From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753647Ab1KPGlw (ORCPT ); Wed, 16 Nov 2011 01:41:52 -0500 Received: from mailout-de.gmx.net ([213.165.64.22]:50902 "HELO mailout-de.gmx.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with SMTP id S1752073Ab1KPGlv (ORCPT ); Wed, 16 Nov 2011 01:41:51 -0500 X-Authenticated: #14349625 X-Provags-ID: V01U2FsdGVkX1+TstJv++2TUrWWk+RD4iktwdoPwp4XHTbvlepFeU qX3Oz9bqRUExVe Subject: Re: [PATCH] sched: avoid unnecessary overflow in sched_clock From: Mike Galbraith To: john stultz Cc: Salman Qazi , Ingo Molnar , LKML , Peter Zijlstra In-Reply-To: <1321398123.2352.29.camel@work-vm> References: <20111115221121.7262.88871.stgit@dungbeetle.mtv.corp.google.com> <1321398123.2352.29.camel@work-vm> Content-Type: text/plain; charset="UTF-8" Date: Wed, 16 Nov 2011 07:41:45 +0100 Message-ID: <1321425705.12017.1.camel@marge.simson.net> Mime-Version: 1.0 X-Mailer: Evolution 2.32.1 Content-Transfer-Encoding: 7bit X-Y-GMX-Trusted: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2011-11-15 at 15:02 -0800, john stultz wrote: > On Tue, 2011-11-15 at 14:12 -0800, Salman Qazi wrote: > > (Added the missing signed-off-by line) > > > > In hundreds of days, the __cycles_2_ns calculation in sched_clock > > has an overflow. cyc * per_cpu(cyc2ns, cpu) exceeds 64 bits, causing > > the final value to become zero. We can solve this without losing > > any precision. > > > > We can decompose TSC into quotient and remainder of division by the > > scale factor, and then use this to convert TSC into nanoseconds. > > > > Signed-off-by: Salman Qazi > > Acked-by: John Stultz This wants a stable tag, no? -Mike > > --- > > arch/x86/include/asm/timer.h | 23 ++++++++++++++++++++++- > > 1 files changed, 22 insertions(+), 1 deletions(-) > > > > diff --git a/arch/x86/include/asm/timer.h b/arch/x86/include/asm/timer.h > > index fa7b917..431793e 100644 > > --- a/arch/x86/include/asm/timer.h > > +++ b/arch/x86/include/asm/timer.h > > @@ -32,6 +32,22 @@ extern int no_timer_check; > > * (mathieu.desnoyers@polymtl.ca) > > * > > * -johnstul@us.ibm.com "math is hard, lets go shopping!" > > + * > > + * In: > > + * > > + * ns = cycles * cyc2ns_scale / SC > > + * > > + * Although we may still have enough bits to store the value of ns, > > + * in some cases, we may not have enough bits to store cycles * cyc2ns_scale, > > + * leading to an incorrect result. > > + * > > + * To avoid this, we can decompose 'cycles' into quotient and remainder > > + * of division by SC. Then, > > + * > > + * ns = (quot * SC + rem) * cyc2ns_scale / SC > > + * = quot * cyc2ns_scale + (rem * cyc2ns_scale) / SC > > + * > > + * - sqazi@google.com > > */ > > > > DECLARE_PER_CPU(unsigned long, cyc2ns); > > @@ -41,9 +57,14 @@ DECLARE_PER_CPU(unsigned long long, cyc2ns_offset); > > > > static inline unsigned long long __cycles_2_ns(unsigned long long cyc) > > { > > + unsigned long long quot; > > + unsigned long long rem; > > int cpu = smp_processor_id(); > > unsigned long long ns = per_cpu(cyc2ns_offset, cpu); > > - ns += cyc * per_cpu(cyc2ns, cpu) >> CYC2NS_SCALE_FACTOR; > > + quot = (cyc >> CYC2NS_SCALE_FACTOR); > > + rem = cyc & ((1ULL << CYC2NS_SCALE_FACTOR) - 1); > > + ns += quot * per_cpu(cyc2ns, cpu) + > > + ((rem * per_cpu(cyc2ns, cpu)) >> CYC2NS_SCALE_FACTOR); > > return ns; > > } > > > > > > > -- > 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/