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Date: Wed, 12 May 2004 13:20:50 -0700
From: Andrew Morton <akpm@osdl.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: davidel@xmailserver.org, jgarzik@pobox.com, greg@kroah.com,
       linux-kernel@vger.kernel.org, netdev@oss.sgi.com
Subject: Re: MSEC_TO_JIFFIES is messed up...
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Ingo Molnar <mingo@elte.hu> wrote:
>
> 
> * Davide Libenzi <davidel@xmailserver.org> wrote:
> 
> > > why is it wrong?
> > 
> > For HZ == 1000 it's fine, even if it'd better to explicitly make it HZ
> > dependent and let the compiler to discard them.
> 
> the compiler cannot discard the multiplication and the division from the
> following:
> 
> 	x * 1000 / 1000
> 
> due to overflows. But we know that HZ is 1000 in the arch-dependent
> param.h, and in sched.c we use the HZ dependent variant:
> 
>  #ifndef JIFFIES_TO_MSEC
>  # define JIFFIES_TO_MSEC(x) ((x) * 1000 / HZ)
>  #endif
>  #ifndef MSEC_TO_JIFFIES
>  # define MSEC_TO_JIFFIES(x) ((x) * HZ / 1000)
>  #endif
> 

Yes, that's a correct optimisation.  This is simply a namespace clash.

How about we do:

#if HZ=1000
#define	MSEC_TO_JIFFIES(msec) (msec)
#define JIFFIES_TO_MESC(jiffies) (jiffies)
#elif HZ=100
#define	MSEC_TO_JIFFIES(msec) (msec * 10)
#define JIFFIES_TO_MESC(jiffies) (jiffies / 10)
#else
#define	MSEC_TO_JIFFIES(msec) ((HZ * (msec) + 999) / 1000)
#define	JIFFIES_TO_MSEC(jiffies) ...
#endif

in some kernel-wide header then kill off all the private implementations?