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

[Juri Lelli]

On 04/23/2012 02:22 PM, Peter Zijlstra wrote:
> On Mon, 2012-04-23 at 14:13 +0200, Juri Lelli wrote:
> > On 04/23/2012 01:32 PM, Peter Zijlstra wrote:
> > > On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
> > > > +       /*
> > > > +        * We Keep moving the deadline away until we get some
> > > > +        * available runtime for the entity. This ensures correct
> > > > +        * handling of situations where the runtime overrun is
> > > > +        * arbitrary large.
> > > > +        */
> > > > +       while (dl_se->runtime<= 0) {
> > > > +               dl_se->deadline += dl_se->dl_deadline;
> > > > +               dl_se->runtime += dl_se->dl_runtime;
> > > > +       }
> > >
> > > Does gcc 'optimize' that into a division? If so, it might need special
> > > glue to make it not do that.
> >
> > I got two adds and a jle, no div here..
>
> Gcc is known to change such loops to something like:
>
>   if (runtime<= 0) {
>     tmp = 1 - runtime / dl_runtime;
>     deadline += tmp * dl_deadline;
>     runtime += tmp * dl_runtime;
>   }

This is what I got for that snippet:

ffffffff81062826 <enqueue_task_dl>:
[...]
ffffffff81062885:       49 03 44 24 20          add    0x20(%r12),%rax
ffffffff8106288a:       49 8b 54 24 28          mov    0x28(%r12),%rdx
ffffffff8106288f:       49 01 54 24 38          add    %rdx,0x38(%r12)
ffffffff81062894:       49 89 44 24 30          mov    %rax,0x30(%r12)
ffffffff81062899:       49 8b 44 24 30          mov    0x30(%r12),%rax
ffffffff8106289e:       48 85 c0                test   %rax,%rax
ffffffff810628a1:       7e e2                   jle    ffffffff81062885 <enqueue_task_dl+0x5f>

So it seems we are fine in this case, right?
It is anyway better to enforce this Gcc behaviour, just to be
on the safe side?

Thanks,

- Juri
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