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

From: Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>
To: Rik van Riel <riel@redhat.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
Date: Tue, 10 Jul 2012 17:16:24 +0530	[thread overview]
Message-ID: <4FFC1610.5060801@linux.vnet.ibm.com> (raw)
In-Reply-To: <4FFB5B81.4040903@redhat.com>

On 07/10/2012 04:00 AM, Rik van Riel wrote:
> 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.
>

Thanks for the review. will do more documentation.