rt_credit <= 0, which you could
>> deal with right away.
> 
> That shouldn't be possible:
> 1. csched_runtime() should only be called on someone about to be 
> scheduled (from csched_schedule()).
> 2. The credit of such an snext should always be higher than anyone else 
> on the queue (or they would be scheduled instead).
> 3. If someone is about to be scheduled with snext->credit <= 0, it will 
> trigger reset_credt(), which will make everyone's credit positive.
> 4. 3 guarantees that snext->credit will be positive
> 5. 2 guatantees that (snext->credit - swait->credit) will be positive
> 6 So rt_credit should never be <= 0.
> 
> I can add an assert to this effect if you like.

That would be nice, because I came up with the whole thing just
because I was trying to understand whether the 2nd argument
to c2t() in the invocation this change removes could go negative.

Jan


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