`Il 23/04/2012 12:55, Peter Zijlstra ha scritto:> 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 :/why not write this straight in asm, i.e., multiply 64*64 then divide by 64 keeping the intermediate result on 128 bits?Something straightforward to write in asm, but not that easy to let gcc understand that I don't want to multiply 128*128 :-).... a few years ago I had a similar issue; perhaps it was a 32/64 version of this problem, and gcc was not optimizing properly the C code with -O3, so I had used asm segments.In this case, if avoiding the division is a major requirement, then we could multiply twice 64*64 in asm, then compare the two results on 128 bits ? Again, a few assembly lines on architectures supporting the 64*64 and 128-bits comparison.     T.-- Tommaso Cucinotta, Computer Engineering PhD, ResearcherReTiS Lab, Scuola Superiore Sant'Anna, Pisa, ItalyTel +39 050 882 024, Fax +39 050 882 003http://retis.sssup.it/people/tommaso`