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Date: Mon, 09 Jul 2012 18:30:25 -0400
From: Rik van Riel <riel@redhat.com>
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To: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
CC: "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>,
        Marcelo Tosatti <mtosatti@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Avi Kivity <avi@redhat.com>, S390 <linux-s390@vger.kernel.org>,
        Carsten Otte <cotte@de.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        KVM <kvm@vger.kernel.org>, chegu vinod <chegu_vinod@hp.com>,
        "Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>, X86 <x86@kernel.org>,
        Gleb Natapov <gleb@redhat.com>, linux390@de.ibm.com,
        Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
        Joerg Roedel <joerg.roedel@amd.com>
Subject: Re: [PATCH RFC 2/2] kvm PLE handler: Choose better candidate for
 directed yield
References: <20120709062012.24030.37154.sendpatchset@codeblue> <20120709062053.24030.52767.sendpatchset@codeblue>
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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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