[PATCH v3 1/2] KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

[Wanpeng Li]

From: Wanpeng Li <wanpengli@xxxxxxxxxxx>

Using a moving average based on per-vCPU lapic_timer_advance_ns to tune 
smoothly, filter out drastic fluctuation which prevents this before, 
let's assume it is 10000 cycles.

Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
---
 arch/x86/kvm/lapic.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index e904ff0..181537a 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -69,6 +69,7 @@
 #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
 /* step-by-step approximation to mitigate fluctuation */
 #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
+#define LAPIC_TIMER_ADVANCE_FILTER 10000
 
 static inline int apic_test_vector(int vec, void *bitmap)
 {
@@ -1484,23 +1485,28 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
 					      s64 advance_expire_delta)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
-	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
-	u64 ns;
+	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns, ns;
+
+	if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER)
+		/* filter out drastic fluctuations */
+		return;
 
 	/* too early */
 	if (advance_expire_delta < 0) {
 		ns = -advance_expire_delta * 1000000ULL;
 		do_div(ns, vcpu->arch.virtual_tsc_khz);
-		timer_advance_ns -= min((u32)ns,
-			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
+		timer_advance_ns -= ns;
 	} else {
 	/* too late */
 		ns = advance_expire_delta * 1000000ULL;
 		do_div(ns, vcpu->arch.virtual_tsc_khz);
-		timer_advance_ns += min((u32)ns,
-			timer_advance_ns / LAPIC_TIMER_ADVANCE_ADJUST_STEP);
+		timer_advance_ns += ns;
 	}
 
+	timer_advance_ns = (apic->lapic_timer.timer_advance_ns *
+		(LAPIC_TIMER_ADVANCE_ADJUST_STEP - 1) + advance_expire_delta) /
+		LAPIC_TIMER_ADVANCE_ADJUST_STEP;
+
 	if (abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_DONE)
 		apic->lapic_timer.timer_advance_adjust_done = true;
 	if (unlikely(timer_advance_ns > 5000)) {
-- 
2.7.4