  `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 Ialways forget (but know where to find it by now), also might be good torefer 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 anoverflow.Ideally we'd use u128 here, but I don't think people will let us :/> +       return dl_time_before(right, left);> +} `   