Re: [PATCH][2/4] sched: add discrete weighted cpu load function

[Peter Williams <pwil3058@bigpond.net.au>]

Con Kolivas wrote:
> On Wednesday 15 March 2006 09:09, Peter Williams wrote:
>
> > Con Kolivas wrote:
> >
> > > Peter Williams writes:
> > >
> > > > Con Kolivas wrote:
> > > >
> > > > > +unsigned long weighted_cpuload(const int cpu)
> > > > > +{
> > > > > +    return (cpu_rq(cpu)->raw_weighted_load);
> > > > > +}
> > > > > +
> > > >
> > > > Wouldn't this be a candidate for inlining?
> > >
> > > That would make it unsuitable for exporting via sched.h.
> >
> > If above_background_load() were implemented inside sched.c instead of in
> > sched.h there would be no need to export weighted_cpuload() would there?
> >  This would allow weighted_cpuload() to be inline and the efficiency
> > would be better as above_background_load() doesn't gain a lot by being
> > inline
>
>
> I don't care about above_background_load() being inline; that's done because 
> all functions in header files need to be static inline to not become a mess.
>
>
> > as having weighted_cpulpad() non inline means that it's doing a 
> > function call several times in a loop i.e. it may save one function call
> > by being inline but requires (up to) one function call for every CPU.
>
>
> I haven't checked but gcc may well inline weighted_cpuload anyway?

It may be doing so for internal uses inside sched.c but I'm pretty sure 
that it won't for external calls.

> We're 
> moving away from inlining most things manually since the compiler is doing it 
> well these days.

OK.  Even without explicit inlining you need to give the compiler a 
chance to optimize function call overhead away.  It cant do that with 
the present arrangement.

> > The other way around the cost would be just one function call.
>
>
> The way you're suggesting adds a function that is never used by anything but 
> swap prefetch which would then need to be 'ifdef'ed out to not be needlessly 
> built on every system. Adding ifdefs is frowned upon already, and to have an 
> mm/ specific ifdef in sched.c would be rather ugly.

Sometimes ugliness is the best option.

Peter
--
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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