linux-kernel - Re: [PATCH v6 2/3] KVM: dynamic halt_poll

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Date:	Thu, 3 Sep 2015 17:23:39 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	David Matlack <dmatlack@...gle.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>,
	kvm list <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 2/3] KVM: dynamic halt_poll_ns adjustment

On 9/3/15 2:09 AM, David Matlack wrote:
[...]
>> +
>>   static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>>   {
>>          if (kvm_arch_vcpu_runnable(vcpu)) {
>> @@ -1929,6 +1963,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>          ktime_t start, cur;
>>          DEFINE_WAIT(wait);
>>          bool waited = false;
>> +       u64 poll_ns = 0, wait_ns = 0, block_ns = 0;
>>
>>          start = cur = ktime_get();
>>          if (vcpu->halt_poll_ns) {
>> @@ -1941,10 +1976,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>                           */
>>                          if (kvm_vcpu_check_block(vcpu) < 0) {
>>                                  ++vcpu->stat.halt_successful_poll;
>> -                               goto out;
>> +                               break;
>>                          }
>>                          cur = ktime_get();
>>                  } while (single_task_running() && ktime_before(cur, stop));
>> +
>> +               if (ktime_before(cur, stop)) {
> You can't use 'cur' to tell if the interrupt arrived. single_task_running()
> can break us out of the loop before 'stop'.
>
>> +                       poll_ns = ktime_to_ns(cur) - ktime_to_ns(start);
> Put this line before the if(). block_ns should always include the time
> spent polling; even if polling does not succeed.

How about something like:

@@ -1941,10 +1976,14 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
                          */
                         if (kvm_vcpu_check_block(vcpu) < 0) {
++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 (ktime_before(cur, stop) && single_task_running())
+                       goto out;
         }

>
>> +                       goto out;
>> +               }
>>          }
>>
>>          for (;;) {
>> @@ -1959,9 +1999,24 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>
>>          finish_wait(&vcpu->wq, &wait);
>>          cur = ktime_get();
>> +       wait_ns = ktime_to_ns(cur) - ktime_to_ns(start);
>>
>>   out:
>> -       trace_kvm_vcpu_wakeup(ktime_to_ns(cur) - ktime_to_ns(start), waited);
>> +       block_ns = poll_ns + wait_ns;
>> +
>> +       if (halt_poll_ns) {
> If you want, you can leave this if() out and save some indentation.

Then we will miss the tracepoint.

Regards,
Wanpeng Li

>
>> +               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);
>>
>> --
>> 1.9.1
>>

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