linux-kernel - Re: [PATCH RFC 0/2] kvm: Improving directed yield in PLE handler

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Date:	Mon, 09 Jul 2012 18:28:03 -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 0/2] kvm: Improving directed yield in PLE handler

On 07/09/2012 02:20 AM, Raghavendra K T wrote:
> Currently Pause Looop Exit (PLE) handler is doing directed yield to a
> random VCPU on PL exit. Though we already have filtering while choosing
> the candidate to yield_to, we can do better.
>
> Problem is, for large vcpu guests, we have more probability of yielding
> to a bad vcpu. We are not able to prevent directed yield to same guy who
> has done PL exit recently, who perhaps spins again and wastes CPU.
>
> Fix that by keeping track of who has done PL exit. So The Algorithm in series
> give chance to a VCPU which has:
>
>   (a) Not done PLE exit at all (probably he is preempted lock-holder)
>
>   (b) VCPU skipped in last iteration because it did PL exit, and probably
>   has become eligible now (next eligible lock holder)
>
> Future enhancemnets:

Your patch series looks good to me. Simple changes with a
significant result.

However, the simple heuristic could use some comments :)

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