On 09/10/2018 03:02, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx>
> Cc: Liran Alon <liran.alon@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> ---
> 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
