Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932228AbaLAQXM (ORCPT ); Mon, 1 Dec 2014 11:23:12 -0500 Received: from mail-wg0-f41.google.com ([74.125.82.41]:37213 "EHLO mail-wg0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753388AbaLAQXJ (ORCPT ); Mon, 1 Dec 2014 11:23:09 -0500 Date: Mon, 1 Dec 2014 17:23:06 +0100 From: Frederic Weisbecker To: Heiko Carstens Cc: LKML , Tony Luck , Peter Zijlstra , Benjamin Herrenschmidt , Thomas Gleixner , Oleg Nesterov , Paul Mackerras , Wu Fengguang , Ingo Molnar , Rik van Riel , Martin Schwidefsky Subject: Re: [RFC PATCH 06/30] s390: Introduce cputime64_to_nsecs() Message-ID: <20141201162304.GC27302@lerouge> References: <1417199040-21044-1-git-send-email-fweisbec@gmail.com> <1417199040-21044-7-git-send-email-fweisbec@gmail.com> <20141201122452.GE4074@osiris> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141201122452.GE4074@osiris> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Dec 01, 2014 at 01:24:52PM +0100, Heiko Carstens wrote: > On Fri, Nov 28, 2014 at 07:23:36PM +0100, Frederic Weisbecker wrote: > > This will be needed for the conversion of kernel stat to nsecs. > > > > Cc: Benjamin Herrenschmidt > > Cc: Heiko Carstens > > Cc: Ingo Molnar > > Cc: Martin Schwidefsky > > Cc: Oleg Nesterov > > Cc: Paul Mackerras > > Cc: Peter Zijlstra > > Cc: Rik van Riel > > Cc: Thomas Gleixner > > Cc: Tony Luck > > Cc: Wu Fengguang > > Signed-off-by: Frederic Weisbecker > > --- > > arch/s390/include/asm/cputime.h | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h > > index 820b38a..75ba96f 100644 > > --- a/arch/s390/include/asm/cputime.h > > +++ b/arch/s390/include/asm/cputime.h > > @@ -59,6 +59,11 @@ static inline cputime64_t jiffies64_to_cputime64(const u64 jif) > > return (__force cputime64_t)(jif * (CPUTIME_PER_SEC / HZ)); > > } > > > > +static inline u64 cputime64_to_nsecs(cputime64_t cputime) > > +{ > > + return (__force u64)cputime * CPUTIME_PER_USEC * NSEC_PER_USEC; > > +} > > + > > This is incorrect. You probably wanted to write something like > > return (__force u64)cputime / CPUTIME_PER_USEC * NSEC_PER_USEC; ? You're right :-) > > However we would still lose a lot of precision. > The correct algorithm to convert from cputime to nanoseconds can be found in > tod_to_ns() - see arch/s390/include/asm/timex.h > > And if you see that rather complex algorithm, I doubt we want to have the > changes you propose. We need to have that calculation three times for each > irq (user, system and steal time) and would still have worse precision than > we have right now. Not talking about the additional wasted cpu cycles... Yeah indeed. So probably it could be better to accumulate the time in cputime_t and flush it as nsecs on tick. > > But I guess Martin wanted to comment on your patches anyway ;) > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/