Re: [PATCH] sched: activate SCHED BATCH expired

[Nick Piggin <nickpiggin@yahoo.com.au>]

Con Kolivas wrote:
> On Saturday 18 March 2006 00:26, Nick Piggin wrote:
>
> > Con Kolivas wrote:
> >
> > > -static inline void __activate_task(task_t *p, runqueue_t *rq)
> > > +static void __activate_task(task_t *p, runqueue_t *rq)
> > > {
> > > -	enqueue_task(p, rq->active);
> > > +	if (batch_task(p))
> > > +		enqueue_task(p, rq->expired);
> > > +	else
> > > +		enqueue_task(p, rq->active);
> > > 	inc_nr_running(p, rq);
> > > }
> >
> > I prefer:
> >
> >   prio_array_t *target = rq->active;
> >   if (batch_task(p))
> >     target = rq->expired;
> >   enqueue_task(p, target);
> >
> > Because gcc can use things like predicated instructions for it.
> > But perhaps it is smart enough these days to recognise this?
> > At least in the past I have seen it start using cmov after doing
> > such a conversion.
> >
> > At any rate, I think it looks nicer as well. IMO, of course.
>
>
> Well on my one boring architecture here is a before and after, gcc 4.1.0 with 
> optimise for size kernel config:

> I'm not attached to the style, just the feature. If you think it's warranted 
> I'll change it.

I guess it isn't doing the cmov because it doesn't want to do the
extra load in the common case, which is fair enough (are you compiling
for a pentiumpro+, without generic x86 support? what about if you
turn off optimise for size?)

At least other archtectures might be able to make better use of it,
and I agree even for i386 the code looks better (and slightly smaller).

--
SUSE Labs, Novell Inc.
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