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KVM: X86: Reduce the overhead when lapic_timer_advance is disabled

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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>
This commit is contained in:
Wanpeng Li 2017-12-01 00:15:10 -08:00 committed by Paolo Bonzini
parent 74c55931c7
commit 9c48d517ce
1 changed files with 2 additions and 1 deletions
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3
arch/x86/kvm/x86.c
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@ -7018,7 +7018,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
} }
trace_kvm_entry(vcpu->vcpu_id); trace_kvm_entry(vcpu->vcpu_id);
wait_lapic_expire(vcpu); if (lapic_timer_advance_ns)
wait_lapic_expire(vcpu);
guest_enter_irqoff(); guest_enter_irqoff();
if (unlikely(vcpu->arch.switch_db_regs)) { if (unlikely(vcpu->arch.switch_db_regs)) {