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

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Date:	Wed, 18 Jul 2012 07:58:05 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Rik van Riel <riel@...hat.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 07/17/2012 02:39 PM, Raghavendra K T wrote:
[...]
>
> 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.
>
> Suppose A is doing really good job and and has not done pause loop
> exit, we will not touch it's dy_eligible flag. Also dy_eligible flag
> will not prevent B doing yield_to to A.
>
> Suppose A has started spinning in the beginning itself, it will do pause
> loop exit if it crosses threshold, and we will now start toggling
> dy_eligible.
>
> Was that you were referring?
>
> And it seems we may still have to set dy_eligible flag to false at the
> beginning of vcpu_on_spin along with cpu_relax_intercepted = true, like
> below, so that we do not have spill-over status from previous PL exits.
>
> vcpu_on_spin()
> {
> cpu_relax_intercepted = true;
> dy_eligible = false;
> .
> .
> .
>
> cpu_relax_intercepted = false;
> }
>
> Let me know if that addresses your concern.
>

Thought you brought in is miraculous. taking care of not having 
spill-over dy_eligible status is needed for making algorithm technically 
more correct. will spin V5 with all these changes.

>>
>> 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.
>>
>

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