linux-kernel - Re: [PATCH RFC 2/2] kvm PLE handler: Choose better candidate for directed yield

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Date:	Mon, 09 Jul 2012 18:30:25 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.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 2/2] kvm PLE handler: Choose better candidate for
 directed yield

On 07/09/2012 02:20 AM, Raghavendra K T wrote:

> +bool kvm_arch_vcpu_check_and_update_eligible(struct kvm_vcpu *vcpu)
> +{
> +	bool eligible;
> +
> +	eligible = !vcpu->arch.plo.pause_loop_exited ||
> +			(vcpu->arch.plo.pause_loop_exited&&
> +			 vcpu->arch.plo.dy_eligible);
> +
> +	if (vcpu->arch.plo.pause_loop_exited)
> +		vcpu->arch.plo.dy_eligible = !vcpu->arch.plo.dy_eligible;
> +
> +	return eligible;
> +}

This is a nice simple mechanism to skip CPUs that were
eligible last time and had pause loop exits recently.

However, it could stand some documentation.  Please
add a good comment explaining how and why the algorithm
works, when arch.plo.pause_loop_exited is cleared, etc...

It would be good to make this heuristic understandable
to people who look at the code for the first time.

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