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Subject[PATCH RESEND v3 2/5] KVM: LAPIC: Periodically revaluate to get conservative lapic_timer_advance_ns
Date
From: Wanpeng Li <wanpengli@tencent.com>

Even if for realtime CPUs, cache line bounces, frequency scaling, presence
of higher-priority RT tasks, etc can still cause different response. These
interferences should be considered and periodically revaluate whether
or not the lapic_timer_advance_ns value is the best, do nothing if it is,
otherwise recaluate again. Set lapic_timer_advance_ns to the minimal
conservative value from all the estimated values.

Testing on Skylake server, cat vcpu*/lapic_timer_advance_ns, before patch:
1628
4161
4321
3236
...

Testing on Skylake server, cat vcpu*/lapic_timer_advance_ns, after patch:
1553
1499
1509
1489
...

Testing on Haswell desktop, cat vcpu*/lapic_timer_advance_ns, before patch:
4617
3641
4102
4577
...
Testing on Haswell desktop, cat vcpu*/lapic_timer_advance_ns, after patch:
2775
2892
2764
2775
...

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 | 37 ++++++++++++++++++++++++++++++-------
arch/x86/kvm/lapic.h | 2 ++
2 files changed, 32 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 2f4a48a..12ade70 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -70,6 +70,7 @@
/* step-by-step approximation to mitigate fluctuation */
#define LAPIC_TIMER_ADVANCE_ADJUST_STEP 8
#define LAPIC_TIMER_ADVANCE_FILTER 10000
+#define LAPIC_TIMER_ADVANCE_RECALC_PERIOD (600 * HZ)

static inline int apic_test_vector(int vec, void *bitmap)
{
@@ -1484,13 +1485,24 @@ static inline void __wait_lapic_expire(struct kvm_vcpu *vcpu, u64 guest_cycles)
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, ns;
+ struct kvm_timer *ktimer = &vcpu->arch.apic->lapic_timer;
+ u32 timer_advance_ns = ktimer->timer_advance_ns, ns;

if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_FILTER)
/* filter out drastic fluctuations */
return;

+ /* periodic revaluate */
+ if (unlikely(ktimer->timer_advance_adjust_done)) {
+ ktimer->recalc_timer_advance_ns = jiffies +
+ LAPIC_TIMER_ADVANCE_RECALC_PERIOD;
+ if (abs(advance_expire_delta) > LAPIC_TIMER_ADVANCE_ADJUST_DONE) {
+ timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
+ ktimer->timer_advance_adjust_done = false;
+ } else
+ return;
+ }
+
/* too early */
if (advance_expire_delta < 0) {
ns = -advance_expire_delta * 1000000ULL;
@@ -1503,17 +1515,24 @@ static inline void adjust_lapic_timer_advance(struct kvm_vcpu *vcpu,
timer_advance_ns += ns;
}

- timer_advance_ns = (apic->lapic_timer.timer_advance_ns *
+ timer_advance_ns = (ktimer->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;
+ ktimer->timer_advance_adjust_done = true;
if (unlikely(timer_advance_ns > 5000)) {
timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
- apic->lapic_timer.timer_advance_adjust_done = false;
+ ktimer->timer_advance_adjust_done = false;
+ }
+
+ ktimer->timer_advance_ns = timer_advance_ns;
+
+ if (ktimer->timer_advance_adjust_done) {
+ if (ktimer->min_timer_advance_ns > timer_advance_ns)
+ ktimer->min_timer_advance_ns = timer_advance_ns;
+ ktimer->timer_advance_ns = ktimer->min_timer_advance_ns;
}
- apic->lapic_timer.timer_advance_ns = timer_advance_ns;
}

static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
@@ -1532,7 +1551,8 @@ 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 (unlikely(!apic->lapic_timer.timer_advance_adjust_done) ||
+ time_before(apic->lapic_timer.recalc_timer_advance_ns, jiffies))
adjust_lapic_timer_advance(vcpu, apic->lapic_timer.advance_expire_delta);
}

@@ -2310,9 +2330,12 @@ int kvm_create_lapic(struct kvm_vcpu *vcpu, int timer_advance_ns)
if (timer_advance_ns == -1) {
apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_ADJUST_INIT;
apic->lapic_timer.timer_advance_adjust_done = false;
+ apic->lapic_timer.recalc_timer_advance_ns = jiffies;
+ apic->lapic_timer.min_timer_advance_ns = UINT_MAX;
} else {
apic->lapic_timer.timer_advance_ns = timer_advance_ns;
apic->lapic_timer.timer_advance_adjust_done = true;
+ apic->lapic_timer.recalc_timer_advance_ns = MAX_JIFFY_OFFSET;
}


diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 50053d2..56a05eb 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -31,6 +31,8 @@ struct kvm_timer {
u32 timer_mode_mask;
u64 tscdeadline;
u64 expired_tscdeadline;
+ unsigned long recalc_timer_advance_ns;
+ u32 min_timer_advance_ns;
u32 timer_advance_ns;
s64 advance_expire_delta;
atomic_t pending; /* accumulated triggered timers */
--
2.7.4
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