linux-kernel - Re: [PATCH] Prevent the loop in timespec_add

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Date:	Thu, 28 Feb 2008 22:58:49 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	john stultz <johnstul@...ibm.com>
Subject: Re: [PATCH] Prevent the loop in timespec_add_ns() to be optimised
 away

On Wed, 27 Feb 2008, Andrew Morton wrote:

> On Fri, 22 Feb 2008 22:40:45 +0100
> Segher Boessenkool <segher@...nel.crashing.org> wrote:
> 
> > ...since some architectures don't support __udivdi3() (and
> > we don't want to use that, anyway).
> > 
> > Signed-off-by: Segher Boessenkool <segher@...nel.crashing.org>
> > ---
> >  include/linux/time.h |    4 ++++
> >  1 files changed, 4 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/time.h b/include/linux/time.h
> > index 2091a19..d32ef0a 100644
> > --- a/include/linux/time.h
> > +++ b/include/linux/time.h
> > @@ -174,6 +174,10 @@ static inline void timespec_add_ns(struct timespec *a, u64 ns)
> >  {
> >  	ns += a->tv_nsec;
> >  	while(unlikely(ns >= NSEC_PER_SEC)) {
> > +		/* The following asm() prevents the compiler from
> > +		 * optimising this loop into a modulo operation.  */
> > +		asm("" : "+r"(ns));
> > +
> >  		ns -= NSEC_PER_SEC;
> >  		a->tv_sec++;
> >  	}
> 
> It's pretty sad that we need to turn this into a loop just because of the
> __udivdi3() thing.
> 
> otoh, it's rarely occurring, and it could be that the number of times it
> loops is usually 1 (if it wasn't zero), so perhaps a loop is faster than a
> divide anyway.
> 
> This code is probably too large to be inlined.
> 
> I queued this patch as needed-in-2.6.25, to-be-merged-via-Thomas.

Are you going to send it or should I grab it from the mailing list
myself ?

Thanks,
	tglx
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