[PATCH] KVM: LAPIC: Tune lapic_timer_advance_ns automatically

[Wanpeng Li <kernellwp@gmail.com>]

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;
+		if (abs(guest_tsc - tsc_deadline) < 100)
+			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 edbf00e..e865d12 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 lapic_timer_advance_ns = 1000;
 module_param(lapic_timer_advance_ns, uint, S_IRUGO | S_IWUSR);
 EXPORT_SYMBOL_GPL(lapic_timer_advance_ns);
 
-- 
2.7.4