linux-kernel - Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire soon

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Date:	Thu, 19 May 2016 19:35:52 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH] KVM: halt-polling: poll if emulated lapic timer will fire soon

2016-05-19 19:23 GMT+08:00 Christian Borntraeger <borntraeger@...ibm.com>:
> On 05/19/2016 11:26 AM, Wanpeng Li wrote:
>
> I think in general a good idea to poll if a timer will expire soon.
>
> Some patch comments:
>
> Same for all non-x86 archs:
>> +static inline unsigned int kvm_arch_timer_remaining(struct kvm_vcpu *vcpu) {}
>
> A function returning int, without a return statement?
> That gives at least a compiler warning.

How about return 0 for all non-x86 archs?

>
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -78,6 +78,9 @@ module_param(halt_poll_ns_grow, uint, S_IRUGO | S_IWUSR);
>>  static unsigned int halt_poll_ns_shrink;
>>  module_param(halt_poll_ns_shrink, uint, S_IRUGO | S_IWUSR);
>>
>> +/* lower-end of message passing workload latency TCP_RR's poll time < 10us */
>> +static unsigned int halt_poll_ns_base = 10000;
>> +
>>  /*
>>   * Ordering of locks:
>>   *
>> @@ -1966,7 +1969,7 @@ static void grow_halt_poll_ns(struct kvm_vcpu *vcpu)
>>       grow = READ_ONCE(halt_poll_ns_grow);
>>       /* 10us base */
>>       if (val == 0 && grow)
>> -             val = 10000;
>> +             val = halt_poll_ns_base;
>>       else
>>               val *= grow;
>>
>> @@ -2015,11 +2018,15 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>>       DECLARE_SWAITQUEUE(wait);
>>       bool waited = false;
>>       u64 block_ns;
>> +     unsigned int delta, remaining;
>>
>> +     remaining = kvm_arch_timer_remaining(vcpu);
>
> and now it causes undefined behaviour, no?

Ditto.

>
>
>>       start = cur = ktime_get();
>> -     if (vcpu->halt_poll_ns) {
>> -             ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
>> +     if (vcpu->halt_poll_ns || (remaining < halt_poll_ns_base)) {
>> +             ktime_t stop;
>>
>> +             delta = vcpu->halt_poll_ns ? vcpu->halt_poll_ns : remaining;
>> +             stop = ktime_add_ns(ktime_get(), delta);
>>               ++vcpu->stat.halt_attempted_poll;
>>               do {
>>                       /*
>>
>
> So you avoid to shrink/grow for these cases? Probably makes sense

I think my patch also shrink/grow for these cases.

Regards,
Wanpeng Li