linux-kernel - Re: [PATCH 2/2] KVM: x86: optimize delivery of TSC deadline timer interrupt

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Date:	Fri, 6 Feb 2015 18:51:37 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, riel@...hat.com,
	rkrcmar@...hat.com, dmatlack@...gle.com
Subject: Re: [PATCH 2/2] KVM: x86: optimize delivery of TSC deadline timer
 interrupt

On Fri, Feb 06, 2015 at 01:16:59PM +0100, Paolo Bonzini wrote:
> The newly-added tracepoint shows the following results on
> the tscdeadline_latency test:
> 
>         qemu-kvm-8387  [002]  6425.558974: kvm_vcpu_wakeup:      poll time 10407 ns
>         qemu-kvm-8387  [002]  6425.558984: kvm_vcpu_wakeup:      poll time 0 ns
>         qemu-kvm-8387  [002]  6425.561242: kvm_vcpu_wakeup:      poll time 10477 ns
>         qemu-kvm-8387  [002]  6425.561251: kvm_vcpu_wakeup:      poll time 0 ns
> 
> and so on.  This is because we need to go through kvm_vcpu_block again
> after the timer IRQ is injected.  Avoid it by polling once before
> entering kvm_vcpu_block.
> 
> On my machine (Xeon E5 Sandy Bridge) this removes about 500 cycles (7%)
> from the latency of the TSC deadline timer.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/x86.c | 14 +++++++++-----
>  1 file changed, 9 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 0b8dd13676ef..1e766033ebff 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6389,11 +6389,15 @@ static inline int __vcpu_run(struct kvm *kvm, struct kvm_vcpu *vcpu)
>  	    !vcpu->arch.apf.halted)
>  		return vcpu_enter_guest(vcpu);
>  
> -	srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> -	kvm_vcpu_block(vcpu);
> -	vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
> -	if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
> -		return 1;
> +	if (kvm_arch_vcpu_runnable(vcpu))
> +		clear_bit(KVM_REQ_UNHALT, &vcpu->requests);
> +	else {

Why the clear_bit? Since only kvm_vcpu_block in the below section
sets it, and that section clears it as well.

Can remove another 300 cycles from do_div when programming LAPIC
tscdeadline timer.

> +		srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> +		kvm_vcpu_block(vcpu);
> +		vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
> +		if (!kvm_check_request(KVM_REQ_UNHALT, vcpu))
> +			return 1;
> +	}
>  
>  	kvm_apic_accept_events(vcpu);
>  	switch(vcpu->arch.mp_state) {
> -- 
> 1.8.3.1
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