On 06/02/2015 21:51, Marcelo Tosatti wrote:
> 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@xxxxxxxxxx>
>> ---
>> 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.
You're right.
> Can remove another 300 cycles from do_div when programming LAPIC
> tscdeadline timer.
Do you mean using something like lib/reciprocal_div.c? Good idea,
though that's not latency, it's just being slow. :)
Paolo
--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
