Messages in this thread | | | Subject | Re: [PATCH 2/2] MN10300: Make sched_clock() report time since boot | From | Peter Zijlstra <> | Date | Fri, 26 Sep 2008 12:58:39 +0200 |
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On Wed, 2008-09-24 at 17:48 +0100, David Howells wrote: > Make sched_clock() report time since boot rather than time since last timer > interrupt. > > Make sched_clock() expand and scale the 32-bit TSC value running at IOCLK > speed (~33MHz) to a 64-bit nanosecond counter, using cnt32_to_63() acquired > from the ARM arch and without using slow DIVU instructions every call.
Right, so since you have to up-scale the 63 bit counter obtained using the smarty pants cnt32_to_63 code, you actually end up with a genuine 64bit counter.. sweet.
I'm not going to pretend to understand the NM10300 details, but the generic idea makes sense.
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: David Howells <dhowells@redhat.com> > --- > > arch/mn10300/kernel/time.c | 52 +++++++++++++++++++++++++++++++++++--------- > 1 files changed, 41 insertions(+), 11 deletions(-) > > > diff --git a/arch/mn10300/kernel/time.c b/arch/mn10300/kernel/time.c > index babb7c2..e460658 100644 > --- a/arch/mn10300/kernel/time.c > +++ b/arch/mn10300/kernel/time.c > @@ -1,6 +1,6 @@ > /* MN10300 Low level time management > * > - * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved. > + * Copyright (C) 2007-2008 Red Hat, Inc. All Rights Reserved. > * Written by David Howells (dhowells@redhat.com) > * - Derived from arch/i386/kernel/time.c > * > @@ -16,6 +16,7 @@ > #include <linux/init.h> > #include <linux/smp.h> > #include <linux/profile.h> > +#include <linux/cnt32_to_63.h> > #include <asm/irq.h> > #include <asm/div64.h> > #include <asm/processor.h> > @@ -40,27 +41,54 @@ static struct irqaction timer_irq = { > .name = "timer", > }; > > +static unsigned long sched_clock_multiplier; > + > /* > * scheduler clock - returns current time in nanosec units. > */ > unsigned long long sched_clock(void) > { > union { > - unsigned long long l; > - u32 w[2]; > - } quot; > + unsigned long long ll; > + unsigned l[2]; > + } tsc64, result; > + unsigned long tsc, tmp; > + unsigned product[3]; /* 96-bit intermediate value */ > + > + /* read the TSC value > + */ > + tsc = 0 - get_cycles(); /* get_cycles() counts down */ > > - quot.w[0] = mn10300_last_tsc - get_cycles(); > - quot.w[1] = 1000000000; > + /* expand to 64-bits. > + * - sched_clock() must be called once a minute or better or the > + * following will go horribly wrong - see cnt32_to_63() > + */ > + tsc64.ll = cnt32_to_63(tsc) & 0x7fffffffffffffffULL; > > - asm("mulu %2,%3,%0,%1" > - : "=r"(quot.w[1]), "=r"(quot.w[0]) > - : "0"(quot.w[1]), "1"(quot.w[0]) > + /* scale the 64-bit TSC value to a nanosecond value via a 96-bit > + * intermediate > + */ > + asm("mulu %2,%0,%3,%0 \n" /* LSW * mult -> 0:%3:%0 */ > + "mulu %2,%1,%2,%1 \n" /* MSW * mult -> %2:%1:0 */ > + "add %3,%1 \n" > + "addc 0,%2 \n" /* result in %2:%1:%0 */ > + : "=r"(product[0]), "=r"(product[1]), "=r"(product[2]), "=r"(tmp) > + : "0"(tsc64.l[0]), "1"(tsc64.l[1]), "2"(sched_clock_multiplier) > : "cc"); > > - do_div(quot.l, MN10300_TSCCLK); > + result.l[0] = product[1] << 16 | product[0] >> 16; > + result.l[1] = product[2] << 16 | product[1] >> 16; > > - return quot.l; > + return result.ll; > +} > + > +/* > + * initialise the scheduler clock > + */ > +static void __init mn10300_sched_clock_init(void) > +{ > + sched_clock_multiplier = > + __muldiv64u(NSEC_PER_SEC, 1 << 16, MN10300_TSCCLK); > } > > /* > @@ -128,4 +156,6 @@ void __init time_init(void) > /* start the watchdog timer */ > watchdog_go(); > #endif > + > + mn10300_sched_clock_init(); > } >
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