Commit 9c48d517 authored Dec 01, 2017 by Wanpeng Li Committed by Paolo Bonzini Dec 14, 2017

KVM: X86: Reduce the overhead when lapic_timer_advance is disabled



When I run ebizzy in a 32 vCPUs guest on a 32 pCPUs Xeon box, I can observe
~8000 kvm_wait_lapic_expire CurAvg/s through kvm_stat tool even if the advance
tscdeadline hrtimer expiration is disabled. Each call to wait_lapic_expire()
will consume ~70 cycles when a timer fires since apic_timer_expire() will
set expired_tscdeadline and then wait_lapic_expire() will do some caculation
before bailing out. So total ~175us per second is lost on this 3.2Ghz machine.
This patch reduces the overhead by skipping the function wait_lapic_expire()
when lapic_timer_advance is disabled.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 74c55931

arch/x86/kvm/x86.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -7018,6 +7018,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
		}

		trace_kvm_entry(vcpu->vcpu_id);
		if (lapic_timer_advance_ns)
		wait_lapic_expire(vcpu);
		guest_enter_irqoff();

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