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SubjectRe: [PATCH] KVM: LAPIC: Tune lapic_timer_advance_ns automatically
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> On 8 Oct 2018, at 13:59, Wanpeng Li <kernellwp@gmail.com> wrote:
>
> On Mon, 8 Oct 2018 at 05:02, Liran Alon <liran.alon@oracle.com> wrote:
>>
>>
>>
>>> On 28 Sep 2018, at 9:12, Wanpeng Li <kernellwp@gmail.com> wrote:
>>>
>>> From: Wanpeng Li <wanpengli@tencent.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 d0659d946be05 (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@redhat.com>
>>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>>> Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
>>> ---
>>> arch/x86/kvm/lapic.c | 7 +++++++
>>> arch/x86/kvm/x86.c | 2 +-
>>> 2 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>>> index fbb0e6d..b756f12 100644
>>> --- a/arch/x86/kvm/lapic.c
>>> +++ b/arch/x86/kvm/lapic.c
>>> @@ -70,6 +70,8 @@
>>> #define APIC_BROADCAST 0xFF
>>> #define X2APIC_BROADCAST 0xFFFFFFFFul
>>>
>>> +static bool __read_mostly lapic_timer_advance_adjust_done = false;
>>> +
>>> static inline int apic_test_vector(int vec, void *bitmap)
>>> {
>>> return test_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
>>> @@ -1492,6 +1494,11 @@ 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) {
>>> + lapic_timer_advance_ns += (s64)(guest_tsc - tsc_deadline) / 8;
>>
>> I don’t understand how this “/ 8” converts between guest TSC units to host nanoseconds.
>
> Oh, I miss it. In addition, /8 here I mean adjust
> lapic_timer_advance_ns step by step. I can observe big fluctuated

If that’s the case, I would also put the “8” as a #define to make it more clear of it’s purpose.

> value between early and late when running real guest os like linux
> instead of kvm-unit-tests. After more testing, I saw
> lapic_timer_advance_ns can be overflow since the delta between
> guest_tsc and tsc_deadline is too huge.
>
>>
>> I think that instead you should do something like:
>> s64 ns = (s64)(guest_tsc - tsc_deadline) * 1000000ULL;
>> do_div(ns, vcpu->arch.virtual_tsc_khz);
>> lapic_timer_advance_ns += ns;
>>
>>> + if (abs(guest_tsc - tsc_deadline) < 100)
>>
>> I would put this “100” hard-coded value as some “#define” to make code more clear.
>
> How about something like below:
>
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index fbb0e6d..354eb13c 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -70,6 +70,9 @@
> #define APIC_BROADCAST 0xFF
> #define X2APIC_BROADCAST 0xFFFFFFFFul
>
> +static bool __read_mostly lapic_timer_advance_adjust_done = false;
> +#define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100
> +
> static inline int apic_test_vector(int vec, void *bitmap)
> {
> return test_bit(VEC_POS(vec), (bitmap) + REG_POS(vec));
> @@ -1472,7 +1475,7 @@ 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 +1495,19 @@ 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) {
> + 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 / 8);
> + } else {
> + 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 / 8);
> + }
> + if (ns < LAPIC_TIMER_ADVANCE_ADJUST_DONE)

Didn’t you meant to compare here that abs(guest_tsc - tsc_deadline) < LAPIC_TIMER_ADVANCE_ADJUST_DONE?
This is also a good example on why I would also rename LAPIC_TIMER_ADVANCE_ADJUST_DONE to something
which indicates it represents a number in guest TSC units.

> + 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);

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