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Date: Thu, 30 May 2019 12:36:53 -0700
From: Sean Christopherson <sean.j.christopherson@...el.com>
To: Wanpeng Li <kernellwp@...il.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
Paolo Bonzini <pbonzini@...hat.com>,
Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH 1/2] KVM: LAPIC: Optimize timer latency consider world
switch time
On Thu, May 23, 2019 at 12:18:50PM +0800, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
>
> Advance lapic timer tries to hidden the hypervisor overhead between the
> host emulated timer fires and the guest awares the timer is fired. However,
> even though after more sustaining optimizations, kvm-unit-tests/tscdeadline_latency
> still awares ~1000 cycles latency since we lost the time between the end of
> wait_lapic_expire and the guest awares the timer is fired. There are
> codes between the end of wait_lapic_expire and the world switch, futhermore,
> the world switch itself also has overhead. Actually the guest_tsc is equal
> to the target deadline time in wait_lapic_expire is too late, guest will
> aware the latency between the end of wait_lapic_expire() and after vmentry
> to the guest. This patch takes this time into consideration.
>
> The vmentry_lapic_timer_advance_ns module parameter should be well tuned by
> host admin, it can reduce average cyclictest latency from 3us to 2us on
> Skylake server. (guest w/ nohz=off, idle=poll, host w/ preemption_timer=N,
> the cyclictest latency is not too sensitive when preemption_timer=Y for this
> optimization in my testing), kvm-unit-tests/tscdeadline_latency can reach 0.
>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Sean Christopherson <sean.j.christopherson@...el.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> arch/x86/kvm/lapic.c | 17 +++++++++++++++--
> arch/x86/kvm/lapic.h | 1 +
> arch/x86/kvm/vmx/vmx.c | 2 +-
> arch/x86/kvm/x86.c | 3 +++
> arch/x86/kvm/x86.h | 2 ++
> 5 files changed, 22 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fcf42a3..6f85221 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -1531,6 +1531,19 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
> apic->lapic_timer.timer_advance_ns = timer_advance_ns;
> }
>
> +u64 get_vmentry_advance_delta(struct kvm_vcpu *vcpu)
Hmm, this isn't a delta, I think get_vmentry_advance_cycles would be more
appropriate.
> +{
> + u64 vmentry_lapic_timer_advance_cycles = 0;
> +
> + if (vmentry_lapic_timer_advance_ns) {
> + vmentry_lapic_timer_advance_cycles = vmentry_lapic_timer_advance_ns *
> + vcpu->arch.virtual_tsc_khz;
> + do_div(vmentry_lapic_timer_advance_cycles, 1000000);
> + }
> + return vmentry_lapic_timer_advance_cycles;
> +}
> +EXPORT_SYMBOL_GPL(get_vmentry_advance_delta);
> +
> void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
> {
> struct kvm_lapic *apic = vcpu->arch.apic;
> @@ -1544,7 +1557,7 @@ void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
>
> tsc_deadline = apic->lapic_timer.expired_tscdeadline;
> apic->lapic_timer.expired_tscdeadline = 0;
> - guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> + guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_delta(vcpu);
> apic->lapic_timer.advance_expire_delta = guest_tsc - tsc_deadline;
>
> if (guest_tsc < tsc_deadline)
> @@ -1572,7 +1585,7 @@ static void start_sw_tscdeadline(struct kvm_lapic *apic)
> local_irq_save(flags);
>
> now = ktime_get();
> - guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
> + guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc()) + get_vmentry_advance_delta(vcpu);
>
> ns = (tscdeadline - guest_tsc) * 1000000ULL;
> do_div(ns, this_tsc_khz);
> diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
> index f974a3d..df2fe17 100644
> --- a/arch/x86/kvm/lapic.h
> +++ b/arch/x86/kvm/lapic.h
> @@ -221,6 +221,7 @@ static inline int kvm_lapic_latched_init(struct kvm_vcpu *vcpu)
> bool kvm_apic_pending_eoi(struct kvm_vcpu *vcpu, int vector);
>
> void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu);
> +u64 get_vmentry_advance_delta(struct kvm_vcpu *vcpu);
>
> bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
> struct kvm_vcpu **dest_vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index da24f18..0199ac3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7047,7 +7047,7 @@ static int vmx_set_hv_timer(struct kvm_vcpu *vcpu, u64 guest_deadline_tsc,
>
> vmx = to_vmx(vcpu);
> tscl = rdtsc();
> - guest_tscl = kvm_read_l1_tsc(vcpu, tscl);
> + guest_tscl = kvm_read_l1_tsc(vcpu, tscl) + get_vmentry_advance_delta(vcpu);
> delta_tsc = max(guest_deadline_tsc, guest_tscl) - guest_tscl;
> lapic_timer_advance_cycles = nsec_to_cycles(vcpu,
> ktimer->timer_advance_ns);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a4eb711..a02e2c3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -145,6 +145,9 @@ module_param(tsc_tolerance_ppm, uint, S_IRUGO | S_IWUSR);
> static int __read_mostly lapic_timer_advance_ns = -1;
> module_param(lapic_timer_advance_ns, int, S_IRUGO | S_IWUSR);
>
> +u32 __read_mostly vmentry_lapic_timer_advance_ns = 0;
> +module_param(vmentry_lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
Hmm, an interesting idea would be to have some way to "lock" this param,
e.g. setting bit 0 locks the param. That would allow KVM to calculate the
cycles value to avoid the function call and the MUL+DIV. If I'm not
mistaken, vcpu->arch.virtual_tsc_khz is set only in kvm_set_tsc_khz().
For example, if get_vmentry_advance_cycles() sees the value is locked, it
caches the value in struct kvm_lapic. The cached value would also need to
be updated in kvm_set_tsc_khz() if it has been set.
static inline u64 get_vmentry_advance_cycles(struct kvm_lapic *lapic)
{
if (lapic->vmentry_advance_cycles)
return lapic->vmentry_advance_cycles;
return compute_vmentry_advance_cycles(lapic);
}
u64 compute_vmentry_advance_cycles(struct kvm_lapic *lapic)
{
u64 val = vmentry_lapic_timer_advance_ns;
u64 cycles = (val & ~1ULL) * lapic->vcpu->arch.virtual_tsc_khz;
do_div(cycles, 1000000);
if (val & 1)
lapic->vmentry_advance_cycles = cycles;
return cycles;
}
> +
> static bool __read_mostly vector_hashing = true;
> module_param(vector_hashing, bool, S_IRUGO);
>
> diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
> index 275b3b6..b0a3b84 100644
> --- a/arch/x86/kvm/x86.h
> +++ b/arch/x86/kvm/x86.h
> @@ -294,6 +294,8 @@ extern u64 kvm_supported_xcr0(void);
>
> extern unsigned int min_timer_period_us;
>
> +extern unsigned int vmentry_lapic_timer_advance_ns;
> +
> extern bool enable_vmware_backdoor;
>
> extern struct static_key kvm_no_apic_vcpu;
> --
> 2.7.4
>
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