linux-kernel - Re: [PATCH v2] KVM: LAPIC: Tune lapic_timer_advance

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Date:   Tue, 9 Oct 2018 18:52:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2] KVM: LAPIC: Tune lapic_timer_advance_ns automatically

On 09/10/2018 03:02, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> In cloud environment, lapic_timer_advance_ns is needed to be tuned for every CPU 
> generations, and every host kernel versions(the kvm-unit-tests/tscdeadline_latency.flat 
> is 5700 cycles for upstream kernel and 9600 cycles for our 3.10 product kernel, 
> both preemption_timer=N, Skylake server).
> 
> This patch adds the capability to automatically tune lapic_timer_advance_ns
> step by step, the initial value is 1000ns as 'commit d0659d946be0 ("KVM: x86: 
> add option to advance tscdeadline hrtimer expiration")' recommended, it will be 
> reduced when it is too early, and increased when it is too late. The guest_tsc 
> and tsc_deadline are hard to equal, so we assume we are done when the delta 
> is within a small scope e.g. 100 cycles. This patch reduces latency 
> (kvm-unit-tests/tscdeadline_latency, busy waits, preemption_timer enabled)
> from ~2600 cyles to ~1200 cyles on our Skylake server.
> 
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Liran Alon <liran.alon@...cle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v1 -> v2:
>  * converts between guest TSC units to host nanoseconds correctly
>  * put hard-coded numbers to #define 
> 
>  arch/x86/kvm/lapic.c | 25 ++++++++++++++++++++++++-
>  arch/x86/kvm/x86.c   |  2 +-
>  2 files changed, 25 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fbb0e6d..197cf5d 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -70,6 +70,11 @@
>  #define APIC_BROADCAST			0xFF
>  #define X2APIC_BROADCAST		0xFFFFFFFFul
>  
> +static bool lapic_timer_advance_adjust_done = false;
> +#define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100
> +/* step-by-step approximation to mitigate fluctuation */
> +#define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
> +
>  static inline int apic_test_vector(int vec, void *bitmap)
>  {
>  	return test_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
> @@ -1472,7 +1477,7 @@ static bool lapic_timer_int_injected(struct kvm_vcpu *vcpu)
>  void wait_lapic_expire(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_lapic *apic = vcpu->arch.apic;
> -	u64 guest_tsc, tsc_deadline;
> +	u64 guest_tsc, tsc_deadline, ns;
>  
>  	if (!lapic_in_kernel(vcpu))
>  		return;
> @@ -1492,6 +1497,24 @@ void wait_lapic_expire(struct kvm_vcpu *vcpu)
>  	if (guest_tsc < tsc_deadline)
>  		__delay(min(tsc_deadline - guest_tsc,
>  			nsec_to_cycles(vcpu, lapic_timer_advance_ns)));
> +
> +	if (!lapic_timer_advance_adjust_done) {
> +		/* too early */
> +		if (guest_tsc < tsc_deadline) {
> +			ns = (tsc_deadline - guest_tsc) * 1000000ULL;
> +			do_div(ns, vcpu->arch.virtual_tsc_khz);
> +			lapic_timer_advance_ns -= min((unsigned int)ns,
> +				lapic_timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> +		} else {
> +		/* too late */
> +			ns = (guest_tsc - tsc_deadline) * 1000000ULL;
> +			do_div(ns, vcpu->arch.virtual_tsc_khz);
> +			lapic_timer_advance_ns += min((unsigned int)ns,
> +				lapic_timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
> +		}
> +		if (abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
> +			lapic_timer_advance_adjust_done = true;
> +	}
>  }
>  
>  static void start_sw_tscdeadline(struct kvm_lapic *apic)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ca71773..1f3f955 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -136,7 +136,7 @@ static u32 __read_mostly tsc_tolerance_ppm = 250;
>  module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
>  
>  /* lapic timer advance (tscdeadline mode only) in nanoseconds */
> -unsigned int __read_mostly lapic_timer_advance_ns = 0;
> +unsigned int __read_mostly lapic_timer_advance_ns = 1000;
>  module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
>  EXPORT_SYMBOL_GPL(lapic_timer_advance_ns);
>  
> 

Queued, thanks.

Paolo