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SubjectRe: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly
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On 28/08/19 10:19, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@tencent.com>
>
> Using a moving average based on per-vCPU lapic_timer_advance_ns to tune
> smoothly, filter out drastic fluctuation which prevents this before,
> let's assume it is 10000 cycles.
>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Cc: Radim Krčmář <rkrcmar@redhat.com>
> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> ---
> arch/x86/kvm/lapic.c | 18 ++++++++++++------
> 1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index e904ff0..181537a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -69,6 +69,7 @@
> #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
> /* step-by-step approximation to mitigate fluctuation */
> #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> +#define LAPIC_TIMER_ADVANCE_FILTER 10000
>
> static inline int apic_test_vector(int vec, void *bitmap)
> {
> @@ -1484,23 +1485,28 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> s64 advance_expire_delta)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
> - u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
> - u64 ns;
> + u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns, ns;
> +
> + if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER)
> + /* filter out drastic fluctuations */
> + return;
>
> /* too early */
> if (advance_expire_delta < 0) {
> ns = -advance_expire_delta * 1000000ULL;
> do_div(ns, vcpu->arch.virtual_tsc_khz);
> - timer_advance_ns -= min((u32)ns,
> - timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> + timer_advance_ns -= ns;
> } else {
> /* too late */
> ns = advance_expire_delta * 1000000ULL;
> do_div(ns, vcpu->arch.virtual_tsc_khz);
> - timer_advance_ns += min((u32)ns,
> - timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> + timer_advance_ns += ns;
> }
>
> + timer_advance_ns = (apic->lapic_timer.timer_advance_ns *
> + (LAPIC_TIMER_ADVANCE_ADJUST_STEP - 1) + advance_expire_delta) /
> + LAPIC_TIMER_ADVANCE_ADJUST_STEP;
> +
> if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
> apic->lapic_timer.timer_advance_adjust_done = true;
> if (unlikely(timer_advance_ns > 5000)) {
>

This looks great. But instead of patch 2, why not remove
timer_advance_adjust_done altogether?

Paolo

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