Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

[Juri Lelli]

On 04/23/2012 01:55 PM, Peter Zijlstra wrote:
> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
> > +/*
> > + * Here we check if --at time t-- an entity (which is probably being
> > + * [re]activated or, in general, enqueued) can use its remaining runtime
> > + * and its current deadline _without_ exceeding the bandwidth it is
> > + * assigned (function returns true if it can).
> > + *
> > + * For this to hold, we must check if:
> > + *   runtime / (deadline - t)<  dl_runtime / dl_deadline .
>
> It might be good to put a few words in as to why that is.. I know I
> always forget (but know where to find it by now), also might be good to
> refer those papers Tommaso listed when Steven asked this a while back.

Ok, I'll fix the comment, extend it and add T.'s references in the
Documentation.

> > + */
> > +static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
> > +{
> > +       u64 left, right;
> > +
> > +       /*
> > +        * left and right are the two sides of the equation above,
> > +        * after a bit of shuffling to use multiplications instead
> > +        * of divisions.
> > +        *
> > +        * Note that none of the time values involved in the two
> > +        * multiplications are absolute: dl_deadline and dl_runtime
> > +        * are the relative deadline and the maximum runtime of each
> > +        * instance, runtime is the runtime left for the last instance
> > +        * and (deadline - t), since t is rq->clock, is the time left
> > +        * 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;
> > +       right = (dl_se->deadline - t) * dl_se->dl_runtime;
>
>
>  From what I can see there are no constraints on the values in
> __setparam_dl() so the above left term can be constructed to be an
> overflow.

Yes, could happen :-\.

> Ideally we'd use u128 here, but I don't think people will let us :/

Do we need to do something about that? If we cannot go for bigger space
probably limit dl_deadline (or warn the user)..

> > +       return dl_time_before(right, left);
> > +}
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