* Re: [PATCH] Use clockevent multiplier and shifter for decrementer
[not found] <1333522311-1390-1-git-send-email-bharat.bhushan@freescale.com>
@ 2012-04-06 19:10 ` Scott Wood
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From: Scott Wood @ 2012-04-06 19:10 UTC (permalink / raw)
To: Bharat Bhushan; +Cc: Bharat Bhushan, linuxppc-dev, kvm, kvm-ppc
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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