linux-kernel - Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed yield

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Date:	Tue, 17 Jul 2012 11:29:38 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Rik van Riel <riel@...hat.com>
CC:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	S390 <linux-s390@...r.kernel.org>,
	Carsten Otte <cotte@...ibm.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	KVM <kvm@...r.kernel.org>, chegu vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 <x86@...nel.org>,
	Gleb Natapov <gleb@...hat.com>, linux390@...ibm.com,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Joerg Roedel <joerg.roedel@....com>
Subject: Re: [PATCH RFC V4 3/3] kvm: Choose better candidate for directed
 yield

On 07/16/2012 07:10 PM, Rik van Riel wrote:
> On 07/16/2012 06:07 AM, Avi Kivity wrote:
> 
>>> +{
>>> +    bool eligible;
>>> +
>>> +    eligible = !vcpu->ple.cpu_relax_intercepted ||
>>> +            (vcpu->ple.cpu_relax_intercepted&&
>>> +             vcpu->ple.dy_eligible);
>>> +
>>> +    if (vcpu->ple.cpu_relax_intercepted)
>>> +        vcpu->ple.dy_eligible = !vcpu->ple.dy_eligible;
>>
>> Probably should assign 'true', since the previous value is essentially
>> random.
> 
> I suspect the intended purpose of this conditional is to
> flip the eligibility of a vcpu for being selected as a
> direct yield target.
> 
> In other words, that bit of the code is correct.

If vcpu A is in a long spin loop and is preempted away, and vcpu B dips
several times in kvm_vcpu_on_spin(), then it will act as intended.  But
if vcpu A is spinning for x% of its time and processing on the other,
then vcpu B will flip its dy_eligible for those x%, and not flip it when
it's processing.  I don't understand how this is useful.

I guess this is an attempt to impose fairness on yielding, and it makes
sense to do this, but I don't know if this is the best way to achieve it.

-- 
error compiling committee.c: too many arguments to function


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