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