linux-kernel - Re: [PATCH 1/9] time: Condense timekeeper.xtime into xtime

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Date:	Sat, 3 Mar 2012 06:54:12 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	John Stultz <john.stultz@...aro.org>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH 1/9] time: Condense timekeeper.xtime into xtime_sec

On Fri, Mar 02, 2012 at 03:13:25PM -0600, Christoph Lameter wrote:
> On Thu, 1 Mar 2012, John Stultz wrote:
> 
> > +static inline void tk_normalize_xtime(struct timekeeper *tk)
> > +{
> > +	while (tk->xtime_nsec >= ((u64)NSEC_PER_SEC << tk->shift)) {
> > +		tk->xtime_nsec -= (u64)NSEC_PER_SEC << tk->shift;
> > +		tk->xtime_sec++;
> > +	}
> > +}
> 
> Could we avoid the loop?
> 
> y = ((u64)NSEC_PER_SEC << tk->shift));
> tk->xtime_sec += tk->xtime_nsec / y;
> tk->xtime_nsec %= y;

But the two divisions are more costly than addition, substraction, and
shift.

(Normally the code loops just once.)

Richard
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