linux-kernel - Re: [PATCH v7 2/3] KVM: dynamic halt-polling

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Date:	Sun, 6 Sep 2015 16:32:03 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...mail.com>
Cc:	David Matlack <dmatlack@...gle.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/3] KVM: dynamic halt-polling



On 05/09/2015 00:38, Wanpeng Li wrote:
>>
>> @@ -1940,11 +1975,16 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>                * arrives.
>>                */
>>               if (kvm_vcpu_check_block(vcpu) < 0) {
>> +                polled = true;
>>                   ++vcpu->stat.halt_successful_poll;
>> -                goto out;
>> +                break;
>>               }
>>               cur = ktime_get();
>>           } while (single_task_running() && ktime_before(cur, stop));
>> +
>> +        poll_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>> +        if (polled)
>> +            goto out;
>>
> 
> Please move poll_ns caculation under if() when you applied, as I
> explained in reply to v6.

You can do much more than just that, the patch reduces to this:

@@ -1929,6 +1963,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	ktime_t start, cur;
 	DEFINE_WAIT(wait);
 	bool waited = false;
+	u64 block_ns;
 
 	start = cur = ktime_get();
 	if (vcpu->halt_poll_ns) {
@@ -1961,7 +1996,21 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
 	cur = ktime_get();
 
 out:
-	trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
+	block_ns = ktime_to_ns(cur) - ktime_to_ns(start);
+
+	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 &&
+			block_ns < halt_poll_ns)
+			grow_halt_poll_ns(vcpu);
+	}
+
+	trace_kvm_vcpu_wakeup(block_ns, waited);
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_block);
 

Paolo
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