Use clockevent multiplier and shifter for decrementer

Time for which the hrtimer is started for decrementer emulation is calculated using tb_ticks_per_usec. While hrtimer uses the clockevent for DEC reprogramming (if needed) and which calculate timebase ticks using the multiplier and shifter mechanism implemented within clockevent layer. It was observed that this conversion (timebase->time->timebase) are not correct because the mechanism are not consistent. In our setup it adds 2% jitter.

With this patch clockevent multiplier and shifter mechanism are used when starting hrtimer for decrementer emulation. Now the jitter is < 0.5%.

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
---
 arch/powerpc/include/asm/time.h |    2 ++
 arch/powerpc/kernel/time.c      |    6 ++++++
 arch/powerpc/kvm/emulate.c      |    5 +++--
 3 files changed, 11 insertions(+), 2 deletions(-)

On 04/04/2012 01:51 AM, Bharat Bhushan wrote:
> Time for which the hrtimer is started for decrementer emulation is
> calculated using tb_ticks_per_usec. While hrtimer uses the clockevent
> for DEC reprogramming (if needed) and which calculate timebase ticks
> using the multiplier and shifter mechanism implemented within
> clockevent layer. It was observed that this conversion
> (timebase->time->timebase) are not correct because the mechanism are
> not consistent. In our setup it adds 2% jitter.
>
> With this patch clockevent multiplier and shifter mechanism are used
> when starting hrtimer for decrementer emulation. Now the jitter is <
> 0.5%.
>
> Signed-off-by: Bharat Bhushan<bharat.bhushan@freescale.com>
> ---
>   arch/powerpc/include/asm/time.h |    2 ++
>   arch/powerpc/kernel/time.c      |    6 ++++++
>   arch/powerpc/kvm/emulate.c      |    5 +++--
>   3 files changed, 11 insertions(+), 2 deletions(-)

Changes to arch/powerpc outside arch/powerpc/kvm need to
Cc: linuxppc-dev@lists.ozlabs.org

> diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
> index 7eb10fb..6d631b2 100644
> --- a/arch/powerpc/include/asm/time.h
> +++ b/arch/powerpc/include/asm/time.h
> @@ -202,6 +202,8 @@ extern u64 mulhdu(u64, u64);
>   extern void div128_by_32(u64 dividend_high, u64 dividend_low,
>   			 unsigned divisor, struct div_result *dr);
>
> +extern void get_clockevent_mult(u64 *multi, u64 *shift);
> +
>   /* Used to store Processor Utilization register (purr) values */
>
>   struct cpu_usage {
> diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> index 567dd7c..d229edd 100644
> --- a/arch/powerpc/kernel/time.c
> +++ b/arch/powerpc/kernel/time.c
> @@ -910,6 +910,12 @@ static void __init init_decrementer_clockevent(void)
>   	register_decrementer_clockevent(cpu);
>   }
>
> +void get_clockevent_mult(u64 *multi, u64 *shift)
> +{
> +	*multi = decrementer_clockevent.mult;
> +	*shift = decrementer_clockevent.shift;
> +}

Maybe just make decrmenter_clockevent non-static?

> diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
> index afc9154..4bfcaa1 100644
> --- a/arch/powerpc/kvm/emulate.c
> +++ b/arch/powerpc/kvm/emulate.c
> @@ -76,6 +76,7 @@ void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
>   {
>   	unsigned long dec_nsec;
>   	unsigned long long dec_time;
> +	u64 mult, shift;
>
>   	pr_debug("mtDEC: %x\n", vcpu->arch.dec);
>   	hrtimer_try_to_cancel(&vcpu->arch.dec_timer);
> @@ -103,9 +104,9 @@ void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
>   	 * host ticks.
>   	 */
>
> +	get_clockevent_mult(&mult,&shift);
>   	dec_time = vcpu->arch.dec;
> -	dec_time *= 1000;
> -	do_div(dec_time, tb_ticks_per_usec);
> +	dec_time = (dec_time<<  shift) / mult;
>   	dec_nsec = do_div(dec_time, NSEC_PER_SEC);
>   	hrtimer_start(&vcpu->arch.dec_timer,
>   		ktime_set(dec_time, dec_nsec), HRTIMER_MODE_REL);

-Scott

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