KVM: LAPIC: Tune lapic_timer_advance_ns automatically
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 'commit d0659d946be0 ("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>
Cc: Liran Alon <liran.alon@oracle.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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);