Re: [PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/09/19 06:02, Wanpeng Li wrote:
> Hi Paolo,
> 
> Something like below? It still fluctuate when running complex guest os
> like linux. Removing timer_advance_adjust_done will hinder introduce
> patch v3 2/2 since there is no adjust done flag in each round
> evaluation.

That's not important, since the adjustment would be continuous.

How much fluctuation can you see?

> I have two questions here, best-effort tune always as
> below or periodically revaluate to get conservative value and get
> best-effort value in each round evaluation as patch v3 2/2, which one
> do you prefer? The former one can wast time to wait sometimes and the
> later one can not get the best latency. In addition, can the adaptive
> tune algorithm be using in all the scenarios contain
> RT/over-subscribe?

I prefer the former, like the patch below, mostly because of the extra
complexity of the periodic reevaluation.

Paolo

> 
> ---8<---
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 685d17c..895735b 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 5000
> 
>  static inline int apic_test_vector(int vec, void *bitmap)
>  {
> @@ -1479,29 +1480,28 @@ 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 ||
> +        abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE) {
> +        /* filter out random 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/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
>      } 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/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)) {
> +    if (unlikely(timer_advance_ns > 5000))
>          timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> -        apic->lapic_timer.timer_advance_adjust_done = false;
> -    }
>      apic->lapic_timer.timer_advance_ns = timer_advance_ns;
>  }
> 
> @@ -1521,7 +1521,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>      if (guest_tsc < tsc_deadline)
>          __wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
> 
> -    if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
> +    if (lapic_timer_advance_ns == -1)
>          adjust_lapic_timer_advance(vcpu,
> apic->lapic_timer.advance_expire_delta);
>  }
> 
> @@ -2298,10 +2298,8 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int
> timer_advance_ns)
>      apic->lapic_timer.timer.function = apic_timer_fn;
>      if (timer_advance_ns == -1) {
>          apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
> -        apic->lapic_timer.timer_advance_adjust_done = false;
>      } else {
>          apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> -        apic->lapic_timer.timer_advance_adjust_done = true;
>      }
> 
> 
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index 50053d2..2aad7e2 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -35,7 +35,6 @@ struct kvm_timer {
>      s64 advance_expire_delta;
>      atomic_t pending;            /* accumulated triggered timers */
>      bool hv_timer_in_use;
> -    bool timer_advance_adjust_done;
>  };
> 
>  struct kvm_lapic {
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 93b0bd4..4f65ef1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -141,7 +141,7 @@
>   * advancement entirely.  Any other value is used as-is and disables adaptive
>   * tuning, i.e. allows priveleged userspace to set an exact advancement time.
>   */
> -static int __read_mostly lapic_timer_advance_ns = -1;
> +int __read_mostly lapic_timer_advance_ns = -1;
>  module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
> 
>  static bool __read_mostly vector_hashing = true;
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 6594020..2c6ba86 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -301,6 +301,7 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
> unsigned long cr2,
> 
>  extern bool enable_vmware_backdoor;
> 
> +extern int lapic_timer_advance_ns;
>  extern int pi_inject_timer;
> 
>  extern struct static_key kvm_no_apic_vcpu;
>