KVM: LAPIC: Tune lapic_timer_advance_ns smoothly

Filter out drastic fluctuation and random fluctuation, remove
timer_advance_adjust_done altogether, the adjustment would be
continuous.
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/lapic.c

@@ -65,7 +65,9 @@
 #define APIC_BROADCAST			0xFF
 #define X2APIC_BROADCAST		0xFFFFFFFFul
 
-#define LAPIC_TIMER_ADVANCE_ADJUST_DONE 100
+static bool lapic_timer_advance_dynamic __read_mostly;
+#define LAPIC_TIMER_ADVANCE_ADJUST_MIN 100
+#define LAPIC_TIMER_ADVANCE_ADJUST_MAX 5000
 #define LAPIC_TIMER_ADVANCE_ADJUST_INIT 1000
 /* step-by-step approximation to mitigate fluctuation */
 #define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
@@ -1485,26 +1487,25 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
 	u32 timer_advance_ns = apic->lapic_timer.timer_advance_ns;
 	u64 ns;
 
+	/* Do not adjust for tiny fluctuations or large random spikes. */
+	if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_ADJUST_MAX ||
+	    abs(advance_expire_delta) < LAPIC_TIMER_ADVANCE_ADJUST_MIN)
+		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/LAPIC_TIMER_ADVANCE_ADJUST_STEP;
 	} 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/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)) {
+	if (unlikely(timer_advance_ns > LAPIC_TIMER_ADVANCE_ADJUST_MAX))
 		timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
-		apic->lapic_timer.timer_advance_adjust_done = false;
-	}
 	apic->lapic_timer.timer_advance_ns = timer_advance_ns;
 }
 
@@ -1524,7 +1525,7 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
 	if (guest_tsc < tsc_deadline)
 		__wait_lapic_expire(vcpu, tsc_deadline - guest_tsc);
 
-	if (unlikely(!apic->lapic_timer.timer_advance_adjust_done))
+	if (lapic_timer_advance_dynamic)
 		adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
 }
 
@@ -2302,13 +2303,12 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
 	apic->lapic_timer.timer.function = apic_timer_fn;
 	if (timer_advance_ns == -1) {
 		apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
-		apic->lapic_timer.timer_advance_adjust_done = false;
+		lapic_timer_advance_dynamic = true;
 	} else {
 		apic->lapic_timer.timer_advance_ns = timer_advance_ns;
-		apic->lapic_timer.timer_advance_adjust_done = true;
+		lapic_timer_advance_dynamic = false;
 	}
 
-
 	/*
 	 * APIC is created enabled. This will prevent kvm_lapic_set_base from
 	 * thinking that APIC state has changed.

arch/x86/kvm/lapic.h

@@ -35,7 +35,6 @@ struct kvm_timer {
 	s64 advance_expire_delta;
 	atomic_t pending;			/* accumulated triggered timers */
 	bool hv_timer_in_use;
-	bool timer_advance_adjust_done;
 };
 
 struct kvm_lapic {