From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758830AbYB1WA7 (ORCPT ); Thu, 28 Feb 2008 17:00:59 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759551AbYB1WAh (ORCPT ); Thu, 28 Feb 2008 17:00:37 -0500 Received: from www.tglx.de ([62.245.132.106]:40648 "EHLO www.tglx.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759140AbYB1WAg (ORCPT ); Thu, 28 Feb 2008 17:00:36 -0500 Date: Thu, 28 Feb 2008 22:58:49 +0100 (CET) From: Thomas Gleixner To: Andrew Morton cc: Segher Boessenkool , linux-kernel@vger.kernel.org, Ingo Molnar , john stultz Subject: Re: [PATCH] Prevent the loop in timespec_add_ns() to be optimised away In-Reply-To: <20080227154341.824efe3c.akpm@linux-foundation.org> Message-ID: References: <6a95d77ad55b29ff2d856c9e1f7dd79682370300.1203715782.git.segher@kernel.crashing.org> <20080227154341.824efe3c.akpm@linux-foundation.org> User-Agent: Alpine 1.00 (LFD 882 2007-12-20) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 27 Feb 2008, Andrew Morton wrote: > On Fri, 22 Feb 2008 22:40:45 +0100 > Segher Boessenkool wrote: > > > ...since some architectures don't support __udivdi3() (and > > we don't want to use that, anyway). > > > > Signed-off-by: Segher Boessenkool > > --- > > 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