Re: [RFC PATCH] KVM: x86: Skip request checking branches in vcpu_enter_guest() more effectively

[Gleb Natapov <gleb@redhat.com>]

On Mon, Sep 24, 2012 at 03:24:47PM +0900, Takuya Yoshikawa wrote:
> This is an RFC since I have not done any comparison with the approach
> using for_each_set_bit() which can be seen in Avi's work.
> 
> 	Takuya
> ---
> 
> We did a simple test to see which requests we would get at the same time
> in vcpu_enter_guest() and got the following numbers:
> 
> |...........|...............|........|...............|.|
>      (N)           (E)         (S)          (ES)      others
>      22.3%         30.7%       16.0%        29.5%     1.4%
> 
> (N) : Nothing
> (E) : Only KVM_REQ_EVENT
> (S) : Only KVM_REQ_STEAL_UPDATE
> (ES): Only KVM_REQ_EVENT and KVM_REQ_STEAL_UPDATE
> 
> * Note that the exact numbers can change for other guests.
> 
Yes, for guests that do not enable steal time KVM_REQ_STEAL_UPDATE
should be never set, but currently it is. The patch (not tested) should
fix this.


diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 901ad00..01572f5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1544,6 +1544,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
 	delta = current->sched_info.run_delay - vcpu->arch.st.last_steal;
 	vcpu->arch.st.last_steal = current->sched_info.run_delay;
 	vcpu->arch.st.accum_steal = delta;
+
+	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 }
 
 static void record_steal_time(struct kvm_vcpu *vcpu)
@@ -1673,8 +1675,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 		accumulate_steal_time(vcpu);
 		preempt_enable();
 
-		kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
-
 		break;
 	case MSR_KVM_PV_EOI_EN:
 		if (kvm_lapic_enable_pv_eoi(vcpu, data))
@@ -2336,7 +2336,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	}
 
 	accumulate_steal_time(vcpu);
-	kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 }
 
 void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
--
			Gleb.