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

[Tommaso Cucinotta]

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, Researcher
ReTiS Lab, Scuola Superiore Sant'Anna, Pisa, Italy
Tel +39 050 882 024, Fax +39 050 882 003
http://retis.sssup.it/people/tommaso

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