Re: [PATCH 08/16] sched: add period support for -deadline tasks.

[Juri Lelli]

On 04/11/2012 10:32 PM, Steven Rostedt wrote:
> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
>
> > @@ -293,7 +293,11 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
> >    * assigned (function returns true if it can).
> >    *
> >    * For this to hold, we must check if:
> > - *   runtime / (deadline - t)<  dl_runtime / dl_deadline .
> > + *   runtime / (deadline - t)<  dl_runtime / dl_period .
> > + *
> > + * Notice that the bandwidth check is done against the period. For
> > + * task with deadline equal to period this is the same of using
> > + * dl_deadline instead of dl_period in the equation above.
>
> First, it seems that the function returns true if:
>
> 	dl_runtime / dl_period<  runtime / (deadline - t)

Right, the comment is wrong! Just reverse the inequality as you did.
 
> I'm a little confused by this. We are comparing the ratio of runtime
> left and deadline left, to the ratio of total runtime to period.
>
> I'm actually confused by this premise anyway. What's the purpose of
> comparing the ratio? If runtime<  (deadline - t) wouldn't it not be able
> to complete anyway? Or are we thinking that the runtime will be
> interrupted proportionally by other tasks?

We are actually applying one of the CBS rules. We want to be able to
"slow down" a deadline task if it is going to exceed its reserved
bandwidth. We are in fact checking here that the bandwidth this task
will consume from t to its deadline is no more than its reserved one.

> >    */
> >   static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
> >   {
> > @@ -312,7 +316,7 @@ static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
> >   	 * to the (absolute) deadline. Therefore, overflowing the u64
> >   	 * type is very unlikely to occur in both cases.
> >   	 */
> > -	left = dl_se->dl_deadline * dl_se->runtime;
> > +	left = dl_se->dl_period * dl_se->runtime;
> >   	right = (dl_se->deadline - t) * dl_se->dl_runtime;
> >
> >   	return dl_time_before(right, left);

Thanks,

- Juri
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