Re: [PATCH] KVM: LAPIC: Tune lapic_timer_advance_ns automatically

[Wanpeng Li]

On Mon, 8 Oct 2018 at 05:02, Liran Alon <liran.alon@xxxxxxxxxx> wrote:
>
>
>
> > On 28 Sep 2018, at 9:12, Wanpeng Li <kernellwp@xxxxxxxxx> 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 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@xxxxxxxxxx>
> > Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> > 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 donât understand how this â/ 8â 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
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 â100â hard-coded value as some â#defineâ 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)
+            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);