linux-kernel - Re: [PATCH 1/5] KVM: simplify branch check in host poll code

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Date:   Mon, 4 Nov 2019 11:49:27 +0800
From:   Zhenzhong Duan <zhenzhong.duan@...cle.com>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        joao.m.martins@...cle.com, rafael.j.wysocki@...el.com,
        rkrcmar@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/5] KVM: simplify branch check in host poll code


On 2019/11/2 5:03, Marcelo Tosatti wrote:
> On Sat, Oct 26, 2019 at 11:23:55AM +0800, Zhenzhong Duan wrote:
>> Remove redundant check.
>>
>> Signed-off-by: Zhenzhong Duan <zhenzhong.duan@...cle.com>
>> ---
>>   virt/kvm/kvm_main.c | 9 ++++-----
>>   1 file changed, 4 insertions(+), 5 deletions(-)
>>
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 67ef3f2..2ca2979 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -2366,13 +2366,12 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>   		} else if (halt_poll_ns) {
>>   			if (block_ns <= vcpu->halt_poll_ns)
>>   				;
>> -			/* we had a long block, shrink polling */
>> -			else if (vcpu->halt_poll_ns && block_ns > halt_poll_ns)
>> -				shrink_halt_poll_ns(vcpu);
>>   			/* we had a short halt and our poll time is too small */
>> -			else if (vcpu->halt_poll_ns < halt_poll_ns &&
> This is not a redundant check: it avoids from calling
> into grow_halt_poll_ns, which will do:
>
> 	1) Multiplication
> 	2) Cap that back to halt_poll_ns
> 	3) Invoke the trace_kvm_halt_poll_ns_grow tracepoint
> 	   (when in fact vcpu->halt_poll_ns did not grow).

In this branch, vcpu->halt_poll_ns < block_ns is true, and if block_ns < 
halt_poll_ns,

then vcpu->halt_poll_ns < halt_poll_ns is always true, isn't it?


I realized I ignored the situation that halt_poll_ns and 
halt_poll_ns_grow could be

updated at runtime, so pls ignore this patch, I'll fix it by following 
the guest haltpoll code.

Thanks

Zhenzhong