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

[Peter Zijlstra <peterz@infradead.org>]

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.

> + */
> +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.

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

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