linux-kernel - Re: [PATCH] sched: Improve readability in update_cpu

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Message-ID: <20080515171143.GA3584@in.ibm.com>
Date:	Thu, 15 May 2008 22:41:43 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, npiggin@...e.de,
	linux-kernel@...r.kernel.org, Srivatsa Vaddagiri <vatsa@...ibm.com>
Subject: Re: [PATCH] sched: Improve readability in update_cpu_load() code

On Thu, May 15, 2008 at 08:22:15PM +0530, Srivatsa Vaddagiri wrote:
> On Thu, May 15, 2008 at 06:34:59PM +0530, Gautham R Shenoy wrote:
> > Author: Gautham R Shenoy <ego@...ibm.com>
> > Date:   Thu May 15 17:55:49 2008 +0530
> > 
> >     sched: Improve readability in update_cpu_load() code
> >     
> >     Currently the cpu_load[i] is calculated as:
> >     	this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
> >     
> >     However, since scale = 2^i, this can be simplified as:
> >     	this_rq->cpu_load[i] = old_load + ((new_load - old_load) >> i);
> >     
> >     Makes it easier to read.
> >     Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 2d7d8f1..e1a6985 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2921,7 +2921,7 @@ static void update_cpu_load(struct rq *this_rq)
> >  		 */
> >  		if (new_load > old_load)
> >  			new_load += scale-1;
> > -		this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
> > +		this_rq->cpu_load[i] = old_load + ((new_load - old_load) >> i);
> 
> This wont work when new_load < old_load ..
>

Sorry for the noise. I must have read the previous if() condition
incorrectly.

> For ex: I tried this prog:
> 
> #include <stdio.h>
> 
> main()
> {
> 	unsigned long old_load = 100, new_load = 90, this_load, this_load1;
> 	int i = 1, scale = 2 << i;
> 
> 	this_load = (old_load*(scale-1) + new_load) >> i;
> 	this_load1 = old_load + ((new_load - old_load) >> i);
> 
> 	printf ("this_load = %u, this_load1 = %u \n", this_load, this_load1);
> }
> 
> This is what I get:
> 
> $ ./a.out
> this_load = 195, this_load1 = 2147483743
> $
> 
> 	
> 
> -- 
> Regards,
> vatsa

-- 
Thanks and Regards
gautham
--
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